Electricity

What you'll learn

This revision guide covers all testable content on electricity from the Pearson Edexcel International IGCSE Physics specification. You'll master the fundamental principles of current, voltage and resistance, learn to analyse series and parallel circuits, understand mains electricity and electrical safety, and apply your knowledge to calculate power and energy transfers.

Key terms and definitions

Current (I) — the rate of flow of charge, measured in amperes (A); one ampere equals one coulomb of charge flowing per second

Potential difference (V) — the energy transferred per unit charge between two points in a circuit, measured in volts (V); also called voltage

Resistance (R) — the opposition to current flow in a component, measured in ohms (Ω)

Power (P) — the rate of energy transfer, measured in watts (W); one watt equals one joule per second

Direct current (d.c.) — current that flows in one direction only, supplied by cells and batteries

Alternating current (a.c.) — current that repeatedly reverses direction, supplied by mains electricity at 50 Hz in the UK

Ohmic conductor — a component that obeys Ohm's law, where current is directly proportional to potential difference at constant temperature

Earth wire — a safety wire that provides a low-resistance path to the ground, preventing the casing of an appliance from becoming live

Core concepts

Electric charge and current

Electric current is the flow of charged particles. In metals, these particles are electrons moving from negative to positive terminals, though conventional current is defined as flowing from positive to negative.

The relationship between charge, current and time is:

Q = I × t

Where:

• Q = charge (coulombs, C)
• I = current (amperes, A)
• t = time (seconds, s)

Key points about current:

• Current is measured using an ammeter connected in series
• In a series circuit, current is the same at all points
• In a parallel circuit, current splits at junctions and the total current entering a junction equals the total current leaving

Potential difference and resistance

Potential difference (voltage) represents the energy transferred by each coulomb of charge passing through a component. It is measured using a voltmeter connected in parallel across the component.

Ohm's law states that for an ohmic conductor at constant temperature:

V = I × R

Where:

• V = potential difference (volts, V)
• I = current (amperes, A)
• R = resistance (ohms, Ω)

Current-voltage (I-V) characteristics:

• Ohmic conductors (e.g., fixed resistors at constant temperature) produce a straight line through the origin
• Filament lamps show a curve because resistance increases with temperature as the filament heats up
• Diodes only allow current to flow in one direction (forward bias), showing very high resistance in reverse bias

Series and parallel circuits

Series circuits:

• Components are connected end-to-end in a single loop
• Current is the same through all components: I₁ = I₂ = I₃
• Total potential difference is shared: V_total = V₁ + V₂ + V₃
• Total resistance: R_total = R₁ + R₂ + R₃
• If one component fails, the whole circuit breaks

Parallel circuits:

• Components are connected across the same two points
• Potential difference is the same across all branches: V₁ = V₂ = V₃
• Total current is the sum of branch currents: I_total = I₁ + I₂ + I₃
• For two resistors in parallel: 1/R_total = 1/R₁ + 1/R₂
• If one component fails, current continues through other branches
• Adding resistors in parallel decreases total resistance

Electrical power and energy

Power represents the rate at which electrical energy is transferred. The fundamental equations are:

P = E / t (power equals energy transferred per second)

P = I × V (power equals current times potential difference)

P = I² × R (derived from combining P = IV and V = IR)

P = V² / R (alternative form)

Energy transferred by an appliance:

E = P × t or E = I × V × t

Where:

• E = energy (joules, J)
• P = power (watts, W)
• t = time (seconds, s)

When using kilowatt-hours (kWh), the standard unit for electrical energy billing:

• Energy (kWh) = Power (kW) × Time (hours)
• Cost = Energy (kWh) × Cost per kWh

Mains electricity and electrical safety

UK mains electricity operates at 230 V, 50 Hz alternating current. The three-core cable contains:

Live wire (brown):

• Alternates between approximately +325 V and -325 V
• Provides the alternating potential difference

Neutral wire (blue):

• Maintained at approximately 0 V
• Completes the circuit

Earth wire (yellow and green stripes):

• Safety wire connected to the metal casing
• At 0 V under normal conditions
• If a fault causes the live wire to touch the casing, a large current flows through the earth wire to ground, blowing the fuse

Fuses are safety devices containing a thin wire that melts when current exceeds its rating, breaking the circuit. The fuse should be rated slightly higher than the normal operating current of the appliance.

Circuit breakers are electromagnetic switches that trip (open) when current exceeds a safe level. Advantages over fuses:

• Reset by pressing a button (no replacement needed)
• Act faster than fuses
• Can be more sensitive to current changes

The fuse or circuit breaker must be connected in the live wire to be effective.

