Electricity in IGCSE Physics 0625: Current, Voltage and Resistance Explained

Electricity in IGCSE Physics 0625: Current, Voltage and Resistance Explained

Quick answer: Electricity in IGCSE Physics 0625 rests on four ideas: charge, current, voltage and resistance. The key equations are Q = It, V = IR, P = IV and E = IVt, and you need to recall them, because the exam doesn't give you an equation sheet. On top of these, you need series and parallel circuit rules, the I–V graphs for a resistor, filament lamp and diode, how LDRs and thermistors behave, and mains safety. Learn the rules, then practise until you can apply them without thinking.

This guide covers the electricity content of Cambridge IGCSE Physics (0625) in a logical order. Along the way you'll find the definitions examiners want, every equation you need, worked examples and the mistakes that cost the most marks.

Key takeaways

• Current is the rate of flow of charge: I = Q / t, measured in amperes (A) with an ammeter in series.

• Voltage comes in two forms. EMF is the energy the source gives each coulomb of charge. Potential difference (p.d.) is the energy each coulomb transfers in a component. Both are measured in volts (V) with a voltmeter in parallel.

• Resistance is R = V / I, in ohms (Ω). A longer wire has more resistance, and a thicker wire has less.

• In series, the current is the same everywhere and the resistances add up. In parallel, the p.d. is the same across each branch, and the combined resistance is less than any single branch.

• Power is P = IV, and energy is E = IVt. Your electricity bill uses the kilowatt-hour (kWh).

• Safety questions test fuses, circuit breakers, earthing and double insulation, and they ask you to explain why, not just name them.

What the 0625 electricity topic covers

In the Cambridge IGCSE Physics syllabus, electricity sits inside the larger "Electricity and magnetism" topic. Magnetism and electromagnetic effects are separate sub-topics, so this guide covers the electricity part only:

Sub-topic	What you need to know
Electric charge	Positive and negative charge, attraction and repulsion, conductors and insulators, electric fields (Extended)
Electric current	Current as flow of charge, Q = It, ammeters, conventional current vs electron flow, d.c. and a.c.
EMF and p.d.	What a volt means in terms of energy, voltmeters, V = W / Q (Extended)
Resistance	R = V / I, how length and thickness change resistance, I–V graphs
Energy and power	P = IV, E = IVt, kilowatt-hours and cost
Circuits	Circuit symbols, series and parallel rules, potential dividers, LDRs, thermistors, diodes
Electrical safety	Hazards, fuses, trip switches, earthing, double insulation

Core students need the basic statements. Extended students need everything, including the extra equations. Extended candidates sit Papers 2 and 4, and Core candidates sit Papers 1 and 3. Check the current syllabus on the Cambridge website, because the wording changes between versions.

What is electric charge?

Electric charge is a property of particles that makes them feel a force from other charged particles. It's measured in coulombs (C).

There are two kinds of charge, positive and negative. Like charges repel and opposite charges attract. That one rule explains most of the static electricity questions you'll see.

When you rub a plastic rod with a cloth, the rod becomes charged. Extended students need the reason: electrons are transferred between the two materials. Positive charges don't move, so a negatively charged object has gained electrons and a positively charged object has lost them. Writing "positive charges moved onto the cloth" is one of the quickest ways to lose a mark in this section.

Conductors (most metals) let electrons move through them easily. Insulators (plastic, rubber, glass) don't. That's why wires have copper cores inside a plastic coat.

Extended students also need electric fields. An electric field is a region where a charge feels a force. Its direction is the direction of the force on a positive charge. So field lines point away from positive charges and towards negative ones.

What is electric current?

Electric current is the rate of flow of charge. In a metal wire, the charge carriers are electrons.

I = \frac{Q}{t}

Here I is current in amperes (A), Q is charge in coulombs (C) and t is time in seconds (s). One ampere means one coulomb of charge passing a point every second.

Three details come up again and again in exams:

1. You measure current with an ammeter connected in series. The current has to flow through the meter to be measured.

2. Conventional current flows from + to −. Electrons actually move the other way, from − to +. Circuit questions use conventional current unless they say otherwise.

3. Direct current (d.c.) flows one way only, as from a cell or battery. Alternating current (a.c.) keeps changing direction, as from mains sockets.

One idea fixes a lot of wrong answers: current is not used up in a circuit. The current leaving the cell equals the current returning to it. Energy is what gets transferred to the components, not current.

What is the difference between EMF and potential difference?

Both are measured in volts, and many students treat them as the same thing. Examiners don't.

