The water-circuit analogy

Electricity is invisible, so physicists picture a circuit as a loop of water pipes. It's not perfect, but it makes every quantity feel concrete:

The pump is the cell — it provides the push (the potential difference / voltage). The flow rate of water is the current. A narrow section is a resistor: it makes it harder for water to flow, so the flow (current) is smaller.

The key insight: the water is never used up — the same water keeps circulating. In the same way, charge (and current) is not used up going round a circuit. Energy is what gets transferred to the components.

Current is a flow of charge

Electric current is the rate of flow of electric charge. For a current to flow, the circuit must be complete (a closed loop) and there must be a source of potential difference (such as a cell) to push the charge.

Charge is measured in coulombs (C), current in amperes (A), time in seconds (s). The bigger the current, the more charge passes a point each second.

Your turn — charge flow

Voltage, resistance and Ohm's law

Potential difference (p.d. or voltage, in volts, V) is the energy transferred per unit charge — it is the "push" driving the current. Resistance (in ohms, Ω) opposes the flow of charge. They are linked by:

Rearrange it to find any quantity: I = V ÷ R and R = V ÷ I. A larger resistance means a smaller current for the same p.d.

Your turn — potential difference

Your turn — finding resistance

What does a bulb take from the charge?

The symbols you must know

AQA expects you to recognise and draw these standard circuit symbols:

Reading a symbol

Ohmic and non-ohmic components

An I–V graph plots current against potential difference. For an ohmic conductor (a fixed resistor at constant temperature) the line is straight through the origin — resistance stays constant. Other components are non-ohmic:

Thermistor: its resistance falls as temperature rises. LDR: its resistance falls as light intensity increases. Both are used in sensing circuits.

Required practicals: investigate the I–V characteristics of a resistor, lamp and diode; and investigate how the resistance of a wire depends on its length, and the resistance of resistor combinations in series and parallel.

What happens to resistance?

How can more resistance mean less?

Two ways to wire components

Series (one single loop): the current is the same everywhere; the p.d. is shared between the components; and the total resistance is the sum — R = R₁ + R₂ + … (adding a resistor always increases total resistance).

Parallel (separate branches): the p.d. is the same across each branch; the current is shared between branches; and the total resistance is LESS than the smallest individual resistance.

Why does adding a resistor in parallel lower the resistance? Think of checkout lanes at a supermarket: opening a second lane gives shoppers another route, so the queue clears faster overall — even though each lane is no wider. Extra parallel paths make it easier for charge to flow, so total resistance drops. (A classic exam trap: students wrongly add parallel resistances.)

Series or parallel feature?

Each statement describes a circuit. Tap whether it is true of a series or a parallel circuit.

Your turn — series resistance

Parallel resistance

How can more resistance mean less?

What saves you from a live kettle?

Direct vs alternating current

Direct current (d.c.) flows in one direction only — this is what a cell or battery supplies. Alternating current (a.c.) constantly changes direction. UK mains is a.c., at about 230 V and a frequency of 50 Hz.

The three-pin plug

A UK mains cable has three cores, colour-coded so they are never confused:

• The live wire carries the alternating p.d. from the supply (~230 V). It is dangerous even when a switch is off, because it is still at a high potential relative to you.
• The neutral wire completes the circuit and stays near 0 V.
• The earth wire + a fuse (or circuit breaker) are safety features: a fault sends a large current to earth, which melts the fuse and disconnects the appliance.

Why is the live wire dangerous?

What saves you from a live kettle?

Hairdryer or bulb: which transfers more energy?

Power in a circuit

Power is the rate of energy transfer (in watts, W). In an electrical circuit you can find it two ways:

The power rating on an appliance (e.g. "2000 W") tells you how fast it transfers energy. A higher current — or, for the same current, a higher resistance — raises the power dissipated as heat.

Your turn — power (P = VI)

Your turn — power (P = I²R)

Energy, work and the grid

Everyday appliances transfer energy electrically: a kettle to a thermal store, a motor to a kinetic store, a lamp by radiation. The energy transferred depends on power and time, or on charge and p.d.:

The more powerful the appliance and the longer it runs, the more energy it transfers — this is what you pay for. Using less power or running for less time both cut the cost.

The National Grid is the network of cables and transformers that carries electricity from power stations to homes. Step-up transformers raise the p.d. for transmission (which lowers the current, reducing energy wasted as heat in the cables), and step-down transformers lower it again for safe use at home.

Your turn — energy transferred

Hairdryer or bulb: which transfers more energy?

Your turn — energy from charge

Where did the charge come from?

Static charge by friction

Physics only: this static-electricity section is examined on the Physics paper only, not Combined Science.

When two insulators are rubbed together, electrons (which are negative) are transferred from one to the other by friction. The object that gains electrons becomes negatively charged; the one that loses electrons becomes positively charged. Only electrons move — never the positive nuclei.

If enough charge builds up, the p.d. between the object and earth can become large enough to make a spark jump — electrons leap across the gap, ionising the air.

Charging by friction

Where did the charge come from?

Conductor or insulator?

Tap a material, then tap the box it belongs in.

The equations to know

You've covered the key ideas in AQA 4.2 — current, p.d. & resistance, circuit symbols and I–V graphs, series & parallel, mains electricity & safety, power and energy, and static electricity. Press Finish to see your score.