How do magnets and electric currents create magnetic fields, and how does the motor effect work?
Permanent and induced magnets, magnetic fields and field lines, the magnetic field around a current-carrying wire and a solenoid, electromagnets and their uses, and the motor effect with the factors affecting the force.
A focused answer to the OCR Gateway GCSE Combined Science A topic P3 on magnetism and electromagnetism, covering permanent and induced magnets, magnetic fields, the field around a wire and a solenoid, electromagnets and their uses, and the motor effect.
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What this topic is asking
OCR wants you to describe permanent and induced magnets, draw magnetic fields, describe the field around a wire and a solenoid, explain electromagnets and their uses, and explain the motor effect and the factors affecting the force.
Magnets and magnetic fields
Magnetic field lines always run from the north pole to the south pole of a magnet, and the closer the lines, the stronger the field. The field is strongest at the poles. Two like poles repel and two unlike poles attract, and this is a non-contact force (it acts at a distance). A plotting compass shows the direction of the field at a point, because the compass needle is itself a small magnet that lines up with the field, which is also how a compass shows that the Earth has a magnetic field.
Electromagnetism and electromagnets
When a current flows through a wire it creates a magnetic field around the wire, in circles centred on the wire; reversing the current reverses the field direction. Winding the wire into a coil (a solenoid) concentrates the field, producing a field pattern like that of a bar magnet, with a strong, fairly uniform field inside the coil.
The ability to turn the magnetism on and off, and to vary its strength, is exactly why electromagnets are used where a permanent magnet would not work, for example to pick up and then release scrap cars.
The motor effect
The motor effect is the force experienced by a current-carrying wire placed in a magnetic field. The magnetic field produced by the current interacts with the external field, and if the wire is at right angles to the field it experiences a force at right angles to both the current and the field. Reversing either the current or the field reverses the direction of the force. The size of the force is increased by increasing the current, increasing the strength of the magnetic field, or increasing the length of wire in the field. This effect is the basis of the electric motor, where the force makes a current-carrying coil rotate.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20194 marksDescribe two ways to increase the strength of an electromagnet, and give one use of an electromagnet that a permanent magnet could not perform.Show worked answer →
A Physics Paper 5 structured question on electromagnets. Reward two ways to increase the strength: increase the current through the coil, increase the number of turns on the coil, or add an iron core (or move the poles closer). A use that needs an electromagnet (not a permanent magnet): a scrapyard crane that picks up and then drops cars (the magnetism can be switched off), an electric bell, or a relay/circuit breaker, because the magnetism can be turned on and off, which a permanent magnet cannot do. Markers credit any two valid strengthening methods and one use that specifically relies on the magnetism being switchable.
OCR 20214 marksExplain the motor effect, and state two factors that increase the size of the force on a current-carrying wire in a magnetic field.Show worked answer →
A P3 question on the motor effect. Reward: the motor effect is the force experienced by a current-carrying wire (conductor) placed in a magnetic field, because the magnetic field of the current interacts with the external magnetic field; if the wire is at right angles to the field, it experiences a force at right angles to both. Two factors that increase the force: increasing the current, and increasing the strength of the magnetic field (magnetic flux density). The length of wire in the field also affects the force. Markers credit the description of a force on a current-carrying wire in a field, and two valid factors (current, field strength, or length in the field).
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