• Electromotive force (EMF) is the energy the source (a cell, battery or power supply) transfers to each unit of charge it drives around the complete circuit.

• Potential difference (p.d.) is the energy transferred by each unit of charge as it passes through a component, such as a lamp or resistor.

Put simply, EMF is energy going in at the source, and p.d. is energy coming out in the components. For Extended, both are defined using:

V = \frac{W}{Q}

W is the energy transferred (work done) in joules (J). So 1 volt = 1 joule per coulomb. A 9 V battery gives each coulomb of charge 9 J of energy.

You measure p.d. with a voltmeter connected in parallel across the component. The voltmeter compares the energy of the charge on each side, so it has to connect to both sides.

What is resistance?

Resistance tells you how hard it is for current to flow through a component. It's measured in ohms (Ω).

R = \frac{V}{I}

For the same p.d., a bigger resistance means a smaller current. You'll rearrange this as V = IR more often than any other equation in the topic.

For a metal wire, resistance depends on its shape:

• Longer wire, more resistance. Resistance is directly proportional to length, so doubling the length doubles the resistance.

• Thicker wire, less resistance. Resistance is inversely proportional to cross-sectional area, so doubling the area halves the resistance.

Extended students should be able to use these proportions in calculations. For example, a wire of 4 Ω cut to half its length has a resistance of 2 Ω.

The three I–V graphs you must know

An I–V graph plots current against p.d. Its shape tells you how a component's resistance behaves.

Component	Graph shape	What it tells you
Fixed resistor (at constant temperature)	Straight line through the origin	Current is proportional to p.d., so resistance stays constant
Filament lamp	Curve that gets less steep as p.d. rises	The filament heats up, so its resistance increases
Diode (Extended)	Almost zero current one way, then a steep rise the other way	Current flows in one direction only

The filament lamp is the one examiners love. A full-mark explanation links three steps: more current, so the filament gets hotter, so its resistance increases. Saying "the resistance changes" without the temperature link usually earns nothing.

How do you calculate electrical power and energy?

Power is how fast energy is transferred. For electrical components:

P = IV

 

E = IVt

P is power in watts (W), E is energy in joules (J), and t is time in seconds.

For homes, joules are too small to be useful, so energy is sold in kilowatt-hours (kWh). To find it, multiply the power in kilowatts by the time in hours. A 2 kW heater left on for 3 hours uses 2 × 3 = 6 kWh. If electricity costs 15 cents per kWh, that's 6 × 15 = 90 cents. Cost questions are usually easy marks, as long as you don't mix up watts with kilowatts or minutes with hours.

Series and parallel circuits: the rules

Almost every electricity question on Paper 4 involves a circuit. These rules let you work out any current, p.d. or resistance in it.

	Series circuit	Parallel circuit
Current	Same at every point	Splits between branches; branch currents add up to the total
Potential difference	Shared between components; the p.d.s add up to the EMF	Same across each branch
Combined resistance	R = R₁ + R₂ + …	Less than the smallest branch resistance
If one lamp breaks	Every lamp goes out	The other lamps stay on

Extended students need the parallel formula for two resistors:

\frac{1}{R} = \frac{1}{R_1} + \frac{1}{R_2}

The branch currents adding up to the total is just conservation of charge: charge can't appear or disappear at a junction.

Why are houses wired in parallel? Each appliance gets the full mains voltage. Each one can be switched on and off on its own. And if one fails, the rest keep working. That's a classic 3-mark answer.

Circuit components you need to know

Beyond cells, lamps and resistors, four components come up often.

• Variable resistor (rheostat). Changing its resistance changes the current, for example to dim a lamp.

• Light-dependent resistor (LDR). Its resistance decreases as light gets brighter. It's used in automatic night lights.

• Thermistor. Its resistance decreases as temperature rises. It's used in fire alarms and thermostats.

• Diode. It lets current flow in one direction only. Extended students should know it can turn a.c. into d.c. (rectification).

Potential dividers (Extended)

A potential divider is two resistors in series, used to split a supply voltage. The p.d. divides in the same ratio as the resistances:

\frac{V_1}{V_2} = \frac{R_1}{R_2}

Swap one of the resistors for an LDR or thermistor and you have a sensor circuit. The output voltage changes with light or temperature. A common question asks what happens to the output when it gets darker. Work it through step by step: the LDR's resistance goes up, so it takes a bigger share of the voltage.

Electrical safety

Safety questions look easy, but they're often marked hard, because examiners want the reason behind each feature.

Hazards to recognise: damaged insulation, cables that overheat because they carry too much current, and water or damp conditions near mains electricity.

