Electricity

Circuit Symbols

Standard symbols are used to draw circuit diagrams — students must recognise and draw them all.

These come up on every circuits question. Ammeter in series, voltmeter in parallel — getting this wrong invalidates the whole circuit.

Circuit Symbols — Key Knowledge

Charge, Current and Potential Difference

Current is the flow of charge. Potential difference is the energy transferred per unit charge.

Q = It, V = W/Q

charge = current × time, voltage = work done ÷ charge

Current is not "used up" — the same current flows out of the battery as back in. Energy is transferred, not charge consumed.

Charge, Current and Potential Difference — Key Knowledge

Current rate of flow of charge, measured in amps
Charge measured in coulombs
Potential difference energy per coulomb, measured in volts

Resistance and Ohm's Law

The most-used equation in electricity — links voltage, current, and resistance. Rearrange it for any of the three.

V = IR

voltage = current × resistance

V = IR underpins almost every electricity calculation. Rearrange: I = V/R, R = V/I.

Resistance and Ohm's Law — Key Knowledge

Resistance opposition to current flow, measured in ohms Ω
Ohm's law only holds when temperature is constant ohmic conductors
Higher resistance means lower current for a given voltage

I-V Characteristics

Different components produce different current-voltage graphs, revealing how their resistance behaves.

Must be able to sketch and interpret all three graphs. The resistor is ohmic; the lamp and diode are non-ohmic.

I-V Characteristics — Key Knowledge

Resistor straight line through origin — constant resistance
Filament lamp curve — resistance increases as it heats up
Diode current flows in forward direction only

LDR and Thermistor

Two components whose resistance changes with environmental conditions.

Both are used in sensing circuits. More light → less resistance. More heat → less resistance. Both go down.

LDR and Thermistor — Key Knowledge

LDR resistance decreases as light intensity increases
Thermistor resistance decreases as temperature increases

Series and Parallel Circuits

Components can be connected in a single loop (series) or on separate branches (parallel).

Adding components in series increases resistance. Adding a parallel branch decreases total resistance and increases total current.

Series and Parallel Circuits — Key Knowledge

Series — current is the same everywhere; voltage is shared between components; total resistance = R₁ + R₂, Parallel — voltage is the same across each branch; current splits at junctions; total resistance is less than the smallest resistor

Electricity in the Home

UK mains is 230V ac at 50Hz, delivered through a three-wire plug with built-in safety devices.

The live wire is dangerous even when the switch is off. If a fault occurs, the earth wire carries a large current, blowing the fuse and breaking the circuit.

Electricity in the Home — Key Knowledge

Live wire brown, 230V
Neutral wire blue, ~0V
Earth wire green/yellow, safety
Fuse melts if current is too high, in the live wire
Circuit breaker trips and can be reset

Electrical Power

Power is the rate of energy transfer. Energy transferred depends on power and time.

P = IV, P = I²R, E = Pt

P = I²R shows that doubling current quadruples power lost as heat — this is why high current is wasteful.

Electrical Power — Key Knowledge

Power measured in watts
Energy transferred measured in joules

The National Grid

The network of cables and transformers that delivers electricity from power stations to consumers.

P = IV: same power at higher voltage needs lower current. P = I²R: lower current means far less heating in the cables.

The National Grid — Key Knowledge

Step-up transformer increases voltage for transmission
Step-down transformer decreases voltage for homes
High voltage means lower current means less energy wasted as heat in cables

Static Electricity*

Rubbing insulating materials together transfers electrons — one object becomes negative, the other positive.

No charge is created or destroyed — it is transferred. *Physics Only topic.

Static Electricity* — Key Knowledge

Only electrons move protons stay fixed
Like charges repel; unlike charges attract, Sparking occurs when charge builds up then discharges

Electric Fields*

A charged object creates an electric field around it. Other charges in that field experience a force.

Explains non-contact forces between charged objects. *Physics Only topic.

Electric Fields* — Key Knowledge

Electric field region where a charge experiences a force
Field is strongest close to the object, Field lines point away from positive and towards negative

Required Practicals — Resistance and I-V

Two key practicals: investigating how wire length affects resistance, and measuring I-V characteristics of components.

Must be able to describe the method, draw the circuit diagram, and identify which variables are independent, dependent, and controlled.

Required Practicals — Resistance and I-V — Key Knowledge

RP3 — vary wire length and measure R using ammeter and voltmeter (R = V/I); control temperature, material, and cross-section, RP4 — use a variable resistor to vary pd; plot I-V graphs for resistor, lamp, and diode

Self-Assessment

Session Complete

Electricity