Electricity and Magnetism
Electricity and magnetism is the study of electric circuits, static electricity and magnetic fields, including how current, potential difference (voltage) and resistance are related in series and parallel circuits. It also covers how magnets and electromagnets produce magnetic fields, and how static charge builds up when materials are rubbed together. This KS3 topic uses the equations Q = I t, V = I R and P = V I.
Before you start
No specific prerequisites - this is a good place to start.
Method
- Learn the standard circuit symbols (cell, resistor, ammeter, voltmeter, lamp, switch) and remember that current is measured in amps with an ammeter in series, and potential difference in volts with a voltmeter in parallel.
- Apply series circuit rules: current is the same everywhere, and total resistance is the sum of the resistors (R_total = R1 + R2 + ...).
- Apply parallel circuit rules: potential difference is the same across each branch, and total current is the sum of the branch currents.
- Use Q = I x t to calculate charge, V = I x R to calculate potential difference, current or resistance, and P = V x I to calculate electrical power.
- Explain static electricity in terms of electrons transferring between materials by friction, and describe how like charges repel while opposite charges attract.
- Recall that magnetic materials (iron, nickel, cobalt) are attracted to magnets, and that an electromagnet's strength increases with a larger current, more coil turns, or an iron core.
Worked example
A current of 4 A flows through a lamp for 25 s. Use Q = I x t. Calculate the charge that flows through the lamp.
- Write down the equation: charge = current x time.
- Substitute the values: Q = 4 x 25.
- Calculate: Q = 100.
- Final answer: 100 C (coulombs) of charge flows.
Practice questions
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Q1State the unit of electric current.Show answer
Answer: Amps (A)
Q2State whether this is true or false: 'In a series circuit, the current is the same at every point.'Show answer
Answer: True
Q3A current of 0.8 A flows through a resistor of resistance 15 ohm. Use V = I x R. Calculate the potential difference across the resistor.Show answer
Answer: 12 V (0.8 x 15)
Q4A series circuit has two resistors, R1 = 5 ohm and R2 = 7 ohm. Calculate the total resistance of the circuit.Show answer
Answer: 12 ohm (5 + 7)
Q5A device is connected to a 230 V supply and draws a current of 2 A. Use P = V x I. Calculate the power of the device.Show answer
Answer: 460 W (230 x 2)
Q6A 12 V supply is connected to two resistors in parallel, R1 = 4 ohm and R2 = 6 ohm. Use I = V / R for each branch. Calculate the total current supplied by the battery.Show answer
Answer: 5 A (3 A through R1 plus 2 A through R2)
Exam-style questions
Written in the style of a KS3 Science exam paper, with a full mark scheme.
A series circuit contains a 9 V battery, an ammeter, and two resistors R1 = 3 ohm and R2 = 6 ohm connected one after another. Use R_total = R1 + R2 and V = I x R_total. (a) Calculate the total resistance of the circuit. (1) (b) Calculate the current shown on the ammeter. (2)
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Before a road tanker begins pumping petrol into an underground storage tank at a filling station, workers first clip an earthing cable between the tanker and a metal point on the ground. (a) Explain, in terms of electrons, how a build-up of static charge could occur as the petrol flows through the delivery pipe. (2) (b) Explain how the earthing cable reduces the risk of a fire during the delivery. (2)
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A student builds an electromagnet by wrapping insulated wire in a coil around an iron nail, and connects it to a variable power supply. (a) State two ways the student could increase the strength of the electromagnet. (2) (b) Explain, in terms of magnetic fields, why adding the iron core increases the strength of the electromagnet compared with the coil alone. (2) (c) The student increases the current from 2 A to 5 A. State what happens to the number of paperclips the electromagnet can pick up. (1)
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Free printable worksheet
Want more practice on paper? Download the electricity and magnetism worksheet pack - 17 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
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