International GCSE Physics · Pearson Edexcel 4PH1 · Topic 4
Energy resources and energy transfers
Original exam-style questions with mark schemes. Questions marked “Paper 2 only” use content assessed only on Paper 2. Notes 4(b) →Notes 4(c) →Notes 4(d) →
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Original questions in the style of each exam boardEvery answer re-solved independently before publishingCreated by Evan Kotronis with AI assistanceUpdated 13 October 2026 · new questions added regularlyHow every question is checked →
1IGCSE-PHY-4-001Energy stores and transfers: firing an arrow from a bowPapers 1 & 2Grade 4–51 mark
An archer pulls back the string of a bow and holds it still. The archer then releases the string and the arrow is fired horizontally. Which of these describes the main energy transfer as the arrow is fired?
Show mark scheme
C (elastic energy store of the bow → kinetic energy store of the arrow);
A is incorrect: the chemical store of the archer was used earlier, while pulling the string back, not as the arrow is fired
B is incorrect: this is the reverse of the transfer that happens
D is incorrect: the arrow is fired horizontally so its height (and gravitational store) does not change significantly
3IGCSE-PHY-4-003Energy stores in a clockwork toy carPapers 1 & 2Grade 4–56 marks
A child turns the key of a clockwork toy car. Turning the key winds up a spring inside the car. The child puts the car on a carpet and lets it go. The car moves forwards, then slows down and stops.
(a)
Name the energy store that decreases in the child as the child turns the key.
(1)
(b)
Name the energy store that increases in the spring as it is wound up.
(1)
(c)
State how energy is transferred from the spring to the kinetic energy store of the car. Choose from: electrically, mechanically, by heating, by radiation.
(1)
(d)
Describe what happens to the energy in the kinetic energy store of the car as it slows down on the carpet.
(2)
(e)
State the principle of conservation of energy.
(1)
Show mark scheme
(a)
chemical;
Do not accept
kinetic
food (without chemical)
(b)
elastic (potential);
Allow
strain
Do not accept
kinetic
spring energy
(c)
mechanically;
Allow
by a force (doing work)
Do not accept
electrically
(d)
kinetic (energy) store (of car) decreases / energy transferred out of kinetic store;
(energy transferred) to thermal store of surroundings/carpet/car (by friction);
Allow
MP2: allow sound transferred to surroundings
MP2: allow 'heats up the carpet/wheels'
Do not accept
energy is lost / used up / destroyed
energy turned into friction
(e)
energy cannot be created or destroyed (only transferred/stored);
Allow
total energy (of a closed system) stays constant
Do not accept
energy is never lost (alone, without reference to creation/destruction or total)
4IGCSE-PHY-4-004Work done and power: pushing a wheelbarrowPapers 1 & 2Grade 4–57 marks
A gardener pushes a wheelbarrow along a level path. The gardener applies a constant horizontal force of 85 N to the wheelbarrow and it moves 24 m in the direction of the force. The journey takes 16 s.
(a)
Calculate the work done by the gardener on the wheelbarrow.
(2)
(b)
State the amount of energy transferred by the gardener to the wheelbarrow.
(1)
(c)
Describe what is meant by power.
(1)
(d)
Calculate the power of the gardener in pushing the wheelbarrow. Give the unit.
(3)
Show mark scheme
(a)
substitution;
evaluation;
e.g.
(W =) 85 × 24
(W =) 2040 (J)
Allow
2000 (J)
(b)
2040 J / same as (a);
Allow
ECF from (a)
(c)
(power is) the rate of transfer of energy / rate of doing work;
Allow
energy transferred per second / work done per unit time
5IGCSE-PHY-4-005Sankey diagram: electric toothbrushPapers 1 & 2Grade 4–56 marks
The Sankey diagram shows the energy transfers in an electric toothbrush in a short time. The diagram is drawn to scale on a grid. The width of each arrow is proportional to the energy it represents. The total electrical energy input is 50 J.
(a)
Use the diagram to determine the energy transferred to the kinetic energy store of the brush head.
(1)
(b)
Use the diagram to determine the energy transferred as sound.
(1)
(c)
State the relationship between efficiency, useful energy output and total energy output.
(1)
(d)
Calculate the efficiency of the toothbrush.
(2)
(e)
Explain why the toothbrush can never have an efficiency of 100%.
(1)
Show mark scheme
(a)
20 J;
(b)
10 J;
(c)
efficiency = useful energy output ÷ total energy output (× 100%);
Allow
total energy input for total energy output
useful power output ÷ total power output/input
standard symbols
useful ÷ total (× 100%)
Do not accept
total ÷ useful
useful ÷ wasted
(d)
substitution;
evaluation;
e.g.
(efficiency =) 20 ÷ 50 (× 100%)
(efficiency =) 40 (%)
Allow
0.4
ECF from (a)
Do not accept
2.5 / 250% (inverted)
(e)
(some) energy is always (wasted/dissipated) to thermal store of surroundings / as sound (because of friction);
Allow
idea that not all input energy is transferred usefully
Do not accept
energy is destroyed / lost (without where it goes)
6IGCSE-PHY-4-021Energy transfer when a mobile phone battery is chargedPapers 1 & 2Grade 4–51 mark
A mobile phone is plugged into a charger connected to the mains supply. Which statement describes the main energy transfer while the phone's battery is charging?
Show mark scheme
B (energy is transferred electrically to the chemical energy store of the battery);
A is incorrect because the useful transfer is electrical; heating the battery is wasted energy
C is incorrect because this describes the battery discharging (the phone being used), not charging
D is incorrect because the charger does not transfer energy by radiation and the battery stores energy chemically
8IGCSE-PHY-4-023Energy stores and transfers in a firework rocketPapers 1 & 2Grade 4–57 marks
A firework rocket is lit. Burning fuel pushes hot gases out of the bottom of the rocket and the rocket accelerates upwards. At the top of its flight the rocket explodes, giving a bright flash of light and a loud bang.
(a)
Name the energy store of the fuel before the rocket is lit.
(1)
(b)
As the rocket accelerates upwards, two energy stores of the rocket increase. Name these two energy stores.
(2)
(c)
Give two ways in which energy is transferred from the explosion to the surroundings.
(2)
(d)
A student says: 'When the firework has finished, all of its energy has been used up.' Explain why the student is wrong.
