Understanding Physics: Electricity: Edexcel IGCSE Science Double Award Explained

Electricity can feel abstract at first, but once you connect the equations to real circuits and real appliances, the whole topic clicks into place. Here is everything you need for the electricity section of the Edexcel IGCSE Science Double Award.

The edexcel igcse science double award physics: electricity section of the 4SD0 specification covers three topics: units, mains electricity, and energy and voltage in circuits. As edexcel igcse science double award explained in the specification, this is one of the most equation-rich parts of the physics paper, but every equation has a practical context that makes it easier to remember. These edexcel igcse science double award revision notes guide you through each topic with worked examples and the kind of clear explanations that build genuine confidence.

Units for electricity

Electricity has its own set of quantities and units that you need to know fluently:

Quantity	Symbol	Unit	Unit symbol
Current	I	ampere (amp)	A
Voltage (potential difference)	V	volt	V
Resistance	R	ohm	Ω
Power	P	watt	W
Energy	E	joule	J
Charge	Q	coulomb	C

Current is the rate of flow of charge: I = Q / t. One ampere means one coulomb of charge flows past a point every second. Voltage (potential difference) is the energy transferred per unit charge: V = E / Q. One volt means one joule of energy is transferred for every coulomb of charge. Getting comfortable with these definitions helps you understand what the equations actually mean, not just how to plug numbers in.

Mains electricity

In the UK, the mains electricity supply is alternating current (AC) at about 230 V and a frequency of 50 Hz. AC means the current changes direction many times per second. Batteries, by contrast, supply direct current (DC), which flows in one direction only. The edexcel igcse science double award specification expects you to know the difference and to understand the structure of a three-pin plug.

A three-pin plug contains three wires:

Live wire (brown): Carries the alternating current to the appliance. It alternates between about +325 V and -325 V.
Neutral wire (blue): Completes the circuit. It is at or near 0 V.
Earth wire (green and yellow stripes): A safety wire connected to the metal case of an appliance. It carries no current during normal operation, but provides a low-resistance path to earth if a fault occurs.

If the live wire touches the metal case of an appliance, the earth wire carries the current safely to earth, causing a large current to flow. This large current blows the fuse (or trips the circuit breaker), which breaks the circuit and prevents electric shock. The fuse is always placed in the live wire. If it were in the neutral wire, the appliance would still be connected to the live supply even after the fuse blew, which would be dangerous.

Exam tip: When explaining the function of the earth wire and fuse, always mention the sequence of events: fault occurs, current flows through earth wire, large current blows fuse, circuit breaks. Stating each step clearly picks up all the available marks. Vague answers like "the earth wire stops you getting electrocuted" miss the mechanism.

Residual current circuit breakers (RCCBs) provide additional protection. An RCCB detects a difference between the current flowing in the live wire and the neutral wire. If current is leaking to earth (for example through a person), the RCCB trips almost instantly, much faster than a fuse. Unlike fuses, RCCBs can be reset and do not need replacing.

A fuse is rated just above the normal operating current of the appliance. To calculate the correct fuse, use P = IV, rearranged to I = P / V. If a kettle is rated at 2300 W and the supply is 230 V, the operating current is 2300 / 230 = 10 A, so a 13 A fuse would be appropriate (the next standard fuse rating above 10 A). Using a fuse that is too high would not blow quickly enough during a fault. Using one that is too low would blow during normal operation.

Energy and voltage in circuits

This is the largest topic in the electricity section, and it contains the equations you will use most on the exam.

Ohm's law: For a conductor at constant temperature, the current through it is directly proportional to the voltage across it. This gives the equation V = IR. From this you can find any one of the three quantities if you know the other two. Rearranging: I = V / R and R = V / I.

Not all components obey Ohm's law. A filament lamp, for example, does not: as the current increases, the filament heats up, its resistance increases, and the I-V graph curves. A diode allows current to flow in one direction only, so its I-V graph shows current on one side and virtually nothing on the other. The igcse 4sd0 physics: electricity specification requires you to recognise and sketch I-V characteristics for a resistor (straight line through the origin), a filament lamp (curve) and a diode (one-sided).

Series circuits: Components are connected in a single loop. The current is the same through all components. The total voltage is shared between the components: V_total = V₁ + V₂. The total resistance is the sum of individual resistances: R_total = R₁ + R₂.

Parallel circuits: Components are connected on separate branches. The voltage across each branch is the same. The total current is shared between the branches: I_total = I₁ + I₂. Adding more resistors in parallel decreases the total resistance, because there are more paths for the current to flow through.

Worked example: Two resistors of 6 Ω and 3 Ω are connected in series to a 9 V battery. What is the current?

R_total = 6 + 3 = 9 Ω
I = V / R = 9 / 9 = 1 A

The voltage across the 6 Ω resistor: V = IR = 1 x 6 = 6 V
The voltage across the 3 Ω resistor: V = IR = 1 x 3 = 3 V
Total: 6 + 3 = 9 V (which confirms it matches the battery voltage).

Power: Electrical power is the rate of energy transfer: P = IV. Combined with V = IR, this gives two further useful forms: P = I²R and P = V²/R. Power is measured in watts (W). One watt means one joule of energy is transferred per second.

Energy: The total energy transferred by an appliance is: E = Pt, where P is power in watts and t is time in seconds. Alternatively, E = IVt. If the question gives time in minutes or hours, convert to seconds first. Energy is measured in joules, but electricity bills use kilowatt-hours (kWh): E (kWh) = P (kW) x t (hours).

Worked example: A 2000 W heater runs for 3 hours. How much energy does it use in kWh?

P = 2000 W = 2 kW
E = 2 x 3 = 6 kWh

If the cost of electricity is 15p per kWh, the cost = 6 x 15 = 90p.

Variable resistors and thermistors: A variable resistor allows you to change the resistance in a circuit, which changes the current and therefore the brightness of a lamp or the speed of a motor. A thermistor is a component whose resistance decreases as temperature increases. It is used in temperature-sensing circuits. A light-dependent resistor (LDR) has a resistance that decreases as light intensity increases, making it useful in automatic lighting circuits.

Current and charge: Remember that current is a flow of charge. In metallic conductors, it is electrons that flow. Electrons are negatively charged, so conventional current (which is defined as the flow of positive charge) goes in the opposite direction to the electron flow. The exam may ask you to distinguish between the two. Charge is calculated from current and time: Q = It. If a current of 0.5 A flows for 120 seconds, the charge = 0.5 x 120 = 60 C.

Common mistakes and self-checks

The physics: electricity edexcel igcse practice questions in the edexcel igcse science double award on electricity often involve multi-step calculations. The most common mistakes are forgetting to convert units, using the wrong equation, and confusing series and parallel rules.

Always check whether the circuit is series or parallel before applying any rules.
Remember: current is the same everywhere in a series circuit, but voltage is the same across each branch in a parallel circuit.
When asked for the function of a fuse, give the full sequence of events, not just "it stops fires."

Self-check: Can you state Ohm's law and sketch the I-V graph for a resistor, a lamp and a diode? Can you calculate the correct fuse rating for a 920 W appliance at 230 V? Can you explain why adding resistors in parallel reduces total resistance? If you can answer all three, you have a solid foundation in this topic.

When studying these edexcel igcse science double award notes, build a formula sheet and practice rearranging every equation for every variable. Use the Green Bridge CBT platform for edexcel igcse science double award practice questions and edexcel igcse science double award revision notes on electricity. The exam will test your ability to apply these equations to unfamiliar circuits, so fluency with the methods is the single best investment of your revision time.