How are magnetism and electricity linked, and how does that make motors work?
Permanent and induced magnets and magnetic fields, the magnetic effect of a current and solenoids, electromagnets, and the motor effect including Fleming's left-hand rule.
A focused answer to the AQA GCSE Combined Science: Trilogy Magnetism and electromagnetism topic, covering permanent and induced magnets and magnetic fields, the magnetic effect of a current and solenoids, electromagnets, and the motor effect with Fleming's left-hand rule.
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What this topic is asking
AQA wants you to describe permanent and induced magnets and magnetic fields, explain the magnetic effect of a current and solenoids, describe electromagnets, and explain the motor effect including Fleming's left-hand rule.
Magnets and magnetic fields
A magnetic field is the region around a magnet where a force acts on another magnet or on a magnetic material (iron, steel, cobalt or nickel). Field lines always run from north to south outside the magnet, never cross, and the field is strongest where the lines are closest together (at the poles). The Earth itself behaves like a giant bar magnet, which is why a compass (a small freely turning magnet) lines up with the Earth's field and points roughly north; this is evidence that the Earth's core is magnetic.
The magnetic effect of a current and electromagnets
The strength of an electromagnet is increased by using a larger current, more turns on the coil, or an iron core. Because it can be controlled (turned on and off and varied), the electromagnet is used in devices such as scrapyard cranes, electric bells, relays and loudspeakers.
The motor effect
The force is greatest when the wire is at 90 degrees to the field and is zero when the wire is parallel to the field. Reversing either the current or the field direction reverses the force. A simple electric motor uses this effect: a current-carrying coil in a magnetic field experiences forces on opposite sides in opposite directions, which makes it rotate, and a split-ring commutator swaps the current direction every half-turn to keep it spinning the same way.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20193 marksA wire of length 0.15 m carries a current of 4 A at right angles to a magnetic field of flux density 0.30 T. Calculate the force on the wire.Show worked answer →
A Physics Paper 2 motor-effect calculation. Method: the force on a conductor in a magnetic field is , where is the magnetic flux density, the current and the length in the field. Substituting, N. Markers award the correct equation, the substitution of all three values, and the answer with the unit (newtons). Note the equation in this form applies only when the wire is at right angles to the field, which the question states; if it were parallel the force would be zero.
AQA 20214 marksDescribe how the strength of an electromagnet can be increased, and explain one advantage of an electromagnet over a permanent magnet.Show worked answer →
A Physics Paper 2 description and explanation. Reward ways to increase the strength: increase the current through the coil, increase the number of turns on the coil, and add or use an iron core. For the advantage: an electromagnet can be switched on and off by switching the current, and its strength can be varied, whereas a permanent magnet is always magnetic and cannot be controlled. This makes electromagnets useful in devices such as scrapyard cranes (pick up and drop metal) and electric bells. Markers credit at least two correct ways to increase strength and a valid, clearly explained advantage.
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