(2)
Show mark scheme
(a)
chemical (energy store);
Do not accept
heat
fuel energy
electrical
(b)
kinetic (energy store);
gravitational (potential energy store);
Allow
GPE / potential energy
thermal (energy store of the rocket) for either mark — max 1 for thermal
Do not accept
elastic
chemical
(c)
any two from:
(by) radiation / light;
sound;
by heating (of the air) / thermal energy;
Allow
infrared for radiation
Do not accept
electrically
mechanically (alone)
(d)
energy cannot be created or destroyed / total energy is conserved;
(energy has been) transferred/dissipated to the (thermal store of the) surroundings / spread out;
10IGCSE-PHY-4-025Heating a room with a hot-water radiatorPapers 1 & 2Grade 4–58 marks
The diagram shows a side view of a room. A hot-water radiator is fixed to the wall below the window. Point X is just above the radiator. Point Y is near the ceiling on the opposite side of the room.
(a)
Name the process by which thermal energy passes through the metal wall of the radiator from the hot water inside it.
(1)
(b)
Describe the movement of the air in the room once the radiator has been on for some time. You may refer to points X and Y.
(2)
(c)
The radiator also transfers energy by radiation. Name the type of electromagnetic radiation mainly emitted by the radiator.
(1)
(d)
The radiator is to be repainted. The owner can choose matt black paint or gloss white paint. Explain which paint would make the radiator emit more radiation at the same temperature.
(2)
(e)
A sheet of shiny aluminium foil is fixed to the wall directly behind the radiator. Explain how this reduces unwanted energy transfer.
(2)
Show mark scheme
(a)
conduction;
Do not accept
convection
radiation
(b)
(warm) air rises above the radiator / at X (and moves across the ceiling towards Y);
(cooler) air sinks on the far side / near Y and moves back along the floor to the radiator / circulates;
Allow
'convection current' with correct direction for 1 mark if neither MP scored
Do not accept
heat rises
cold air rises
(c)
infrared / IR;
Do not accept
ultraviolet
microwaves
(d)
matt black;
(matt) black / dull surfaces are better emitters (of infrared) (than white/shiny);
Allow
MP2 can be scored from a comparison: gloss white is a poorer emitter
Do not accept
matt black because it absorbs more heat (absorption, not emission)
(e)
(shiny foil) reflects infrared/radiation back into the room;
(so) less energy absorbed by / conducted through the wall (to the outside);
Allow
foil is a poor absorber (of infrared)
Do not accept
foil insulates by trapping air
foil reflects heat (alone) — needs radiation/infrared or into the room
11IGCSE-PHY-4-006Efficiency from total input and wasted energyPapers 1 & 2Grade 6–71 mark
An electric motor in a food mixer is supplied with 250 J of electrical energy. 90 J of this energy is wasted as thermal energy and sound. What is the efficiency of the motor?
Show mark scheme
B (64%);
A is incorrect: 90 ÷ 250 × 100 = 36% is the percentage wasted, not the useful percentage
C is incorrect: 250 ÷ 160 × 100 = 156% inverts the ratio (efficiency cannot exceed 100%)
D is incorrect: 160 ÷ 250 = 0.64 is a ratio; writing it with a % sign forgets to multiply by 100
13IGCSE-PHY-4-008Efficiency of LED and filament lampsPapers 1 & 2Grade 6–77 marks
A hotel replaces its filament lamps with LED lamps. Each LED lamp has an input power of 8.0 W and an efficiency of 35%. Each filament lamp has an input power of 60 W and a useful light output power of 3.0 W.
(a)
Calculate the useful light output power of one LED lamp.
(2)
(b)
Calculate the efficiency of one filament lamp.
(2)
(c)
Calculate the energy wasted by one filament lamp when it is switched on for 2.0 hours. Give your answer in kJ.
14IGCSE-PHY-4-009Practical: comparing thermal conduction in rods of different materialsPapers 1 & 2Grade 6–79 marks
A student investigates how well different materials conduct thermal energy. She uses four rods of the same length. One end of each rod is placed in a container of hot water. A pin is stuck to the other end of each rod with a small blob of wax. She measures the time taken for each pin to fall off. Rod / material: copper, aluminium, steel, glass Time for pin to fall / s: 38, 55, 160, over 600
(a)
State the independent variable in this investigation.
(1)
(b)
State two variables that the student should control.
(2)
(c)
Describe what the results show about the four materials.
(2)
(d)
Describe how thermal energy is transferred by conduction along the copper rod.
(2)
(e)
Suggest one reason why the time for the steel rod may not be reliable, and how the student could improve this.
(2)
Show mark scheme
(a)
(type of) material (of rod);
Do not accept
time
temperature
(b)
any two from:
thickness/diameter of rod;
length of rod;
amount/mass of wax (or size of wax blob);
(starting) temperature of water;
length of rod in the water;
mass of pins;
Do not accept
material
time
(c)
copper is the best conductor / shortest time;
glass is the worst conductor / insulator / longest time;
Allow
full ranking copper > aluminium > steel > glass scores 2 marks
rate of conduction greatest in copper
Do not accept
copper heats slowest
(d)
particles at the hot end gain (kinetic) energy / vibrate more;
(vibrations) pass energy to neighbouring particles / free electrons carry energy along rod;
Allow
MP2: allow delocalised electrons collide with particles further along
Do not accept
heat rises along the rod
particles move along the rod
(e)
any one reason:
hot water cools down during the (long) time / wax amount varies / judging when pin falls;
matching improvement: keep water at constant temperature (e.g. top up / use kettle/water bath) / repeat and find mean / weigh the wax;
15IGCSE-PHY-4-010Practical: convection current in water using a coloured crystalPapers 1 & 2Grade 6–78 marks
A teacher places a small crystal of potassium permanganate at the bottom of a beaker of cold water, close to one side. The crystal dissolves slowly and colours the water purple. The teacher gently heats the beaker with a small Bunsen flame directly below the crystal.
(a)
Describe the movement of the purple colour that the students should see.
(2)
(b)
Explain why the water above the flame moves in this way.
(3)
(c)
The heating element of an electric kettle is at the bottom of the kettle. Explain why this allows all the water in the kettle to be heated.
(2)
(d)
Suggest why the teacher uses a small crystal and a small flame.
(1)
Show mark scheme
(a)
(purple colour) rises above the flame/crystal;
moves across (top) and down the other side (forming a loop/circulation);
Allow
MP2: allow description of a circular path
Do not accept
colour spreads evenly in all directions
(b)
water (above flame) is heated and expands / particles move further apart;
(so) density (of this water) decreases;
less dense water rises AND cooler/denser water sinks (to replace it);
Allow
MP3: allow hot water floats on cooler water
Do not accept
heat rises
particles expand
particles become less dense
(c)
(heated) water near element becomes less dense and rises;
cooler (denser) water sinks to the element / convection current mixes all the water;
Do not accept
heat rises
element heats water by radiation only
(d)
any one from:
colour (trail) is easier to see / not too dark;
convection current is slow (enough) to observe / water does not boil;
16IGCSE-PHY-4-011Reducing unwanted energy transfer: a picnic cool boxPapers 1 & 2Grade 6–75 marks
A picnic cool box keeps food cold on a sunny day. The walls of the box are plastic with a thick layer of polystyrene foam inside. The outside of the box is shiny white. The lid fits tightly.