Safety features to explain:

• Fuse. A thin wire in the live wire melts if the current gets too high, which breaks the circuit. Choose a fuse rating just above the appliance's normal current.

• Trip switch (circuit breaker). It does the same job as a fuse but switches off automatically and can be reset.

• Earth wire. It connects a metal case to earth. If a fault makes the case live, a large current flows to earth, which blows the fuse or trips the breaker. That stops anyone getting a shock from touching the case.

• Double insulation. The appliance has a plastic outer case, so the user can't touch live metal, and no earth wire is needed.

Worked examples

Try each one before reading the answer.

1. Charge and current. A current of 0.4 A flows for 2 minutes. How much charge passes? Convert first: 2 minutes = 120 s. Then Q = It = 0.4 × 120 = 48 C.

2. Series circuit. A 6 V battery is connected to a 4 Ω resistor and an 8 Ω resistor in series. Find the current and the p.d. across the 8 Ω resistor. Total R = 4 + 8 = 12 Ω. I = V / R = 6 / 12 = 0.5 A. The p.d. across 8 Ω = IR = 0.5 × 8 = 4 V. Check: the 4 Ω resistor takes 2 V, and 4 + 2 = 6 V, which matches the EMF.

3. Parallel circuit (Extended). A 6 Ω and a 3 Ω resistor are connected in parallel across 6 V. Find the combined resistance and the current from the battery. 1/R = 1/6 + 1/3 = 1/6 + 2/6 = 3/6, so R = 2 Ω. The current from the battery is I = 6 / 2 = 3 A. Check: the branch currents are 6/6 = 1 A and 6/3 = 2 A, which add up to 3 A.

4. Choosing a fuse. A 2.3 kW kettle runs on a 230 V supply. Fuses come in 3 A, 5 A and 13 A. Which should you use? I = P / V = 2300 / 230 = 10 A. The 13 A fuse is the smallest rating above 10 A, so choose 13 A.

The most common mistakes in IGCSE electricity questions

1. Connecting a voltmeter in series or an ammeter in parallel. Ammeters go in series and voltmeters go in parallel. Draw them that way every time.

2. Forgetting to convert units. 250 mA is 0.25 A, 2 minutes is 120 s and 3 kΩ is 3000 Ω. Convert before you substitute into an equation.

3. Adding parallel resistances. 6 Ω and 3 Ω in parallel is 2 Ω, not 9 Ω. If your answer is bigger than either resistor, you've used the wrong rule.

4. Saying current is "used up". Energy is transferred, but current isn't consumed.

5. Saying "voltage through" a component. Current flows through a component. P.d. is measured across it.

6. Explaining the filament lamp graph without temperature. You need the full chain: current, then heating, then resistance.

We've gathered more of these across the whole course in Common Mistakes in IGCSE Physics Exams.

How to revise electricity for Paper 2 and Paper 4

Knowing the rules isn't enough. You need to have used them on real exam questions. Here's a plan that works:

1. Learn the six equations by heart: Q = It, V = W/Q, R = V/I, P = IV, E = IVt and the parallel formula. Write them from memory every day for a week.

2. Do topic-by-topic past paper questions. Work through every electricity question from recent years in one go, so you can see which question styles keep coming back. Our guide to the benefits of IGCSE past papers explains how to get the most from them.

3. Mark yourself strictly against the mark scheme. Pay close attention to the exact phrases the mark scheme rewards on safety and I–V graph questions.

4. Finish with full timed papers. In the spring, move on to full papers and predicted papers. Our guide on how to use predicted papers for May/June 2027 shows how to fit them into your revision.

sortmyprep's free plan gives you five years of topic-by-topic IGCSE Physics past papers with mark schemes. When a circuit question stumps you, sorty, the AI tutor built into every question, walks you through it one step at a time. Start practising electricity questions free.

Frequently asked questions

Is electricity hard in IGCSE Physics?

Electricity feels hard when you try to memorise answers instead of understanding them. Once current, p.d. and resistance make sense, most questions follow the same few rules. Many students find it one of the most predictable topics to score in.

Do you get an equation sheet in IGCSE Physics 0625?

No. Cambridge doesn't provide a list of equations for 0625, so you have to recall them. For electricity that means Q = It, V = IR, P = IV and E = IVt, plus V = W/Q and the parallel resistance formula for Extended.

Why does a filament lamp's resistance increase?

As the current increases, the filament gets hotter. In a hotter metal, electrons collide more often with the vibrating ions, so the resistance rises. That's why the lamp's I–V graph curves instead of staying straight.