Magnetism and Electromagnetism
Magnetism and electromagnetism is the GCSE physics topic covering how magnets and magnetic fields behave, and how a magnetic field is created by an electric current. A magnetic field is the region around a magnet, or a current-carrying wire, where a force acts on another magnet or magnetic material, represented by field lines running from north to south, and current flowing through a coil, a solenoid, produces its own magnetic field, the basis of an electromagnet.
Method
- Learn the basic rules of magnetic poles: like poles (N-N or S-S) repel, unlike poles (N-S) attract, and both the size of the force and its range depend on distance, getting weaker further from the magnet.
- Distinguish permanent magnets (made from a magnetically hard material such as steel; produce their own magnetic field at all times) from induced magnets (made from a magnetically soft material such as iron; only become a magnet when placed in an external magnetic field, and lose most of their magnetism quickly when removed). Remember that an induced magnet always experiences a force of attraction towards the magnet that induced it, never repulsion.
- Draw magnetic field diagrams correctly: field lines leave the north pole, curve round, and enter the south pole, with an arrow showing this direction; draw lines closer together near the poles (strongest field) and further apart away from the magnet (weaker field); between two opposite poles facing each other, draw evenly spaced parallel lines to show a uniform field.
- To find a field pattern experimentally, use a plotting compass: place it at a point near the magnet, mark the direction the needle points, move it along that direction to the next point, and repeat to build up a field line. A compass is itself a small bar magnet, and it always points along the field it is in, which is also why compasses point north on the surface of the Earth, since the Earth behaves as if it has a giant bar magnet inside it.
- For a wire carrying a current, remember the field is a series of concentric circles centred on the wire, in a plane at right angles to the wire; the field reverses direction if the current reverses, and gets stronger with a larger current or closer to the wire.
- For a solenoid (a coil of wire), remember the field pattern outside the coil is the same shape as a bar magnet's field, and the field inside the coil is strong and uniform, with field lines running parallel, close together and evenly spaced. Adding an iron core through the centre of the coil makes it into a stronger electromagnet, since the iron becomes an induced magnet as well.
- When comparing a permanent magnet to an electromagnet, remember an electromagnet's field can be switched on and off, reversed, and made stronger or weaker just by changing the current, which is why electromagnets rather than permanent magnets are used in devices such as cranes for lifting scrap steel.
Worked example
A student places a plotting compass at several points around a bar magnet and records the direction the compass needle points at each position, then joins the points to build up a field line. Describe and explain what the student should observe as they move the compass from close to the magnet's north pole to a point far away.
- Place the plotting compass close to the magnet's north pole first; the compass needle aligns with the field, pointing away from the north pole, in the direction field lines leave the magnet.
- Mark a dot at the tip of the needle, then move the compass so its tail sits on that dot, and read the new direction.
- Repeat this process, marking a new dot each time, until the line of dots reaches the south pole of the magnet; joining the dots gives one complete field line, with an arrow added pointing from north to south to show field direction.
- Repeat the whole process starting from different points around the magnet to build up the full field pattern.
- Observe that close to the poles, field lines plotted from nearby starting points are close together, since the magnetic force, and therefore the compass's deflection, is strongest there.
- Final answer: far from the magnet, the field lines plotted are much further apart and the compass needle deflects only weakly, showing that the magnetic field, and the force it can exert, becomes weaker with distance from the magnet.
Practice questions
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Q1Two bar magnets are placed with their north poles facing each other. State what happens to the magnets.Show answer
Answer: They repel each other (push apart), because like poles repel.
Q2State the difference between a permanent magnet and an induced magnet.Show answer
Answer: A permanent magnet produces its own magnetic field at all times; an induced magnet only becomes magnetic when placed in an existing magnetic field, and loses most of its magnetism when removed from that field.
Q3State the type of force that always acts between an induced magnet and the magnet that induced it.Show answer
Answer: A force of attraction (never repulsion).
Q4Name a typical material used to make a permanent magnet, and a typical material used to make an induced (temporary) magnet.Show answer
Answer: Permanent magnet: steel. Induced magnet: iron.
Q5A student wraps a coil of insulated wire around an iron nail and connects it to a battery. Name the device this creates, and state one way to increase its magnetic field strength.Show answer
Answer: An electromagnet. Increase the field strength by increasing the current through the coil, or by adding more turns to the coil.
Q6Describe the shape of the magnetic field produced around a long straight wire carrying a current.Show answer
Answer: A pattern of concentric circles, centred on the wire, in a plane at right angles to the wire.
Q7State what happens to the magnetic field around a straight current-carrying wire if the direction of the current is reversed.Show answer
Answer: The direction of the magnetic field reverses (the concentric field lines point the opposite way around the wire), although the shape and strength of the field pattern, for the same size of current, stay the same.
Q8Explain why a compass needle placed just outside one end of a current-carrying solenoid lines up parallel to the solenoid's axis.Show answer
Answer: Outside the coil, a solenoid's magnetic field has the same shape as a bar magnet's field, with field lines running from the north end, curving round, to the south end, so a compass placed there aligns with this external field just as it would near a bar magnet.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A bar magnet is placed on a bench and a plotting compass is used to investigate the magnetic field around it. (a) State what a plotting compass actually is. (b) State the direction of the magnetic field at a point just outside the magnet's north pole. (c) State how the spacing of the field lines close to a pole compares with the spacing far from the magnet, and what this shows about the field strength.
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A student sets up a circuit in which a straight vertical wire passes through a horizontal sheet of card. The student sprinkles iron filings onto the card and passes a current through the wire. (a) Describe the pattern the iron filings form on the card, and explain what this pattern shows about the magnetic field around the wire. (b) The student replaces the iron filings with a small plotting compass placed next to the wire, then reverses the direction of the current in the wire. State and explain what happens to the direction the compass needle points.
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A crane at a scrapyard uses an electromagnet to lift and move pieces of scrap steel from one pile to another, then release them. Explain why an electromagnet, rather than a permanent magnet, is used for this job, referring to the construction of an electromagnet and how its magnetic field can be controlled.
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Free printable worksheet
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This topic is chapter 7 of GCSE Physics Workbook, the whole course as one free printable PDF.
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