(a)
Explain how the polystyrene foam reduces the energy transferred into the box.
(2)
(b)
Explain why the outside of the box is shiny white rather than matt black.
(2)
(c)
Explain why the food stays cold for longer if the lid fits tightly.
(1)
Show mark scheme
(a)
(foam/air) is a poor conductor / insulator;
air is trapped (in bubbles) so convection (currents) cannot occur;
Allow
MP1: allow 'good insulator'
Do not accept
foam keeps the cold in
foam reflects heat
(b)
shiny/white surfaces are poor absorbers / good reflectors of (infrared) radiation;
so less energy is absorbed (from the Sun) than with matt black / matt black is a good absorber;
Do not accept
white reflects the cold
shiny surface absorbs more
(c)
prevents (warm) air entering / prevents convection (currents) into the box;
17IGCSE-PHY-4-012Two-stage Sankey diagram: charging and riding an electric bicyclePapers 1 & 2Grade 6–78 marks
The battery of an electric bicycle is charged from the mains. The battery is then used to drive the motor. The Sankey diagram shows the energy transfers for both stages. It is drawn to scale: each small square on the grid is 40 J.
(a(i))
Use the diagram to determine the total energy taken from the mains.
(1)
(a(ii))
Use the diagram to determine the energy stored in the battery.
(1)
(b)
Calculate the efficiency of the motor.
(2)
(c)
Calculate the overall efficiency of the whole process, from the mains to the kinetic energy store.
(2)
(d)
A student says the overall efficiency is found by adding the efficiencies of the charger and the motor. Explain why the student is wrong.
(2)
Show mark scheme
(a(i))
400 J;
(a(ii))
320 J;
Allow
ECF: (a)(i) − 80 J
(b)
substitution;
evaluation;
e.g.
(efficiency =) 240 ÷ 320 (× 100)
(efficiency =) 75 (%)
Allow
0.75
ECF from (a)(ii)
Do not accept
60% (uses mains input)
(c)
substitution;
evaluation;
e.g.
(efficiency =) 240 ÷ 400 (× 100)
(efficiency =) 60 (%)
Allow
0.80 × 0.75 (× 100) for substitution
ECF from (a)(i)
Do not accept
40% (wasted fraction)
(d)
(adding gives 155%, but) efficiency cannot be more than 100% / output cannot exceed input;
energy is wasted in both stages, so overall efficiency is less than either / is the product of the two efficiencies;
Allow
MP2: allow motor's input is only the energy stored in the battery, not the mains input
18IGCSE-PHY-4-016Power and efficiency of a crane from a height–time graphPapers 1 & 2Grade 6–79 marks
A building-site crane lifts a load of mass 450 kg vertically. The graph shows how the height of the load changes with time. The electric motor of the crane has an input power of 9.0 kW while the load is moving.
(a)
Use the graph to determine the speed of the load while it is moving upwards.
(2)
(b)
Calculate the gravitational potential energy gained by the load.
(2)
(c)
Calculate the useful power output of the crane while the load is moving.
(2)
(d)
Calculate the efficiency of the crane.
(3)
Show mark scheme
(a)
use of gradient / height ÷ time (for moving section);
19IGCSE-PHY-4-026Comparing kinetic energy of an empty and a loaded truckPapers 1 & 2Grade 6–71 mark
An empty truck has a mass of 6000 kg and travels at 20 m/s. When it is fully loaded, the truck has a mass of 12 000 kg and travels at 10 m/s. Which statement about the kinetic energy of the loaded truck is correct?
Show mark scheme
A (half the kinetic energy of the empty truck);
B is incorrect because it assumes doubling the mass cancels halving the speed (speed is not squared)
C is incorrect because it applies the effect of halving the speed (× ¼) but ignores doubling the mass
D is incorrect because it squares the mass factor instead of the speed factor (2² × ½ = 2)
20IGCSE-PHY-4-027Speed of a dropped phone from energy conservationPapers 1 & 2Grade 6–71 mark
A mobile phone of mass 0.20 kg falls from a shelf 1.8 m above the floor. Air resistance can be ignored. g = 10 N/kg. What is the speed of the phone just before it hits the floor?
Show mark scheme
C (6.0 m/s);
A is incorrect because 36 is v² (2 × 10 × 1.8); the square root has not been taken
B is incorrect because it uses v² = g × h (= 18), leaving out the factor of 2 from ½mv²
D is incorrect because it uses v = g × h (10 × 1.8) with no square root and no factor of 2
21IGCSE-PHY-4-028Sankey diagram: hairdryer with two useful outputsPapers 1 & 2Grade 6–76 marks
The Sankey diagram shows the energy transfers in a hairdryer in 1.0 s when it is on its hot setting. The diagram is drawn to scale on a grid. The width of each arrow is proportional to the energy it represents. In 1.0 s, 180 J of energy is transferred to the kinetic energy store of the air.
(a)
Determine the energy represented by the width of one small square on the grid.
(1)
(b)
Determine the total electrical energy input to the hairdryer in 1.0 s.
(1)
(c)
Both the thermal energy store of the air and the kinetic energy store of the air are useful outputs. Calculate the efficiency of the hairdryer.
(2)
(d)
Another student calculates the efficiency of the hairdryer as 80%. Suggest the mistake this student made.
(1)
(e)
Determine the power input to the hairdryer.
(1)
Show mark scheme
(a)
60 J;
(b)
1200 J;
Allow
ECF from (a): 20 × (a)
sum of outputs 960 + 180 + 60
(c)
substitution;
evaluation;
e.g.
(efficiency =) (960 + 180) ÷ 1200 (× 100%)
(efficiency =) 95 (%)
Allow
0.95
ECF from (a) and (b)
(1200 − 60) ÷ 1200
Do not accept
80 (%) (thermal output only)
15 (%) (kinetic output only)
105 (%) / 1.05 (inverted)
(d)
only counted the thermal (energy store of the air) as useful / ignored the kinetic energy (of the air);
23IGCSE-PHY-4-030Work done by a winch from a force–distance graphPapers 1 & 2Grade 6–78 marks
An electric winch pulls a sailing dinghy up a concrete slipway at a steady speed, starting from the water's edge. The slipway is steeper near the water and less steep further up. The graph shows how the force exerted by the winch on the dinghy, along the slipway, changes with the distance moved along the slipway. The mass of the dinghy is 240 kg. Over the whole 14 m, the dinghy rises through a vertical height of 1.5 m.
(a)
Use the graph to calculate the work done by the winch in the first 6.0 m.
(2)
(b)
Calculate the total work done by the winch over the whole 14 m.
