Reading a circuit diagram

Edexcel expects you to draw and use circuit diagrams with the standard symbols, using the conventions of positive (+) and negative (−) terminals. Conventional current flows from + to − outside the cell.

Watch out: the ammeter is connected in series with a component, but the voltmeter is connected in parallel with it. Mixing these up is one of the most common diagram errors.

Which symbol?

Current is the rate of flow of charge

An electric current is the rate of flow of charge. In a metal, that current is a flow of electrons. The more charge that passes a point each second, the bigger the current:

The water-circuit analogy helps: think of the cell as a pump, the current as the rate of water flow, and a resistor as a narrow section of pipe that holds the flow back.

Watch out: current is not "used up" by components. The reading on an ammeter before a lamp equals the reading after it — what changes is the energy the charges carry, not how many there are.

Your turn — charge

Potential difference moves energy

Potential difference (p.d., or voltage) is the energy transferred per unit charge passing through a component. So the volt is a joule per coulomb (1 V = 1 J/C):

A voltmeter is connected in parallel across a component to measure the p.d. across it — the energy each coulomb of charge gives up as it passes through.

Watch out: p.d. is measured across a component (parallel), current is measured through it (series). Saying "the voltage through a resistor" is a giveaway error.

Resistance and V = I R

Resistance opposes the flow of charge. The bigger the resistance, the smaller the current for a given p.d. The three are linked by:

You can change the current in a circuit with a variable resistor (a rheostat): increasing its resistance decreases the current.

Your turn — resistance

What does a bulb take from the charge?

How can more resistance mean less?

Series vs parallel

In a series circuit there is one loop: the same current flows everywhere, and the p.d. of the supply is shared between components. In a parallel circuit components sit on separate branches: each gets the full supply p.d., and the branch currents add up to the total (current is conserved at a junction).

Series resistance rule: R_total = R₁ + R₂ + … Two resistors in parallel always give a combined resistance smaller than the smallest single one.

Your turn — series resistance

Current at a junction

How can more resistance mean less?

What notices that it is getting dark?

I–V graphs tell you the resistance

Plotting current against potential difference shows how a component behaves. A steeper line means a lower resistance (more current per volt).

Why the lamp curves: as current heats the filament, the metal ions vibrate more, electrons collide with them more often, so resistance increases — the line bends towards the p.d. axis.

Reading the graph

LDRs and thermistors

Two components change their resistance in response to their surroundings:

• LDR (light-dependent resistor): resistance falls as light intensity rises (bright = low R, dark = high R). Used in light sensors, e.g. automatic street lights and camera light meters.
• Thermistor (NTC — negative temperature coefficient): resistance falls as temperature rises (hot = low R, cold = high R). Used in temperature sensors, e.g. thermostats and fire alarms.

Watch out: for GCSE Edexcel you only need the NTC thermistor — resistance decreases with rising temperature. Don't write the opposite.

Component → behaviour

Tap a component, then tap its correct behaviour.

What notices that it is getting dark?

The core practicals

Edexcel sets one named Core Practical on circuits. You construct electrical circuits to:

• (a) investigate the relationship between p.d., current and resistance for a resistor and a filament lamp — varying the p.d., recording I and V, and plotting an I–V graph;
• (b) test series and parallel circuits using resistors and filament lamps.

Tip: take readings both ways round (reverse the cell) so you can plot the graph through negative values, and, for the fixed resistor, keep currents low so it doesn't heat up between readings.

Hairdryer or bulb: which transfers more energy?

Electrical energy and power

When charge flows through a component, energy is transferred. Over a time t:

Power is the energy transferred per second (measured in watts). For any circuit device:

Heating effect: when there's a current in a resistor, energy is dissipated as thermal energy — caused by electrons colliding with the ions in the metal lattice. Useful in a kettle or toaster, wasteful in long cables.

Your turn — power (P = I V)

Your turn — power (P = I²R)

Your turn — energy transferred

Hairdryer or bulb: which transfers more energy?

What saves you from a live kettle?

a.c. vs d.c.

In direct current (d.c.) the charge moves in one direction only. Cells and batteries supply d.c. In alternating current (a.c.) the movement of charge keeps changing direction. The UK mains supply is a.c.

UK mains and the three-pin plug

The UK domestic supply is a.c. at about 230 V and 50 Hz. A three-pin plug has three wires you must know — their colours, their jobs and their potential differences:

• The live wire carries the alternating supply (~230 V). The neutral completes the circuit and stays near 0 V.
• The earth wire (0 V) is a safety wire: if a fault makes the metal case live, charge flows safely to earth.
• A fuse (or circuit breaker) melts/trips if the current gets too big, cutting off the supply. It goes in the live wire so the appliance is isolated from the dangerous voltage.
• Touching the live wire while connected to earth gives a large p.d. across you — a dangerous shock.

Safe or dangerous?

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

What saves you from a live kettle?

Energy in appliances

The equations to know

You've covered the key ideas in Edexcel Topic 10 — symbols, current and charge, p.d. and resistance, series & parallel, I–V graphs, LDRs & thermistors, the core practicals, electrical energy & power, a.c./d.c. and mains safety. Press Finish to see your score.