Understanding Electricity: Edexcel IGCSE Physics Explained

Electricity runs through almost every part of daily life, and it runs through a large part of the Edexcel IGCSE Physics exam too. Here is how to master every circuit, every calculation and every concept.

The electricity section of the edexcel igcse physics electricity specification covers four areas: units, mains electricity, energy and voltage in circuits, and electric charge. Together, they form one of the most calculation-heavy parts of the course. If you enjoy working through problems step by step, you will find this section rewarding. If calculations make you nervous, the worked examples below will build your confidence.

These edexcel igcse physics revision notes are designed to take you through every key concept, show you exactly how to set up and solve the most common question types, and highlight the mistakes that trip students up. Treat them as your electricity edexcel igcse companion for revision.

Units for electricity

You will use all of these regularly, so know them by heart:

Mains electricity and safety

Mains electricity in most countries is alternating current (a.c.), meaning the direction of current flow reverses many times per second. Cells and batteries supply direct current (d.c.), where current flows in one direction only. You need to know this distinction clearly for the exam.

Safety devices

Electrical appliances use several safety features:

• Insulation: Plastic coating around wires prevents the user touching live conductors.
• Double insulation: The appliance has no exposed metal parts and no earth wire is needed. The case is made of an insulating material.
• Earthing: A wire connecting the metal case of an appliance to the ground. If a fault causes the live wire to touch the case, current flows to earth, blowing the fuse or tripping the circuit breaker.
• Fuses: A thin wire that melts and breaks the circuit if the current exceeds a safe value. You select a fuse rated just above the normal operating current.
• Circuit breakers: Automatic switches that break the circuit when the current is too high. They can be reset, unlike fuses which must be replaced.

Heating effect of current

When current flows through a resistor, electrical energy is transferred to thermal energy. The resistor heats up. This principle is used in kettles, toasters, immersion heaters and electric fires. The energy transferred depends on current, voltage and time.

Power, current and voltage

The relationship is: power = current x voltage, P = I x V. Power is measured in watts (W). This equation is essential for fuse selection: calculate the normal operating current using I = P / V, then choose a fuse rated just above that value.

Energy transferred

Energy transferred = current x voltage x time, E = I x V x t. The energy is in joules (J), current in amperes (A), voltage in volts (V), and time in seconds (s).

Energy and voltage in circuits

Current, charge and electrons

Current is the rate of flow of charge: I = Q / t, or rearranged, Q = I x t. Charge is measured in coulombs (C). In solid metallic conductors, current is a flow of negatively charged electrons. The electrons move from negative to positive, but conventional current is drawn from positive to negative.

Voltage is the energy transferred per unit charge: V = E / Q, so E = Q x V. One volt means one joule of energy transferred per coulomb of charge.

Resistance and Ohm's law

Voltage = current x resistance, V = I x R. This is the most-used equation in the electricity section of the igcse 4ph1 electricity specification.

I-V characteristics

Different components have different current-voltage relationships:

• Wire/resistor (at constant temperature): Straight line through the origin. Current is proportional to voltage. Constant resistance.
• Metal filament lamp: Curve that flattens at higher voltages. As the filament heats up, resistance increases, so current increases more slowly.
• Diode: No current flows in one direction (reverse bias). In the forward direction, current flows once the threshold voltage is reached, then increases steeply.

LDRs and thermistors

A light-dependent resistor (LDR) has high resistance in the dark and low resistance in bright light. A thermistor has high resistance at low temperatures and low resistance at high temperatures. Both are commonly tested in circuit analysis questions on the edexcel exam.

Series and parallel circuits

In a series circuit: current is the same through every component; voltage is shared between components (the voltages add up to the supply voltage); total resistance = R₁ + R₂.

In a parallel circuit: voltage is the same across each branch; current splits at junctions (total current = sum of branch currents); total resistance is less than the smallest individual resistance.

Current is conserved at every junction. What flows in must flow out. This is because charge cannot be created or destroyed.

Domestic lighting uses parallel circuits so that each lamp can be switched on and off independently and receives the full mains voltage.

Electric charge (static electricity)

Some materials can be charged by friction. When you rub an insulating material (like a polythene rod with a cloth), electrons transfer from one surface to the other. The material that gains electrons becomes negatively charged; the one that loses electrons becomes positively charged.

• Like charges repel.
• Unlike charges attract.

Common conductors include metals. Common insulators include plastics. Conductors allow charge to flow through them; insulators do not (which is why only insulators can hold a static charge on their surface).

Dangers and uses of static electricity

Dangers: When fuelling aircraft or tankers, friction can build up static charge. A spark could ignite the fuel vapour. To prevent this, the vehicle and fuel nozzle are earthed before refuelling.

Uses: Photocopiers use static charge to attract toner to the paper in the pattern of the image. Inkjet printers use charged plates to direct tiny droplets of ink onto the paper with precision.

Common mistakes in electricity

1. Mixing up current and voltage. Current is the flow of charge (measured in amps). Voltage is the energy transferred per unit charge (measured in volts). They are related by V = IR but they are not the same thing.
2. Getting series and parallel rules backwards. In series, current is the same everywhere and voltage splits. In parallel, voltage is the same across branches and current splits. Drawing a quick circuit diagram before answering helps.
3. Forgetting that a fuse protects the wiring, not the user directly. The fuse blows when current is too high, breaking the circuit before the wiring overheats. The earth wire protects the user by providing a low-resistance path to ground.
4. Not converting units. Energy in kJ must be converted to J. Time in minutes must be converted to seconds. Power in kW must be converted to W.
5. Saying electrons flow from positive to negative. Electrons flow from negative to positive. Conventional current is drawn from positive to negative. The exam expects you to know the difference.

Self-check: edexcel igcse physics practice questions

1. A 6 V battery drives a current of 0.2 A through a resistor. Calculate the resistance.
2. A current of 3 A flows for 120 s. How much charge passes a point in the circuit?
3. A 920 W microwave operates on a 230 V supply. Calculate the current and select an appropriate fuse (3 A, 5 A or 13 A).
4. Two resistors of 8 Ω and 12 Ω are connected in series to a 10 V supply. Calculate the current and the voltage across each resistor.
5. Explain why a polythene rod becomes negatively charged when rubbed with a cloth.

Answers: (1) R = V / I = 6 / 0.2 = 30 Ω; (2) Q = I x t = 3 x 120 = 360 C; (3) I = 920 / 230 = 4 A, so a 5 A fuse; (4) total R = 20 Ω, I = 10 / 20 = 0.5 A, V across 8 Ω = 4 V, V across 12 Ω = 6 V; (5) Friction transfers electrons from the cloth to the rod, giving the rod an excess of negative charge.

These edexcel igcse physics notes cover the full electricity section of the specification. Every equation, every concept, and every question type you are likely to see on the exam is here. The edexcel igcse physics explained approach strips each idea down to what you actually need to write in an answer. Master these notes, attempt the practice questions without looking at the answers first, and this section of the exam will feel straightforward.