Physics: Electricity, Magnetism and Waves
Electricity, magnetism and waves covers three linked physics topics in 13+ Common Entrance Science: current electricity and circuits, magnetism and electromagnets, and the properties and behaviour of waves.
Before you start
Make sure you're comfortable with these topics first:
Method
- Learn the standard UK circuit symbols (cell, battery, switch, lamp, resistor, ammeter, voltmeter, fuse) so you can read and draw a circuit diagram without hesitating.
- Learn the series circuit rules: current is the same at every point in the circuit, and the supply voltage is shared between the components. Learn the parallel circuit rules: voltage is the same across every parallel branch, and current splits between the branches, with the current entering a junction equal to the current leaving it.
- For resistance questions, rearrange V = I x R into whichever form you need (R = V / I or I = V / R) before substituting numbers, and always give resistance in ohms.
- For magnetism questions, remember magnetic field lines run from the north pole to the south pole outside a magnet, and like poles repel while unlike poles attract.
- For electromagnet questions, identify which variable has changed (current or number of coil turns) and state the effect this has on the strength of the magnetic field.
- Identify whether a wave is transverse (vibrations at right angles to the direction of travel, e.g. light or a wave on a rope) or longitudinal (vibrations along the direction of travel, e.g. sound), since this is a common short-answer question.
- For wave calculations, write down wave speed = frequency x wavelength before substituting values, and check your final unit is metres per second (m/s) if you have used hertz (Hz) and metres (m).
Worked example
A circuit contains a 9 V battery connected in series to two resistors, one of 4 ohms and one of 2 ohms. (a) Calculate the total resistance of the circuit. (b) Use V = I x R to calculate the current flowing through the circuit.
- In a series circuit, resistances add together, so total resistance = 4 + 2 = 6 ohms.
- Write down the equation linking voltage, current and resistance: V = I x R.
- Rearrange to find current: I = V / R.
- Substitute the values: I = 9 / 6 = 1.5.
- Final answer: (a) total resistance = 6 ohms; (b) current = 1.5 A.
Practice questions
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Q1Name the circuit component whose symbol is a circle with a cross inside it.Show answer
Answer: A lamp (bulb).
Q2In a series circuit, an ammeter placed before a bulb reads 0.6 A. State the current a second ammeter, placed elsewhere in the same series circuit, would read.Show answer
Answer: 0.6 A - current is the same at every point in a series circuit.
Q3A torch bulb has a resistance of 15 ohms and a current of 0.2 A flows through it. Use V = I x R to calculate the voltage across the bulb.Show answer
Answer: 3 V (0.2 x 15 = 3).
Q4State whether the north pole of one bar magnet placed near the north pole of a second bar magnet will attract or repel.Show answer
Answer: Repel - like poles repel.
Q5State one way, other than increasing the current, to increase the strength of an electromagnet made from a coil of wire around an iron core.Show answer
Answer: Increase the number of turns in the coil.
Q6A wave has a frequency of 50 Hz and a wavelength of 4 m. Use wave speed = frequency x wavelength to calculate its speed.Show answer
Answer: 200 m/s (50 x 4 = 200).
Q7State whether sound is a transverse wave or a longitudinal wave.Show answer
Answer: Longitudinal.
Q8A student doubles the frequency of a wave while keeping its speed the same. State what happens to the wavelength.Show answer
Answer: It halves - since wave speed = frequency x wavelength stays constant, doubling the frequency must halve the wavelength.
Exam-style questions
Written in the style of a 13+ Common Entrance exam paper, with a full mark scheme.
A pupil sets up a circuit with a 12 V battery connected to two identical lamps in parallel. Each lamp has a resistance of 8 ohms. (a) State the voltage across each lamp. (b) Use V = I x R to calculate the current through one lamp. (c) State whether the total current leaving the battery is greater than, equal to, or less than the current through one lamp, and explain why.
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A pupil builds an electromagnet by wrapping insulated wire around an iron nail and connecting it to a cell, a switch and an ammeter. She tests how many paperclips the electromagnet can pick up using different numbers of coil turns, keeping the current constant at 0.5 A each time. Turns: 10, 20, 30, 40. Paperclips picked up: 3, 6, 9, 12. (a) Describe the pattern shown by these results. (b) Predict how many paperclips the electromagnet would pick up with 50 turns, if the pattern continues. (c) State one variable, other than the number of turns, that the pupil kept constant to make this a fair test, and explain why this was necessary.
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A wave machine at a swimming pool produces water waves with a wavelength of 2.5 m. A pupil counts 8 wave crests passing a fixed point in 20 seconds, giving the waves a frequency of 0.4 Hz. (a) Use wave speed = frequency x wavelength to calculate the speed of the water waves. (b) State whether a water wave is a transverse wave or a longitudinal wave, and give one reason for your answer.
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Free printable worksheet
Want more practice on paper? Download the physics: electricity, magnetism and waves worksheet pack - 13 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.
This topic is chapter 12 of 13+ Science Workbook, the whole course as one free printable PDF.
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