(2)
(c)
Calculate the gravitational potential energy gained by the dinghy.
(2)
(d)
The total work done by the winch is greater than the gravitational potential energy gained by the dinghy. Explain why.
(2)
Show mark scheme
(a)
substitution using values from the graph;
evaluation;
e.g.
(W =) 900 × 6.0
(W =) 5400 (J)
Allow
5.4 kJ
Do not accept
150 (N/m) (force ÷ distance)
(b)
work for second section = 650 × 8.0 (= 5200 J);
total evaluated;
e.g.
(W =) 5400 + 5200
(W =) 10 600 (J)
Allow
ECF from (a)
10.6 kJ / 1.06 × 10⁴ J
Do not accept
12 600 (J) (900 × 14)
9100 (J) (650 × 14)
10 850 (J) (775 × 14, average of the two forces)
(c)
substitution;
evaluation;
e.g.
(GPE =) 240 × 10 × 1.5
(GPE =) 3600 (J)
Allow
3530 (J) using g = 9.8 or 9.81
Do not accept
33 600 (J) (uses 14 m)
360 (J) (g omitted)
(d)
work (also) done against friction / resistive forces (between dinghy and slipway);
(this) energy is transferred to the thermal energy store of the dinghy/slipway/surroundings;
Allow
work done against resistive forces / friction (alone) for MP1
dissipated as sound for MP2 only if thermal also mentioned
24IGCSE-PHY-4-031Practical: infrared emission from the faces of a Leslie cubePapers 1 & 2Grade 6–79 marks
A Leslie cube is a hollow metal cube. Each of its four vertical faces has a different surface. A student fills the cube with hot water from a kettle. She places an infrared detector 10 cm from one face and records the reading. She then turns the cube so that each face in turn is 10 cm from the detector. Surface / type: matt black, shiny black, dull silver, shiny silver Detector reading / arbitrary units: 9.2, 7.8, 3.0, 1.1
(a)
Explain why all four faces of the cube are at the same temperature.
(1)
(b)
State two other variables that the student should keep the same.
(2)
(c)
Describe what the results show about how the type of surface affects the emission of infrared radiation.
(2)
(d)
The student repeats all four readings 15 minutes later. Every reading is smaller than before. Explain why.
(2)
(e)
The cooling pipes on the back of a refrigerator transfer energy to the air in the kitchen. Use the results to suggest the best surface for these pipes. Give a reason for your answer.
(2)
Show mark scheme
(a)
(all faces are) in contact with the same (hot) water;
Allow
cube contains one body of water at one temperature
(b)
any two from:
distance between detector and face;
angle of detector to the face / detector pointing straight at face;
time between filling and taking readings / readings taken quickly;
same detector;
same room / surroundings / no draughts;
Do not accept
temperature of the water (already stated in a)
type of surface
(c)
black/dark surfaces emit more (infrared) than silver/light surfaces (with the same texture);
matt/dull surfaces emit more than shiny surfaces of the same colour;
Allow
matt black best emitter AND shiny silver worst emitter for 1 mark if neither MP scored
quoting data in support
Do not accept
black absorbs more (absorption not tested)
(d)
the water/cube has cooled / is at a lower temperature;
a cooler surface emits (infrared) radiation at a lower rate / less energy per second;
Do not accept
the cube has run out of heat
the surfaces have changed
(e)
matt black;
best emitter (of infrared) so transfers energy (to the kitchen) fastest;
Allow
MP2 from data: matt black gave the highest reading
25IGCSE-PHY-4-032Practical: position of the heater in a model hot-water cylinderPapers 1 & 2Grade 6–79 marks
A student uses a tall tube of water as a model of a hot-water cylinder in a house. Three thermometers measure the water temperature at the top, middle and bottom of the tube. In experiment A, a small electric heater is placed at the bottom of the tube. In experiment B, the same heater is placed near the top. In each experiment the water starts at 20 °C and the heater is switched on for 10 minutes. The water is not stirred. Thermometer / position: top, middle, bottom Temperature in experiment A / °C: 39, 37, 35 Temperature in experiment B / °C: 68, 22, 20
(a)
State the independent variable in this investigation.
(1)
(b)
Compare the results of experiment A and experiment B.
(2)
(c)
Explain the results of experiment B.
(3)
(d)
In a house, hot water is taken from the top of the cylinder. Suggest one advantage of having the heater near the top of the cylinder when only a small amount of hot water is needed quickly.
(2)
(e)
Explain why the student must not stir the water before taking the readings.
(1)
Show mark scheme
(a)
position of the heater;
Allow
height of the heater
Do not accept
temperature
time
(b)
A: all three temperatures similar / whole tube warmed (35–39 °C);
B: top much hotter (68 °C) but middle and bottom almost unchanged (22 °C and 20 °C);
Allow
comparative statements e.g. top hotter in B than A, bottom hotter in A than B for 1 mark
(c)
heated water (near the top) expands / becomes less dense;
so it stays at the top / does not sink / no convection current reaches the lower water;
water is a poor (thermal) conductor so little energy reaches the bottom by conduction;
Do not accept
heat rises (alone)
cold rises
(d)
only a small volume of water (at the top) is heated;
so it reaches a high temperature quickly / less energy is needed (than heating the whole cylinder);
Do not accept
the heater uses less power
(e)
stirring would mix the water / remove the temperature differences (being measured);
Allow
stirring would make a forced current that is not convection
26IGCSE-PHY-4-035Practical: absorption of infrared by matt black and shiny silver platesPapers 1 & 2Grade 6–79 marks
A student investigates how the type of surface affects the absorption of infrared radiation. She uses two identical aluminium plates. The front of one plate is painted matt black and the front of the other is polished shiny silver. A temperature sensor is attached to the back of each plate. The plates are placed facing an electric radiant heater, one on each side of it, both 15 cm from the heater. She switches on the heater and records the temperature of each plate every minute. The table shows her results. The graph shows the same results plotted. Time / min: 0, 1, 2, 3, 4, 5, 6, 7, 8 Matt black plate temperature / °C: 20, 26, 31, 35, 38, 40, 41.5, 42, 42 Shiny silver plate temperature / °C: 20, 22, 24, 25.5, 27, 28, 28.5, 29, 29
(a)
Explain why both plates are placed the same distance from the heater.
(1)
(b)
Use the graph to determine the increase in temperature of the matt black plate • during the first minute • between 4 minutes and 5 minutes. Use your answers to describe how the rate of temperature rise of the matt black plate changes.
(3)
(c)
The student concludes that matt black surfaces are better absorbers of infrared radiation than shiny silver surfaces. The heater might not emit infrared radiation equally from both of its sides. Describe how the student could check that her conclusion is valid, and state the result that would support it.
