Electric Circuits

What is an electric circuit?

An electric circuit is a closed path through which electric current can flow. For current to flow, the circuit must be complete (no gaps), have a source of energy (like a battery), and have something to use the energy (like a bulb or resistor).

Electric current (I) is the flow of electrons through a conductor. It is measured in Amperes (A) using an ammeter.

Key quantities

Voltage (V) — the "push" that drives current around the circuit. Measured in Volts (V) using a voltmeter.
Current (I) — the rate of flow of charge. Measured in Amperes (A).
Resistance (R) — how much a component opposes the flow of current. Measured in Ohms (Ω).

Ohm's Law

The relationship between voltage, current, and resistance is described by Ohm's Law:

V = I × R | I = V ÷ R | R = V ÷ I

Where V = Voltage (Volts), I = Current (Amperes), R = Resistance (Ohms). If you increase the voltage, more current flows. If you increase resistance, less current flows.

Example: V = 6 V, R = 3 Ω → I = 6 ÷ 3 = 2 A

Circuit symbols

Component	Symbol description	Function
Battery (cell)	Long line (+) and short line (−)	Provides electrical energy
Bulb (lamp)	Circle with X inside	Converts electrical energy to light
Resistor	Rectangle	Opposes current flow
Switch	Gap with a line that can open/close	Opens or closes the circuit
Ammeter	Circle with A	Measures current (in series)
Voltmeter	Circle with V	Measures voltage (in parallel)
Wire	Straight line	Connects components

Series vs Parallel circuits

Components can be connected in two ways:

Series circuit: Components are connected one after another in a single loop. The same current flows through all components. If one bulb breaks, they all go out.
Parallel circuit: Components are connected across the same two points (side by side). Each component has the same voltage across it. If one bulb breaks, the others stay on.

Home wiring uses parallel circuits so that each appliance can be switched on/off independently and receives the full mains voltage.

Answer in your exercise book. Show all calculations clearly.

Question 1 · (1 mark)

State Ohm's Law in words.

Question 2 · (3 marks)

A circuit has a battery of 9 V and a resistor of 3 Ω. Calculate: (a) the current through the resistor, (b) the power dissipated, (c) what happens to the current if the resistance is doubled.

Question 3 · (2 marks)

What is the difference between a series circuit and a parallel circuit? Give ONE advantage of a parallel circuit in home wiring.

Question 4 · (2 marks)

Name the instrument used to measure (a) current and (b) voltage. Describe how each is connected in a circuit.

Question 5 · (4 marks)

In a series circuit with two bulbs and a 12 V battery: each bulb has a resistance of 4 Ω. Calculate the total resistance and the current flowing. What voltage does each bulb experience? Show all working.

Question 6 · (5 marks)

A learner tests a resistor by connecting it to different battery voltages and measuring the current each time. Her results are shown below.

Voltage (V)	Current (A)
2	0.5
4	1.0
6	1.5
8	2.0

(a) Use Ohm's Law to calculate the resistance (R = V ÷ I) for each of the four readings. Show your working. (2)
(b) What do you notice about the resistance value across all four readings? (1)
(c) Based on your answer to (b), describe what a graph of Current (y-axis) against Voltage (x-axis) would look like for this resistor. (1)
(d) Predict the current that would be measured if the learner connected the same resistor to a 10 V battery. (1)

What is an electric circuit?

Key quantities

Ohm's Law

Circuit symbols

Series vs Parallel circuits

Circuit Simulator

Circuit Diagram