Double insulation (indicated by the ⧈ symbol) uses non-conducting plastic casing, eliminating the need for an earth wire.

Electricity generation and transmission

The National Grid distributes electrical energy across the country. Key features:

Step-up transformers at power stations increase voltage to typically 400,000 V for transmission. This reduces current for a given power (since P = IV), which:

• Reduces energy loss as heat in cables (since energy lost = I²Rt)
• Allows thinner, lighter cables to be used
• Makes long-distance transmission economically viable

Step-down transformers at substations reduce voltage progressively to safe levels for industrial use (33,000 V), then domestic use (230 V).

Power stations generate electricity by rotating electromagnets near coils of wire, producing alternating current. Energy sources include:

• Fossil fuels (coal, oil, gas)
• Nuclear fuel
• Renewable sources (wind, solar, hydroelectric, tidal, geothermal)

Worked examples

Example 1: Calculating charge flow

Question: A current of 2.5 A flows through a lamp for 3 minutes. Calculate the charge that flows through the lamp. [3 marks]

Solution:

• First convert time to seconds: t = 3 × 60 = 180 s [1 mark]
• Use Q = I × t [1 mark]
• Q = 2.5 × 180 = 450 C [1 mark]

Example 2: Series and parallel resistance

Question: A circuit contains three resistors: a 6 Ω resistor in series with two resistors (4 Ω and 12 Ω) connected in parallel to each other. Calculate the total resistance of the circuit. [4 marks]

Solution:

• For the parallel combination, use 1/R_parallel = 1/R₁ + 1/R₂ [1 mark]
• 1/R_parallel = 1/4 + 1/12 = 3/12 + 1/12 = 4/12
• R_parallel = 12/4 = 3 Ω [1 mark]
• Total resistance = series resistor + parallel combination [1 mark]
• R_total = 6 + 3 = 9 Ω [1 mark]

Example 3: Choosing the correct fuse

Question: A kettle is rated at 230 V, 2.3 kW. Available fuses are rated at 3 A, 5 A and 13 A. Which fuse should be used? Show your working. [3 marks]

Solution:

• Convert power to watts: P = 2.3 kW = 2300 W
• Use P = I × V, rearranged to I = P / V [1 mark]
• I = 2300 / 230 = 10 A [1 mark]
• Choose the 13 A fuse (the lowest rating above the operating current) [1 mark]

Note: The 3 A and 5 A fuses would blow during normal operation.

Common mistakes and how to avoid them

• Confusing series and parallel rules: Remember that current is constant in series (like water in a single pipe) but voltage is constant across parallel branches (like the same height difference down multiple waterfalls). Create a comparison table and memorize it.

• Forgetting to convert units: Always convert minutes to seconds, milliamps to amps, kilowatts to watts, and kilojoules to joules before substituting into equations. Write conversions as a separate step.

• Misapplying Ohm's law to non-ohmic components: Ohm's law only applies to ohmic conductors. Filament lamps and diodes do not have constant resistance — their I-V graphs are not straight lines through the origin.

• Incorrect parallel resistance calculations: The total resistance of parallel resistors is always less than the smallest individual resistor. If your answer is larger, check your reciprocal calculation.

• Selecting inappropriate fuses: The fuse must be rated slightly above the normal operating current, not below it. Calculate the operating current using I = P/V first.

• Mixing up live, neutral and earth wires: Use the mnemonic "BLue for Neutral, BRown for Live" and remember earth is always green-yellow striped for safety visibility. Only the live wire alternates potential.

Exam technique for "Electricity"

• Show all working in calculations: Marks are awarded for correct method even if the final answer is wrong. Write the equation, substitute values with units, then calculate. For 3-mark questions, expect: formula (1 mark), substitution (1 mark), answer with unit (1 mark).

• Use the correct equation: Questions asking to "calculate" power may require P = IV, P = I²R, or P = V²/R depending on the data provided. Select the equation that uses the values given in the question.

• Command word precision: "Explain" requires reasons (not just descriptions), "calculate" requires numerical working, "state" needs a concise answer without justification, and "describe" requires a detailed account of features or processes.

• Circuit diagram questions: When asked to add components, ensure ammeters are in series (in the path of current) and voltmeters are in parallel (across components). Draw circuit symbols clearly using a ruler.

Quick revision summary

Current is the rate of charge flow (Q = It). Potential difference is energy per unit charge. Resistance opposes current flow (V = IR). In series circuits, current is constant and voltages add; in parallel, voltage is constant and currents add. Power is the rate of energy transfer (P = IV = I²R = V²/R). UK mains supply is 230 V, 50 Hz a.c. Safety features include fuses, circuit breakers and earth wires. The National Grid uses high voltage transmission to reduce energy losses.