Electricity flashcards

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What is electric current and how is it calculated?
Current is the rate of flow of charge. I = Q ÷ t, where I is current in amperes (A), Q is charge in coulombs (C) and t is time in seconds (s).
A current of 3 A flows for 20 s. How much charge passes?
Q = I × t = 3 × 20 = 60 C.
What is potential difference and what is its unit?
Potential difference (voltage) is the energy transferred per unit charge between two points: V = W ÷ Q. One volt is one joule per coulomb.
What is resistance? Give the equation and its unit.
Resistance is how much a component opposes current. R = V ÷ I, with R in ohms (Ω), V in volts and I in amperes.
What is the resistance of a component with 12 V across it and a current of 0.50 A?
R = V ÷ I = 12 ÷ 0.50 = 24 Ω.
How are an ammeter and a voltmeter connected to a component?
An ammeter is connected in series (it has very low resistance). A voltmeter is connected in parallel across the component (it has very high resistance).
What are the rules for current and p.d. in a series circuit?
The current is the same everywhere. The supply p.d. is shared between components, so the p.d.s add up to the supply p.d. Total resistance is the sum of the resistances.
What are the rules for current and p.d. in a parallel circuit?
The p.d. across every branch equals the supply p.d. The currents in the branches add up to the total current from the supply. Adding branches lowers the total resistance.
Two 5 Ω resistors are connected in series. What is the total resistance?
R = R₁ + R₂ = 5 + 5 = 10 Ω.
A 12 V supply is connected across a 4 Ω and an 8 Ω resistor in series. Find the current and the p.d. across the 8 Ω resistor.
Total R = 4 + 8 = 12 Ω. I = V ÷ R = 12 ÷ 12 = 1.0 A. p.d. across the 8 Ω resistor = IR = 1.0 × 8 = 8.0 V.
Describe the I–V graph of a resistor at constant temperature.
A straight line through the origin: current is directly proportional to p.d., so the resistance is constant. Such a component is called ohmic.
Describe the I–V graph of a filament lamp and explain its shape.
The line curves so the gradient falls as p.d. rises. The filament heats up, its atoms vibrate more and collide more with electrons, so its resistance increases.
Describe the I–V graph of a diode.
A diode lets current flow in one direction only. In the forward direction the current is almost zero until about 0.6 V and then rises steeply. In reverse the current is almost zero.
How does the resistance of an LDR change?
The resistance of a light-dependent resistor falls as light intensity increases. It is used in light-sensing circuits such as automatic street lights.
How does the resistance of a thermistor change?
For an NTC thermistor, resistance falls as temperature rises. It is used in temperature sensors such as thermostats and fire alarms.
Give the equations for electrical power.
P = IV, and combining with V = IR gives P = I²R. P is power in watts (W), I is current in A, V is p.d. in V, R is resistance in Ω.
A kettle runs on 230 V and draws 5.0 A. What is its power?
P = IV = 5.0 × 230 = 1150 W.
State two equations for energy transferred by an electrical device.
E = P × t (energy in J, power in W, time in s) and E = Q × V (charge in C, p.d. in V).
A 2.0 kW heater runs for 3.0 minutes. How much energy does it transfer?
E = Pt = 2000 × 180 = 360 000 J (360 kJ).
Why are lamps in a house usually wired in parallel rather than in series?
Each lamp gets the full supply p.d. and works independently, so one lamp failing does not switch the others off. In series, the p.d. would be shared and one failure would break the whole circuit.
What happens to the current at a junction in a circuit?
Current is conserved: the total current flowing into a junction equals the total current flowing out. For example, 3.0 A splitting into two branches might give 1.2 A and 1.8 A.
What are the main features of UK mains electricity?
It is alternating current (a.c.) at about 230 V and a frequency of 50 Hz.
What is the difference between direct and alternating current?
Direct current (d.c.) flows in one direction only, e.g. from a cell. Alternating current (a.c.) repeatedly reverses direction, e.g. from the mains.
What are the colours and roles of the three wires in a UK plug?
Brown is the live wire (at high p.d., carries current to the appliance). Blue is neutral (completes the circuit). Green and yellow is earth (safety wire).
What does a fuse do and how do you choose its rating?
A fuse melts and breaks the circuit if the current is too high. Choose the standard fuse (3 A, 5 A, 13 A) just above the normal current: a 1150 W, 230 V device draws 5 A, so use a 5 A fuse.
How does the earth wire protect the user?
It connects the metal case to earth. If the live wire touches the case, a large current flows to earth, blowing the fuse and disconnecting the appliance so the case is not live.
What is double insulation?
Appliances with plastic casings and insulated internal parts cannot make the outer case live, so they need no earth wire. They carry a double-square symbol.
Why is a fuse or circuit breaker placed in the live wire?
So that when it breaks, the appliance is disconnected from the high-voltage live supply and is safe to touch. A circuit breaker works the same way but can be reset.
(Paper 2) How do objects become charged by friction?
Electrons are transferred from one insulator to the other when rubbed. The one that gains electrons becomes negative and the one that loses them becomes positive.
(Paper 2) How do charged objects interact?
Like charges repel and unlike charges attract. A charged insulator also attracts neutral objects, because it induces opposite charge on the nearer surface.
(Paper 2) Give one use and one danger of static electricity.
Use: electrostatic paint spraying gives an even coat. Danger: sparks when refuelling or from lightning can ignite fuel. Earthing the objects removes the charge safely.
Which way does conventional current flow compared with electron flow?
Conventional current is drawn from the positive terminal to the negative terminal. Electrons actually move the opposite way, from negative to positive.

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