Magnetism & electromagnetism
Permanent and induced magnets, magnetic fields, electromagnets, the motor effect and the force on a current-carrying conductor.
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Magnetism & electromagnetism, explained point by point
Everything the GCSE specification expects you to be able to do, and how to actually do it - the same lesson a signed-in student studies from.
Compare permanent and induced magnets
A permanent magnet always produces its own magnetic field, with a north and south pole. An induced magnet only becomes magnetic when placed in a magnetic field, and loses most of its magnetism when removed. Like poles repel and unlike poles attract, and magnetic materials such as iron are attracted to both poles.
Describe magnetic fields and how to plot them
A magnetic field is the region where a magnet affects magnetic materials, drawn as field lines running from north to south. The lines are closest where the field is strongest - near the poles. You can plot a field with a small plotting compass, which always lines up along the field, or reveal it with iron filings.
Describe electromagnets and the solenoid
A current through a wire creates a magnetic field around it. Coiling the wire into a solenoid concentrates the field, giving a strong, uniform field inside - much like a bar magnet. Adding an iron core makes an electromagnet, whose strength can be changed with the current and switched on and off, unlike a permanent magnet.
Explain the motor effect and calculate its force
When a current-carrying wire sits in a magnetic field, the two fields interact and the wire experiences a force - the motor effect. The force is given by F = BIL, where B is the magnetic flux density, I the current and L the length of wire in the field. The force is largest when the wire is at right angles to the field.
Describe how an electric motor works
A motor uses the motor effect: a current-carrying coil in a magnetic field feels forces on its opposite sides in opposite directions, which turn the coil. A split-ring commutator reverses the current every half turn so the coil keeps spinning the same way. This turns electrical energy into movement in everything from fans to trains.
Describe uses of the motor effect and electromagnetism
Electromagnetism runs many everyday devices. Electromagnets lift scrap metal and work the switches inside relays; loudspeakers use the motor effect to turn an electrical signal into sound. Because an electromagnet can be switched and its strength varied, it is far more flexible than a permanent magnet for these jobs.
Magnetism & electromagnetism key terms
The words the specification and the mark schemes use, each defined the way an examiner wants it.
- Magnetic field
- The region around a magnet where a force acts on another magnet or on a magnetic material (iron, steel, cobalt, nickel).
- Permanent magnet
- A magnet that produces its own magnetic field all the time.
- Induced magnet
- A material that becomes a magnet while it is inside a magnetic field, and loses its magnetism quickly when removed.
- Solenoid
- A coil of current-carrying wire whose magnetic field has the same shape as a bar magnet's. The field inside is strong and uniform.
- Electromagnet
- A solenoid with an iron core, which makes the field much stronger. Its magnetism can be switched on and off with the current.
- Motor effect
- The force experienced by a current-carrying wire placed in a magnetic field, which is what makes electric motors turn.
Practice
Try a Magnetism & electromagnetism question
A GCSE-style original question from this topic. Have a go before you open the working - deciding on an answer first is what makes the working stick.
A 0.6 m length of wire carrying a current of 3 A sits at right angles to a magnetic field. The force on the wire is 0.9 N. Use F = B × I × L to work out the magnetic flux density, in tesla (T).
- 1.62
- 0.5
- 0.3
- 2
Show the answer and the working
Answer: 0.5
Rearranging F = B × I × L gives B = F ÷ (I × L) = 0.9 ÷ (3 × 0.6) = 0.9 ÷ 1.8 = 0.5 T.
- Rearrange F = B × I × L for flux density: B = F ÷ (I × L).
- Substitute F = 0.9 N, I = 3 A and L = 0.6 m.
- The denominator is 3 × 0.6 = 1.8.
- B = 0.9 ÷ 1.8 = 0.5, so the flux density is 0.5 T.