Electricity

This guide covers Topic 2, Electricity, in full — sub-topics (a) Units, (b) Mains electricity, (c) Energy and voltage in circuits and (d) Electric charge — from the Pearson Edexcel International GCSE in Physics (4PH1), Issue 4 specification. Statements marked “P” are Physics-only content, not shared with the International GCSE in Science (Double Award).

Before studying this

This resource assumes basic numeracy and rearranging equations; no prior electricity study is required.

Syllabus coverage

PEARSON EDEXCEL INTERNATIONAL GCSE PHYSICS (4PH1) — Topic 2

(a) Units — using ampere (A), coulomb (C), joule (J), ohm (Ω), second (s), volt (V) and watt (W).

(b) Mains electricity — understanding how insulation, double insulation, earthing, fuses and circuit breakers protect devices and users; understanding why current in a resistor causes electrical heating and its uses; using power = current × voltage and applying it to fuse selection; using energy transferred = current × voltage × time; knowing the difference between a.c. mains and d.c. from a cell or battery.

(c) Energy and voltage in circuits — explaining when series or parallel circuits suit particular applications; understanding how current in a series circuit depends on voltage and other components; describing how current varies with voltage for wires, resistors, filament lamps and diodes, investigated practically; describing qualitatively how changing resistance affects current; describing how LDR and thermistor resistance vary with illumination and temperature; using voltage = current × resistance; knowing current is the rate of flow of charge; using charge = current × time; knowing current in metals is a flow of electrons; understanding current conservation at a junction; calculating series circuit currents, voltages and resistances; knowing voltage as energy transferred per unit charge; using energy transferred = charge × voltage.

(d) Electric charge — [P] identifying conductors and insulators; [P] investigating charging by friction; [P] explaining how positive and negative charges arise from electron loss and gain; [P] knowing like charges repel and unlike charges attract; [P] explaining electrostatic phenomena and their dangers and uses.

Mains electricity and safety

Mains electricity is alternating current (a.c.), which periodically reverses direction, unlike the one-directional direct current (d.c.) supplied by a cell or battery. Safety features such as insulation, double insulation, earthing, fuses and circuit breakers all protect users and devices from electrical faults. A current flowing through a resistor transfers electrical energy, raising its temperature — the basis of devices such as electric heaters and kettles.

Power in a circuit is related to current and voltage by:

power = current × voltage
P = I × V

This relationship is used to select an appropriately rated fuse for a given appliance. The total energy transferred over time is:

energy transferred = current × voltage × time
E = I × V × t

Series and parallel circuits

In a series circuit, all components share the same current, and the total voltage is shared between components. In a parallel circuit, each branch has the same voltage, and current divides between branches — which is why domestic lighting is wired in parallel (so one bulb failing doesn’t switch off the others).

Current, voltage and resistance

Current is the rate of flow of electric charge:

charge = current × time
Q = I × t

A lamp lighting up, or an LED glowing, indicates the presence of a current in the circuit — a simple qualitative test used before any meter reading is taken.

Voltage, current and resistance are related by:

voltage = current × resistance
V = I × R

Worked example. A 6.0 V battery drives a current of 0.50 A through a resistor. The resistor’s resistance:

R = V/I = 6.0/0.50 = 12 Ω

Different components show different current-voltage relationships: a resistor at constant temperature gives a straight-line (proportional) relationship; a filament lamp’s resistance increases as it heats up, curving the graph; a diode conducts in one direction only, with almost no current in reverse. Light-dependent resistors (LDRs) decrease in resistance as illumination increases; thermistors decrease in resistance as temperature increases.

Energy and voltage

Voltage is defined as the energy transferred per unit charge passed:

energy transferred = charge × voltage
E = Q × V

A volt is a joule per coulomb (1 V = 1 J/C) — one volt means one joule of energy is transferred for every coulomb of charge that passes.

Electric charge (Physics only)

Materials can be classified as electrical conductors (e.g. metals) or insulators (e.g. plastics). Insulating materials can become charged by friction: rubbing transfers electrons between surfaces, leaving one surface with a net positive charge (having lost electrons) and the other with a net negative charge (having gained electrons). Like charges repel; unlike charges attract. Electrostatic charge has practical applications (e.g. photocopiers, inkjet printers) but also dangers (e.g. sparks when fuelling aircraft or tankers), which is why fuelling equipment is designed to safely dissipate static charge.

Common mistakes

• Applying V = IR to non-ohmic components without qualification — a filament lamp or diode does not have a constant resistance across all voltages, unlike an ideal resistor.
• Confusing series and parallel voltage/current rules — voltage is shared in series, current is shared (divided) in parallel; current is shared (equal throughout) in series, voltage is equal across parallel branches.
• Mixing up LDR and thermistor behaviour — both decrease in resistance as their respective condition (light/temperature) increases, but they respond to different physical quantities.
• Forgetting a.c. and d.c. are fundamentally different current types — mains is a.c.; batteries and cells supply d.c.

Quick revision checklist

• P = IV and E = IVt for mains electricity and fuse selection
• Series vs parallel circuits, and why domestic lighting uses parallel wiring
• V = IR, Q = It, and E = QV
• Current-voltage behaviour of resistors, filament lamps, diodes, LDRs and thermistors
• Charging by friction, and like/unlike charge interactions (Physics only)

Related resources

• Forces and Motion — the previous topic
• Waves — the next topic
• Pearson Edexcel International GCSE Physics hub

Written against the Pearson Edexcel International GCSE in Physics (4PH1) specification, Issue 4. Always check the current specification for your examination year.