What is the difference between permanent and induced magnets, and how do we represent magnetic fields?
Permanent and induced magnets, magnetic materials, attraction and repulsion between poles, the magnetic field around a bar magnet and the Earth, and how a compass shows field direction.
A focused answer to OCR Gateway GCSE Physics A topic P4 on magnets, covering permanent and induced magnets, magnetic materials, attraction and repulsion between poles, the magnetic field around a bar magnet and the Earth, and how a compass shows field direction.
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What this topic is asking
OCR wants you to distinguish permanent and induced magnets, identify magnetic materials, describe attraction and repulsion between poles, and represent the magnetic field around a magnet and the Earth, including using a compass. This is topic P4.1 of the OCR Gateway Physics A (J249) specification.
Permanent and induced magnets
This gives the definitive test for a permanent magnet: only a permanent magnet can repel another magnet. A magnetic material (or an induced magnet) is only ever attracted, so if two objects repel, both must be permanent magnets.
Magnetic materials and poles
Steel is used for permanent magnets because it keeps its magnetism; soft iron is used where the magnetism needs to come and go (as in an electromagnet) because it loses its magnetism easily.
Magnetic fields
The field around a bar magnet curves from the north pole round to the south pole. A plotting compass placed in the field lines up with it: the needle points along the field line, so moving the compass around and marking the direction at each point traces the field's shape.
The Earth's magnetic field
The Earth behaves as if it has a giant bar magnet inside it, producing a magnetic field. A compass needle is a small magnet that lines up with the Earth's field, which is why it points roughly north. This is evidence that the Earth's core is magnetic, and it is the basis of navigation by compass. Note that the compass points to the Earth's magnetic north, which is near but not exactly at the geographic North Pole.
Try this
Q1. State which is the only reliable test that an object is a permanent magnet. [1 mark]
- Cue. It can repel another magnet (magnetic materials are only attracted).
Q2. State the direction of the magnetic field lines outside a bar magnet. [1 mark]
- Cue. From the north pole to the south pole.
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 20184 marksExplain the difference between a permanent magnet and an induced magnet, and describe a test that would show whether a bar of iron is a permanent magnet or just a magnetic material.Show worked answer →
A P4 question worth four marks. A permanent magnet produces its own magnetic field all the time and cannot be switched off (2 marks for it always being magnetic and producing its own field). An induced magnet becomes magnetic only when placed in a magnetic field, and loses most or all of its magnetism when removed (1 mark). A test: bring one end of the iron bar near the repelling behaviour of a known magnet; only a permanent magnet repels as well as attracts, because an induced magnet (or magnetic material) is only ever attracted, never repelled (1 mark). Markers reward the always-on field for the permanent magnet, the field-dependent magnetism of the induced magnet, and using repulsion as the definitive test.
OCR 20213 marksDescribe the magnetic field around a bar magnet, and explain how a plotting compass can be used to show the direction of the field.Show worked answer →
A P4 question worth three marks. The magnetic field around a bar magnet runs from the north pole to the south pole outside the magnet, is shown by field lines that never cross, and is strongest where the lines are closest together, which is at the poles (2 marks for direction north to south, field lines and strongest at the poles). A plotting compass placed near the magnet lines up with the field, and its needle points along the field line; moving it around and marking the direction at each point traces the field's shape and direction (1 mark). Markers reward the north-to-south direction, the field-line representation, and using a compass to find the direction.
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