(2)
(d)
Explain why the temperature of each plate eventually stops rising, even though the heater is still on.
(3)
Show mark scheme
(a)
so both receive the same amount of (infrared) radiation per second / same intensity;
Allow
fair test because intensity depends on distance
Do not accept
so it is a fair test (alone)
(b)
first minute: 6 (°C);
between 4 and 5 minutes: 2 (°C);
rate of temperature rise decreases (as the plate gets hotter / as time goes on);
e.g.
26 − 20 = 6 (°C) and 40 − 38 = 2 (°C)
Allow
MP3: allow the plate heats up more slowly later / rise in the 5th minute is a third of the first
MP3: ECF from incorrectly read values if the comparison is consistent
unit not required
Do not accept
26 (°C) / 40 (°C) (temperature read instead of the increase)
rate increases
(c)
swap the positions of the two plates (and repeat the experiment);
matt black plate still has the faster temperature rise / reaches the higher temperature;
Allow
MP1: test each plate in turn in the same position (each starting at room temperature)
MP2: matt black still gives the larger temperature rise in the same time
Do not accept
repeat the experiment (alone, with no change of position)
use a more accurate thermometer
(d)
as temperature rises the plate loses/emits energy (to surroundings) at a greater rate;
(eventually) rate of energy loss = rate of energy absorbed (from heater);
so no net energy gain / energy in = energy out, so temperature stays constant;
Allow
energy lost by radiation, conduction or convection to the air
27IGCSE-PHY-4-013Energy and work against friction on a skateboard half-pipePapers 1 & 2Grade 8–99 marks
A skateboarder of total mass 55 kg (including the skateboard) starts from rest at the top of a half-pipe. The top is 3.2 m above the lowest point. He rolls down the curved ramp without pushing. The distance he travels along the ramp to the lowest point is 5.0 m. At the lowest point his speed is 7.4 m/s.
(a)
Show that the gravitational potential energy lost by the skateboarder is about 1800 J.
(2)
(b)
Calculate the average resistive force acting on the skateboarder as he moves along the ramp to the lowest point.
(5)
(c)
Explain, without further calculation, why the skateboarder cannot reach a height of 3.2 m on the other side of the half-pipe if he does not push.
(2)
Show mark scheme
(a)
substitution;
evaluation to at least 3 s.f.;
e.g.
(GPE =) 55 × 10 × 3.2
(GPE =) 1760 (J)
Allow
use of g = 9.8 / 9.81 giving 1720 / 1730 (J)
Do not accept
1800 (J) without working
(b)
substitution into KE = ½ × m × v²;
evaluation of KE;
work done against resistive force = GPE lost − KE gained;
substitution into W = F × d (with d = 5.0 m);
evaluation of force;
e.g.
KE = 0.5 × 55 × 7.4² = 1506 (J)
work = 1760 − 1506 = 254 (J)
254 = F × 5.0
(F =) 51 (N)
Allow
ECF from (a) and from calculated KE
50 (N) from rounded values (1760 − 1510) ÷ 5.0
58 (N) from (1800 − 1510) ÷ 5.0 scores 4 marks
use of vertical drop 3.2 m for d giving 79 (N) scores 4 marks
KE not squared (204 J) used correctly thereafter: max 3 marks
Do not accept
352 (N) (GPE ÷ 5.0, ignores KE)
(c)
(at the bottom) KE is less than the GPE needed to rise 3.2 m / some energy already transferred to thermal store;
(and) work is done against resistive forces/friction on the way up too, (so even less energy available for GPE);
28IGCSE-PHY-4-014Practical: rate of cooling of matt black and shiny silver cansPapers 1 & 2Grade 8–910 marks
A student investigates how the type of surface affects the rate of energy transfer by radiation. She uses two identical aluminium cans. One is painted matt black and the other is polished shiny silver. She pours hot water into each can and measures the temperature every 2 minutes. The graph and table show her results. Time / min: 0, 2, 4, 6, 8, 10 Matt black can temperature / °C: 80, 72, 65, 59, 54, 50 Shiny silver can temperature / °C: 80, 76, 72, 68.5, 65.5, 63
(a)
State two variables the student must keep the same to make this a fair test.
(2)
(b)
Calculate the average rate of decrease in temperature of the water in the matt black can over the 10 minutes. Give the unit.
(3)
(c)
The student claims: 'The matt black can transfers energy to the surroundings twice as fast as the shiny silver can.' Evaluate this claim. Use data from the table in your answer.
(3)
(d)
Explain why the rate of decrease in temperature of each can becomes smaller as time goes on.
(2)
Show mark scheme
(a)
any two from:
volume/mass of water;
starting temperature of water;
size/shape/material of can;
room temperature / position in room;
both with lid or both without lid;
Do not accept
colour of can
time
(b)
(temperature drop =) 30 (°C);
evaluation (30 ÷ 10);
°C/min (or °C per minute);
e.g.
(rate =) (80 − 50) ÷ 10
(rate =) 3.0
Allow
0.05 °C/s with unit scores 3 marks
unit mark independent
Do not accept
5 (final temperature ÷ time)
(c)
rate (or drop) for shiny can calculated: 1.7 °C/min OR 17 °C;
comparison: black/shiny ratio ≈ 1.8 / black drop is less than twice shiny drop (30 < 34);
conclusion: black can cools faster but claim (of twice as fast) is not supported;
Allow
ECF from (b)
MP3: allow comparison at a single time interval if correctly calculated, e.g. first 2 min: 8 vs 4 °C so twice at start but not overall
MP3: allow 'not a valid conclusion because some energy transferred by conduction/convection, not only radiation'
Do not accept
black is better (alone) without data
(d)
(as water cools) the temperature difference between can and surroundings decreases;
(cooler) surface emits (infrared) radiation at a lower rate / transfers energy more slowly;
29IGCSE-PHY-4-015Practical: planning an investigation of insulating materialsPapers 1 & 2Grade 8–96 marks
A student wants to find out which of four materials is the best thermal insulator: cotton wool, bubble wrap, newspaper and felt. She has access to standard laboratory equipment, a kettle and a supply of water.
(a)
Describe an investigation the student could carry out to find out which material is the best thermal insulator. You should include the equipment she should use, the measurements she should take, how she makes it a fair test and how she uses her results to reach a conclusion.
(6)
Show mark scheme
(a)
Indicative content:
MP1. use identical beakers/cans, each wrapped in one of the materials;
MP2. same thickness / same number of layers of each material;
MP3. same volume/mass of hot water (measured with measuring cylinder) in each;
MP4. measure starting temperature with a thermometer / same starting temperature;
MP5. measure temperature after a fixed time (e.g. 10 minutes) using a stopwatch / at regular intervals;
MP6. cover with lid (of same material) / same room temperature / same surface;
MP7. include an unwrapped beaker as a control (for comparison);
MP10. smallest temperature drop / slowest rate of cooling = best insulator;
MP11. safety: care with hot water / stand beakers on heatproof mat;
Allow
award 1 mark per MP to a maximum of 6, but use the levels to judge the overall quality
MP5: allow data logger with temperature probe
Do not accept
best insulator is the one that gets hottest
measure temperature of the material instead of the water
Level 0 (0)
No rewardable content.
