Current is the flow of charge

An electric current is the rate of flow of charge. For charge to flow you need a source of potential difference and a closed circuit. In a single closed loop the current has the same value at every point.

Charge is measured in coulombs (C), current in amperes (A). Current is the charge that flows past a point each second, so a current of 1 A is 1 coulomb per second. An ammeter measures current and is always connected in series.

Watch out: current is not "used up" by a bulb. In a series loop the same current flows through every component — the bulb transfers energy, but the charge keeps going round.

Your turn — charge flow

Potential difference and resistance

Potential difference (p.d., or voltage) is the energy transferred per unit charge as charge passes between two points. It is measured in volts (V) with a voltmeter, connected in parallel (across) the component. Adding more cells increases the p.d.

Resistance opposes the flow of charge and is measured in ohms (Ω). The current through a component depends on both its resistance and the p.d. across it:

Rearranging: from V = I R you get R = V ÷ I and I = V ÷ R. A larger resistance means a smaller current for the same p.d.

Your turn — resistance

I–V characteristics

By varying the p.d. with a variable resistor and plotting current against voltage, you find each component's I–V characteristic:

• Ohmic resistor (at constant temperature): current ∝ p.d., so the graph is a straight line through the origin and R is constant.
• Filament lamp: as current rises the filament gets hotter, so its resistance increases — an S-shaped curve.
• Diode: very high resistance in reverse, so current flows in one direction only.

Specified practical SP7.1: use an ammeter (series) and voltmeter (parallel) with a variable resistor to record I and V; switch off between readings so components don't heat up and distort results.

Reading the graph

LDRs and thermistors

Two components whose resistance changes with their surroundings:

• LDR (light-dependent resistor): as light gets brighter, its resistance falls. In the dark its resistance is highest — used in automatic lights.
• Thermistor (ntc): as temperature rises, its resistance falls — used in thermostats and temperature sensors.

Memory aid: both LDR and ntc thermistor work the "opposite" way you might guess — more light or more heat means less resistance.

Sensing the surroundings

Electrical power and energy

The power transferred by any circuit device depends on the p.d. across it and the current through it:

Over a time t, the energy transferred is power × time, and it also equals the charge that flowed multiplied by the p.d.:

Higher tier: the forms P = I²R and E = Q V are Higher-tier-only statements.

Your turn — electrical power

Your turn — energy transferred

Series vs parallel circuits

A series circuit has a single path; a parallel circuit branches. The rules for current, p.d. and resistance are different:

• Series — current: the same at every point. p.d.: the supply p.d. is shared (the p.d.s add up to the supply). Resistance: R_total = R₁ + R₂ (adding resistors increases total R).
• Parallel — current: the total current is shared between branches (branch currents add up to the total). p.d.: the same across every branch. Resistance: adding resistors in parallel decreases the total resistance.

Watch out — two classic mistakes: in series the current is the same everywhere but the p.d. splits; in parallel the p.d. is the same on each branch but the current splits. Don't mix them up!

Higher tier: calculating the net resistance of two resistors in parallel (which is always less than the smaller one) is a Higher-tier skill.

Series rules

Your turn — series resistance

Static electricity

When two insulators are rubbed together, electrons are transferred from one to the other by friction. The object that gains electrons becomes negative; the one that loses electrons is left positive.

• Like charges (++ or −−) repel; unlike charges (+ −) attract — a non-contact force.
• Only electrons move — never protons. The charge is created by where the electrons end up.
• If enough charge builds up, the air can break down and a spark (discharge) jumps. This is more likely at points on a charged conductor, where charge concentrates.

Watch out: static is always about electron transfer, not protons moving. Protons are locked in the nuclei; it is electrons that are gained or lost.

Electric fields

An electric field is a region where a charge feels a force. We draw field lines to show it. The field lines around an isolated charge are radial, and by convention they point from + to − (the direction of the force on a small positive test charge).

Watch out: field lines always go from positive to negative — the direction of the force on a positive test charge. Closer lines mean a stronger field.

Field lines & charge

The mains supply

The UK domestic supply is alternating current (a.c.) at 230 V and a frequency of 50 Hz. In a.c. the direction of the current keeps changing (50 times a second). Direct current (d.c.), from a battery or cell, flows in one direction only.

A mains cable has three wires, each with its own job and colour:

• Live (brown): carries the alternating p.d. (about 230 V relative to earth). It is dangerous even when the switch is open, because it is still at a high p.d.
• Neutral (blue): completes the circuit and is at about 0 V.
• Earth (green/yellow): a safety wire at 0 V connected to the metal case. Any connection between live and earth is dangerous — a large current would flow.

Fuses keep the cable safe

A fuse is a thin wire in the live side of the plug. If too large a current flows, the fuse wire heats up and melts, breaking the circuit before the cable can overheat. Fuses come in standard ratings (e.g. 3 A, 5 A, 13 A) — you choose the smallest one above the appliance's normal current.

Watch out: a fuse protects the cable and appliance from too large a current — it does not on its own protect a person from a shock. Choose a rating just above the normal current.

Pick the fuse

a.c. or d.c.?

The equations & rules to know

You've covered all four parts of Eduqas Topic 7 — current/p.d./resistance, series & parallel circuits, static electricity & electric fields, and domestic uses & safety. Press Finish to see your score.