Topic 2: Electricity

Electricity is the controlled movement of charge that powers nearly everything around us. This topic builds from the basic ideas of current and potential difference up to the equations that describe energy and power. By the end you should be able to analyse circuits, explain how everyday components behave, and understand how electricity reaches your home safely.

Electric charge and current (Q = It)

Electric current is the rate of flow of electric charge around a circuit. For current to flow there must be a source of potential difference and a closed (complete) loop with no gaps. Current is measured in amperes (A) using an ammeter connected in series. Charge is measured in coulombs (C). The two quantities are linked by the equation charge = current x time, written Q = It, where Q is in coulombs, I in amperes and t in seconds. Worked example: if a current of 3 A flows for 20 s, the charge transferred is Q = It = 3 x 20 = 60 C. Rearranging lets you find current or time, for example I = Q / t. The size of the current depends on the potential difference of the supply and the total resistance of the circuit.

Potential difference and resistance (V = IR)

Potential difference (p.d.), also called voltage, is the energy transferred per unit charge as charge moves between two points. It is measured in volts (V) using a voltmeter connected in parallel across a component. Resistance opposes the flow of charge and is measured in ohms. The key relationship is potential difference = current x resistance, written V = IR. Worked example: a 12 V battery drives a current of 0.5 A through a lamp, so the lamp's resistance is R = V / I = 12 / 0.5 = 24 ohm. For a fixed resistor at constant temperature, current is directly proportional to potential difference, which means the resistance stays constant. The greater the resistance of a component, the smaller the current for a given p.d.

I-V characteristics of components

An I-V graph plots current against potential difference and shows how a component behaves. For a fixed resistor at constant temperature the graph is a straight line through the origin: current is proportional to p.d., so resistance is constant (this component is described as ohmic). For a filament lamp the line is an S-shaped curve that gets shallower as p.d. increases; this happens because the filament heats up, the metal ions vibrate more, and resistance rises. For a diode the graph shows that current only flows in one direction (the forward direction) and is almost zero in the reverse direction, because a diode has very high resistance one way round. To investigate these characteristics you use a variable resistor to change the current and record matching values of p.d. and current.

Series and parallel circuits

In a series circuit components are joined one after another in a single loop. The current is the same everywhere, the total potential difference of the supply is shared between the components, and the total resistance is the sum of the individual resistances: R total = R1 + R2 + ... Adding more resistors in series increases the total resistance. In a parallel circuit components are connected on separate branches. The potential difference across each branch is the same and equal to the supply, while the total current is shared between the branches. Adding more resistors in parallel decreases the total resistance, because charge has more paths to flow through, so the total resistance is less than the smallest single resistor. Worked example: two 10 ohm resistors in series give 20 ohm; the same two in parallel give 5 ohm.

Resistors and special components (LDR, thermistor)

Some components change their resistance in response to their surroundings, which makes them useful in sensing and control circuits. A light dependent resistor (LDR) has a high resistance in the dark and a low resistance in bright light, so it can switch lights on automatically at dusk. A thermistor has a high resistance when cold and a low resistance when hot, so it is used in thermostats and temperature sensors. A standard fixed resistor keeps a constant resistance and is used to control the current in a circuit, while a variable resistor lets you change resistance to dim a lamp or change motor speed. These behaviours let circuits respond to light level or temperature without any moving parts.

Mains electricity (ac/dc and the plug)

Cells and batteries supply direct current (dc), where the current flows in one direction only. Mains electricity is alternating current (ac), where the direction of the current reverses repeatedly. In the UK the mains supply has a frequency of 50 Hz and a potential difference of about 230 V. A three-core cable connects most appliances. The live wire (brown) carries the alternating p.d. from the supply and is dangerous. The neutral wire (blue) completes the circuit and stays close to 0 V. The earth wire (green and yellow stripes) is a safety wire that carries current away if a fault makes the metal case live, and it works with the fuse to cut off the supply. Even when a switch is off, the live wire can give a shock because there is a large p.d. between it and earth.

Power and energy transfer (P = IV, P = I^2 R, E = Pt, E = QV)

Power is the rate at which energy is transferred and is measured in watts (W). The electrical power of a device can be found from power = potential difference x current, written P = IV, or from power = current squared x resistance, written P = I^2 R. Worked example: a heater with current 5 A and p.d. 230 V has power P = IV = 5 x 230 = 1150 W. The energy transferred over time is energy = power x time, E = Pt, in joules when t is in seconds. Energy can also be found from energy = charge x potential difference, E = QV, which shows how much work is done moving the charge. Worked example: a 60 W lamp left on for 120 s transfers E = Pt = 60 x 120 = 7200 J. Higher-power appliances transfer more energy each second.

The National Grid and transformers

The National Grid is the network of cables and transformers that carries electricity from power stations to homes, factories and other users. Transmitting energy over long distances wastes some as heat in the cables. To reduce these losses, step-up transformers increase the potential difference to very high values (sometimes over 100,000 V) before transmission, which lowers the current for the same power. A lower current means much less energy is wasted heating the wires, because power loss depends on the square of the current. Near the point of use, step-down transformers reduce the p.d. back to safer levels for homes. Transformers only work with alternating current, which is one reason the mains supply is ac.

Static electricity and electric fields

When two insulating materials are rubbed together, electrons are transferred from one to the other. The material that gains electrons becomes negatively charged and the one that loses electrons becomes positively charged; only electrons move, never the positive charges. Objects with the same type of charge repel each other, while objects with opposite charges attract. If enough charge builds up, the potential difference can become large enough for charge to jump across a gap as a spark. A charged object is surrounded by an electric field, drawn as field lines pointing away from a positive charge and towards a negative charge. The field is strongest where the lines are closest together, and a second charged object placed in the field feels a force from it.