Level 1 (1–2)
Basic method with little detail, e.g. wrap beakers and measure temperature. Few or no control variables; no clear way of reaching a conclusion.
Level 2 (3–4)
A workable method with measurements of temperature and time and at least one control variable; conclusion may be stated but not fully linked to the measured data.
Level 3 (5–6)
Detailed, logically sequenced method including measuring instruments, several control variables (or a control beaker/repeats) and a clear way of using temperature drop or rate of cooling to identify the best insulator.
30IGCSE-PHY-4-033Useful power of a cyclist climbing a hill against resistive forcesPapers 1 & 2Grade 8–98 marks
A cyclist and her bicycle have a total mass of 80 kg. She cycles up a hill at a steady speed. The road is 1200 m long and rises through a vertical height of 60 m. She takes 4.0 minutes to reach the top. The average resistive force (air resistance and friction) on the cyclist and bicycle is 25 N, acting along the road.
(a)
Calculate the average useful power output of the cyclist as she climbs the hill.
(5)
(b)
The efficiency of the cyclist's muscles is 25%. Calculate the rate at which energy is transferred from the chemical energy store of her body while she climbs.
(2)
(c)
Explain why a change in kinetic energy is not included in your answer to (a).
(1)
Show mark scheme
(a)
GPE gained = 80 × 10 × 60 (= 48 000 J);
work done against resistive force = 25 × 1200 (= 30 000 J);
total useful energy = 78 000 (J);
time converted to 240 s;
evaluation of power;
e.g.
(P =) 78 000 ÷ 240
(P =) 325 (W)
Allow
330 (W)
200 (W) (GPE only, 48 000 ÷ 240) scores 3 marks
125 (W) (resistive work only) scores 3 marks
19 500 (W) (time not converted) scores 4 marks
Do not accept
work against resistance calculated with 60 m (25 × 60)
(b)
substitution into efficiency relationship (useful ÷ total);
evaluation;
e.g.
(rate =) 325 ÷ 0.25
(rate =) 1300 (W)
Allow
ECF from (a)
1320 (W) from 330 W
1.3 kW / 1300 J/s
Do not accept
81 (W) (325 × 0.25)
(c)
(steady speed so) kinetic energy (store) does not change / no change in KE;
31IGCSE-PHY-4-034Regenerative braking and supercapacitor storage in a city tramPapers 1 & 2Grade 8–99 marks
A city tram has a mass of 32 000 kg. It is travelling at 15 m/s when it brakes and comes to rest at a tram stop. The tram uses regenerative braking: its motors act as generators, and 60% of the kinetic energy of the tram is transferred electrically to a bank of supercapacitors on board the tram. The supercapacitors store the energy.
(a)
Calculate the energy transferred to the supercapacitors as the tram comes to rest.
(3)
(b)
When the tram leaves the stop, all of the energy stored in the supercapacitors is transferred to the motors. The motors transfer 75% of this energy to the kinetic energy store of the tram. Calculate the speed of the tram when all of the stored energy has been used. Ignore any other energy transfers.
(4)
(c)
Describe what happens to the other 40% of the kinetic energy of the tram during braking.
32IGCSE-PHY-4-036Thermal design of an astronaut's spacesuitPapers 1 & 2Grade 8–96 marks
An astronaut on a spacewalk wears a spacesuit. Space is almost a vacuum. In direct sunlight, the Sun's radiation can make the outside of the suit very hot. In the Earth's shadow, the suit can lose energy to cold space. The suit has: • a white outer layer • several thin sheets of shiny, aluminium-coated plastic inside the outer layer, with vacuum between the sheets • an inner garment containing thin tubes, through which cool water is pumped next to the astronaut's skin.
(a)
Explain how the design of the spacesuit keeps the astronaut at a safe temperature. Your answer should refer to conduction, convection and radiation.
(6)
Show mark scheme
(a)
Indicative content:
MP1. space is a vacuum / has no particles so no conduction or convection between suit and space;
MP2. so energy is transferred to or from the suit (in space) only by radiation;
MP3. white outer layer is a poor absorber / good reflector of the Sun's radiation, reducing heating in sunlight;
MP4. shiny (aluminium) sheets are poor emitters of infrared, reducing energy loss in shadow;
MP5. shiny sheets reflect infrared back (towards the astronaut or away from the inside);
MP6. vacuum between sheets has no particles so prevents conduction and convection between layers;
MP7. thin sheets with little contact reduce conduction through the suit;
MP8. the astronaut's body continually transfers energy to its thermal store, which cannot escape through the insulated suit;
MP9. cool water absorbs energy from the skin by conduction (heating) and carries it away as it flows;
MP10. overall, the rate of energy gain is balanced by the rate of energy removal so temperature stays constant;
Allow
award 1 mark per MP to a maximum of 6, but use the levels to judge the overall quality
MP3: allow 'white surface reflects light/radiation from the Sun'
MP9: allow 'water carries energy away by convection/flow'
Do not accept
vacuum reflects heat
cold is kept out by the suit
white surface is a good absorber
shiny sheets trap air (there is vacuum, not air, between them)
Level 0 (0)
No rewardable content.
Level 1 (1–2)
Isolated statements about one or two features, e.g. 'white reflects heat' or 'the vacuum stops heat', with little or no correct reference to the type of transfer.
Level 2 (3–4)
Explains how at least two features reduce a named type of energy transfer (e.g. vacuum prevents conduction and convection; white/shiny surfaces reduce absorption or emission of radiation). Some links may be incomplete.
Level 3 (5–6)
A coherent explanation covering conduction, convection and radiation, linking each design feature to the transfer it reduces, recognising that only radiation occurs in space AND explaining why the water cooling is needed to remove energy produced by the body.
34IGCSE-PHY-4-037Nuclear power station: correcting a description of energy transfersPaper 2 onlyGrade 4–57 marks
The block diagram shows the main parts of a nuclear power station. In the reactor, the fission of uranium fuel heats water in the boiler to make high-pressure steam. The steam drives a turbine, which turns a generator.
(a)
Name the energy store of the uranium fuel, and state how energy is transferred from the reactor to the water in the boiler.
(2)
(b)
A student writes: 'In the turbine, the thermal energy of the steam is changed into electrical energy, which is stored in the generator.' Identify two mistakes in the student's statement.
(2)
(c)
Name one energy resource that can generate electricity without heating water to make steam.
