Energy resources and energy transfers flashcards

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State the principle of conservation of energy.
Energy cannot be created or destroyed, only transferred from one store to another or dissipated. The total energy stays the same.
Name the main energy stores.
Kinetic, gravitational potential, elastic (strain), chemical, thermal (internal), nuclear and electrostatic stores.
What does it mean when energy is dissipated?
It is spread out into the surroundings, usually as thermal energy, and cannot be usefully recovered. It is wasted energy.
How is work done calculated and what is its unit?
Work done = force × distance moved in the direction of the force: W = Fd. Work is in joules (J). Work done equals energy transferred.
A 50 N force pushes a box 4.0 m along the floor. How much work is done?
W = Fd = 50 × 4.0 = 200 J.
State the equation for kinetic energy.
KE = ½ m v². KE is in joules, m is mass in kg and v is speed in m/s.
Find the kinetic energy of a 2.0 kg trolley moving at 6.0 m/s.
KE = ½ × 2.0 × 6.0² = ½ × 2.0 × 36 = 36 J.
State the equation for gravitational potential energy.
GPE = m g h. m is mass in kg, g is gravitational field strength (9.8 N/kg) and h is the change in height in m.
What is the change in GPE when a 5.0 kg mass is lifted 4.0 m? Use g = 9.8 N/kg.
ΔGPE = mgh = 5.0 × 9.8 × 4.0 = 196 J.
A 1.0 kg ball falls 5.0 m from rest. Find its speed just before landing (g = 9.8 N/kg, no air resistance).
GPE lost = KE gained: mgh = ½ m v², so v² = 2gh = 2 × 9.8 × 5.0 = 98, v = 9.9 m/s.
Define power and give the equation.
Power is the rate of transferring energy or doing work. P = E ÷ t = W ÷ t. The unit is the watt (W), which is one joule per second.
A motor transfers 600 J in 12 s. What is its power?
P = E ÷ t = 600 ÷ 12 = 50 W.
Give the equation for efficiency.
Efficiency = useful energy output ÷ total energy input (× 100% for a percentage). It can also be written with power in place of energy.
A device gets 200 J and gives 80 J of useful energy. What is its efficiency?
Efficiency = 80 ÷ 200 = 0.40, or 40%. The other 120 J is wasted, mostly as thermal energy.
Why is no device 100% efficient?
Some energy is always dissipated as heat or sound through friction, resistance or air resistance. Useful output is therefore always less than the input.
What does a Sankey diagram show?
Energy transfers: the arrow width is drawn to scale to show the total input, useful output and wasted energy at each stage.
Describe how thermal energy transfers by conduction.
In solids, particles vibrate more when heated and pass energy to neighbours by collisions. In metals, free electrons also carry energy, so metals are good conductors.
Describe how convection transfers energy.
In a fluid, the heated part expands, becomes less dense and rises. Cooler, denser fluid sinks to replace it, setting up a convection current. It cannot happen in solids.
Describe thermal radiation and which surfaces are best at it.
All objects emit infrared radiation, and it needs no medium. Matt black surfaces are the best emitters and absorbers; shiny white surfaces are the worst.
How can heat loss from a house be reduced?
Loft and cavity wall insulation (trapped air reduces conduction and convection), double glazing, draught excluders, and foil behind radiators to reflect radiation.
What affects the rate of cooling of an object?
A larger temperature difference with the surroundings, a larger surface area, a better conducting or darker surface, and less insulation all increase the rate of cooling.
(Paper 2) Name the fossil fuels and give two disadvantages of using them.
Coal, oil and natural gas. They release carbon dioxide (contributing to global warming), can cause acid rain from sulfur, and are non-renewable.
(Paper 2) What is a renewable energy resource? Give examples.
One that is replaced as fast as it is used: wind, solar, hydroelectric, tidal, wave, geothermal and biofuels.
(Paper 2) Compare wind power with fossil fuel power stations.
Wind produces no carbon dioxide in use and has no fuel cost, but depends on the wind, spoils scenery and makes noise. Fossil stations give reliable output on demand but pollute.
(Paper 2) How does a hydroelectric power station generate electricity?
Water stored behind a dam flows down through turbines. GPE becomes KE, the turbines drive generators, and electricity is produced. It can respond quickly to demand.
(Paper 2) Describe how a solar cell and a solar water heater differ.
A solar cell converts light directly into electrical energy. A solar water heater absorbs infrared and visible radiation to heat water through pipes.
(Paper 2) Describe how a nuclear power station generates electricity.
Nuclear fission of uranium or plutonium releases thermal energy that makes steam. The steam drives turbines linked to generators. It makes no carbon dioxide but produces radioactive waste.
(Paper 2) How do tidal and wave power work and what limits them?
Moving water turns turbines connected to generators. Tidal power is predictable, but barrages harm wildlife and habitats; wave power depends on weather and has high set-up cost.
(Paper 2) Why do power stations have a national grid with high-voltage transmission?
Raising the voltage lowers the current for the same power. Lower current means less energy wasted as heat (P = I²R) in the cables, so transmission is more efficient.
(Paper 2) Explain why reliability and cost matter when comparing energy resources.
Some, like solar and wind, vary with weather so cannot meet demand at all times. Others have high building costs but low running costs, so the whole-life cost must be compared.

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