(1)
(d)
Give one advantage and one disadvantage of using nuclear power stations to generate electricity on a large scale.
(2)
Show mark scheme
(a)
nuclear (energy store);
(transferred) by heating;
Allow
thermally for by heating
Do not accept
chemical (store)
radioactive energy
by radiation (alone)
mechanically
(b)
any two from:
(steam/turbine) energy goes to the kinetic (energy store) of the turbine / is transferred mechanically, not into electrical energy;
the generator does not store (electrical) energy / electrical energy is not a store;
(it is) the generator (not the turbine) that transfers energy electrically (to the grid);
Allow
a correct rewritten statement that removes both mistakes scores 2 marks
Do not accept
the steam is not thermal energy (alone)
the turbine stores kinetic energy (alone) with no reference to the mistake
(c)
wind / hydroelectric / water / tidal / wave / solar cells;
Allow
photovoltaic for solar cells
Do not accept
geothermal
solar heating
coal / oil / gas / biofuel / nuclear
solar (unqualified)
(d)
advantage: no CO₂/greenhouse gases (when generating) / reliable or constant output / large energy from small mass of fuel;
disadvantage: radioactive waste (long-lived, needs storage) / high building or decommissioning cost / risk of accident or leak / non-renewable fuel;
Allow
'does not contribute to global warming' for advantage
35IGCSE-PHY-4-018Geothermal power and solar heating systemsPaper 2 onlyGrade 6–78 marks
The diagram shows a geothermal power station. Cold water is pumped down to hot rocks deep underground. Hot water and steam return to the surface.
(a)
Describe the energy transfers involved in generating electricity in a geothermal power station.
(3)
(b)
Give one disadvantage of geothermal power stations.
(1)
(c)
A house has a solar heating system and a solar cell panel on its roof. Describe how the energy transfer in the solar heating system is different from the energy transfer in the solar cells.
(2)
(d)
On a sunny day, the power of the sunlight reaching the solar cell panel is 1.6 kW. The electrical power output of the panel is 0.30 kW. Calculate the efficiency of the solar cell panel.
(2)
Show mark scheme
(a)
energy (from thermal store of) hot rocks transferred by heating to water / turns water to steam;
steam turns turbine / kinetic store of turbine;
generator transfers energy electrically (to the grid);
Allow
MP3: allow 'generator produces electricity'
Do not accept
rocks burn
steam turns generator directly with no turbine (MP2)
(b)
any one from:
only available in a few places / volcanic regions / where hot rocks near surface;
high cost of drilling deep wells;
can release harmful gases (e.g. hydrogen sulfide/CO₂) from underground;
Do not accept
it is non-renewable
does not work at night
(c)
solar heating system: (infrared) radiation absorbed to heat water / increase thermal store of water;
solar cells: (light) radiation transferred (directly) electrically / produces electricity;
36IGCSE-PHY-4-038Pumped-storage hydroelectric power stationPaper 2 onlyGrade 6–79 marks
The diagram shows a pumped-storage hydroelectric power station. When electricity is needed, water flows from the upper reservoir down a pipe to a turbine and generator, then into the lower reservoir. At times of low demand, electric pumps lift water back up to the upper reservoir. g = 10 N/kg
(a)
Describe the energy transfers that take place as water flows down from the upper reservoir and electricity is generated.
(3)
(b)
When the station is generating, 90 000 kg of water passes through the turbines every second. The water falls through a vertical height of 300 m. Calculate the gravitational potential energy lost by the water each second.
(2)
(c)
The electrical power output of the station is 230 MW. Calculate the efficiency of the station when generating.
(2)
(d)
Give one advantage and one disadvantage of a pumped-storage scheme for electricity supply.
(2)
Show mark scheme
(a)
gravitational (potential energy store of water) decreases and kinetic (store of water) increases;
(moving water) transfers energy mechanically to the kinetic (energy store) of the turbine;
generator transfers energy electrically (to the grid);
advantage: stores energy at times of low demand (e.g. surplus from wind/nuclear at night) and can supply it very quickly at peak demand / no fuel or CO₂ at the station;
disadvantage: more energy needed to pump water up than is generated / few suitable sites / flooding of land or harm to habitats / high building cost;
37IGCSE-PHY-4-019Evaluating replacement of a gas-fired power station with offshore wind turbinesPaper 2 onlyGrade 8–911 marks
A gas-fired power station supplies a steady electrical power of 600 MW. Engineers propose replacing it with offshore wind turbines. The table shows the mean power output of one turbine in each of 8 weeks. Week / number: 1, 2, 3, 4, 5, 6, 7, 8 Mean power of one turbine / MW: 5.6, 1.2, 3.8, 0.6, 2.9, 4.4, 5.1, 2.0
(a)
Describe the energy transfers in a gas-fired power station, from the fuel to the electricity supplied to the grid.
(3)
(b)
Calculate the mean power output of one turbine over the 8 weeks.
(2)
(c)
Calculate the number of turbines needed so that their mean power output over the 8 weeks equals 600 MW.
(2)
(d)
A council member says: 'If we build the number of turbines calculated in (c), they will always supply as much electricity as the gas-fired power station.' Evaluate this statement. Use a calculation in your answer.
(4)
Show mark scheme
(a)
chemical (energy) store of gas/fuel transferred by burning/combustion (to thermal store);
water heated / turned into steam (thermal store of steam), which turns the turbine (kinetic store);
generator transfers energy electrically;
Do not accept
gas turns the turbine directly (MP2)
(b)
sum ÷ 8 / substitution;
evaluation;
e.g.
(mean =) 25.6 ÷ 8
(mean =) 3.2 (MW)
(c)
substitution;
evaluation rounded up to a whole number;
e.g.
(N =) 600 ÷ 3.2 = 187.5
(N =) 188
Allow
ECF from (b) (rounded up)
187.5 scores 1 mark
187 scores 1 mark
(d)
calculation of output in a low week, e.g. week 4: 188 × 0.6 = 113 (MW) / week 2: 188 × 1.2 = 226 (MW);
comparison: much less than 600 MW (e.g. about 19%) / output sometimes exceeds 600 MW (e.g. week 1: 1053 MW);
statement is incorrect because wind (output) varies / depends on the weather (only the mean matches);
need backup / energy storage / keep (part of) gas station for calm periods;
Allow
ECF from (c)
MP4: allow balanced point that wind is renewable/no CO₂ emissions/no fuel cost but this does not make the statement correct
38IGCSE-PHY-4-020Evaluating energy resources to replace diesel generators on an islandPaper 2 onlyGrade 8–96 marks
A small island gets all of its electricity from diesel generators. Diesel is a fossil fuel brought to the island by ship. The island council is considering three alternatives: • a large array of solar cells (a solar farm) • a tidal barrage across a bay • a small nuclear power station
(a)
Discuss the advantages and disadvantages of each of the three alternatives compared with the diesel generators.
(6)
Show mark scheme
(a)
Indicative content:
MP1. diesel: available on demand / reliable BUT non-renewable and releases CO₂ (greenhouse gas) / pollution;
MP2. diesel: fuel must be shipped to island (cost/supply can be interrupted / spill risk);
MP3. solar cells: renewable / no fuel cost / no CO₂ released when generating;
MP4. solar cells: no output at night and less on cloudy days, so storage (e.g. batteries) or backup needed;
MP5. solar cells: need a large area of land on a small island / high initial cost for the output obtained;
MP6. tidal: renewable and predictable / tides occur twice a day and times known in advance;
MP7. tidal: high building cost / damages habitats in the bay / affects shipping; output varies through the tidal cycle;
MP8. nuclear: reliable (large constant output) and no CO₂ released when generating / small mass of fuel;
MP9. nuclear: radioactive waste must be stored safely for a long time / risk of accident / non-renewable;
MP10. nuclear: high cost of building and decommissioning / may be too large for small island demand;
MP11. sensible conclusion supported by points, e.g. solar + tidal with some backup or storage;
Allow
award 1 mark per MP to a maximum of 6, but use the levels to judge the overall quality
MP4: allow 'depends on the weather / time of day'
Do not accept
nuclear power releases CO₂ / causes global warming when generating
tides are unpredictable
solar cells generate electricity at night
Level 0 (0)
No rewardable content.
Level 1 (1–2)
Isolated advantages or disadvantages, mostly for one alternative, with little comparison to diesel.
Level 2 (3–4)
Advantages and disadvantages given for at least two alternatives with some comparison to diesel; reasoning partly developed.
Level 3 (5–6)
Balanced discussion of all three alternatives, with both advantages and disadvantages linked to the island's situation and compared with diesel; may reach a supported conclusion.
39IGCSE-PHY-4-039Wind turbine: kinetic energy extracted from moving airPaper 2 onlyGrade 8–910 marks
On a windy day, air approaches a wind turbine at a speed of 12 m/s. Every second, 8000 kg of air passes through the area swept by the blades. Behind the turbine, the speed of this air has fallen to 9.0 m/s. Assume that all of this air slows from 12 m/s to 9.0 m/s. The electrical power output of the turbine's generator is 200 kW.
(a)
Describe the energy transfers involved in generating electricity using a wind turbine.
(3)
(b)
Calculate the efficiency with which the turbine transfers the kinetic energy lost by the air into electrical energy.
(5)
(c)
An engineer suggests redesigning the blades so that the air leaves the turbine at 0 m/s, so that all of the kinetic energy of the wind is transferred. Explain why this is not possible.
(2)
Show mark scheme
(a)
kinetic (energy store) of the air/wind decreases;
energy transferred mechanically to the kinetic (energy store) of the blades/turbine;
generator transfers energy electrically (to the grid);
Allow
some energy dissipated to thermal store / as sound
Do not accept
wind energy store
chemical
(b)
KE of air arriving per second = 0.5 × 8000 × 12² (= 576 000 J);
KE of air leaving per second = 0.5 × 8000 × 9.0² (= 324 000 J);
KE lost per second = 252 000 (J);
substitution into efficiency relationship (200 000 ÷ 252 000);
evaluation;
e.g.
(efficiency =) 200 000 ÷ 252 000 (× 100%)
(efficiency =) 79 (%)
Allow
0.79
79.4 (%)
ECF for an arithmetic slip in KE lost
35 (%) (200 000 ÷ 576 000, leaving air ignored) scores 2 marks
use of (12 − 9)² giving 36 000 J and an efficiency > 100%: max 1 mark (efficiency substitution)
Do not accept
126 (%) / 1.26 (inverted)
0.079 (%) (kW and W mixed)
(c)
the air would stop / pile up at (or behind) the blades;
so no more air could flow through the turbine / the air must keep moving (with some KE) to leave the turbine;
40IGCSE-PHY-4-040Sankey diagram of a coal-fired power station and combined heat and powerPaper 2 onlyGrade 8–911 marks
The Sankey diagram shows the energy transfers each second in a coal-fired power station. It is drawn to scale on a grid. The width of each arrow is proportional to the power it represents. The chemical energy from the coal is transferred at a rate of 2000 MW.
(a)
The coal is burned to heat water in a boiler. Name the energy store that decreases as the coal burns and the energy store of the cooling water that increases.
(2)
(b)
Use the diagram to determine the electrical power output, and calculate the efficiency of the power station.
(3)
(c)
The power station is converted to 'combined heat and power'. Hot water from the cooling system is piped to heat nearby homes. After the conversion, the coal input is still 2000 MW, the electrical output falls to 600 MW and 800 MW is supplied to the homes for heating. Before the conversion, these homes were heated by gas boilers with an efficiency of 80%. The electrical output that is lost is now generated by another power station with an efficiency of 40%. A local newspaper claims: 'The conversion reduces the total rate at which energy from fuels is used.' Evaluate this claim. Use calculations in your answer.
(5)
(d)
Give one disadvantage of generating electricity using coal compared with using renewable resources.
(1)
Show mark scheme
(a)
chemical (energy store of the coal) decreases;
thermal (energy store of the cooling water) increases;
Allow
internal energy for thermal
Do not accept
heat energy store
nuclear
(b)
electrical output = 700 MW (from the diagram);
substitution into efficiency relationship;
evaluation;
e.g.
(efficiency =) 700 ÷ 2000 (× 100%)
(efficiency =) 35 (%)
Allow
0.35
ECF for a misread output value
Do not accept
2.9 / 286 (%) (inverted)
50 (%) (cooling water used as useful)
(c)
fuel (gas) energy no longer needed for boilers = 800 ÷ 0.80 = 1000 (MW);
extra fuel energy needed to replace lost electricity = (700 − 600) ÷ 0.40 = 250 (MW);
net reduction = 1000 − 250 = 750 (MW);
conclusion: claim supported because fuel use falls (by 750 MW) while coal input is unchanged;
limitation: heating only needed in cold weather / homes must be near the station / energy lost from the hot-water pipes, so actual saving smaller;
Allow
ECF throughout, including from (b) for the lost electrical output
MP4 for a correct conclusion based on their values
overall efficiency (600 + 800) ÷ 2000 = 70% compared with 35% before scores MP4 only if MP1–MP3 not scored
Do not accept
640 (MW) (800 × 0.80, multiplying by efficiency)
40 (MW) (100 × 0.40)
claim supported because electrical output increases
(d)
any one from:
releases carbon dioxide / greenhouse gas (contributes to global warming / climate change);
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