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Topic 4 · 4.5.3–4.5.5

Magnetic effect of a current and motors

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Describe the field pattern and direction around a straight wire and a solenoid, and an experiment to show it; describe how field strength varies EXTENDED.
  • Describe how the magnetic effect of a current is used in relays and loudspeakers.
  • Describe an experiment showing the force on a current-carrying conductor, and the effect of reversing the current or the field; use Fleming’s left-hand rule EXTENDED.
  • Determine the direction of the force on beams of charged particles in a magnetic field EXTENDED.
  • Know what increases the turning effect on a coil; describe the d.c. motor and its split-ring commutator EXTENDED.

📚The physics

Magnetic field of a current

Two straight wires seen end on. Current into the page, shown as a cross, has circular field lines going clockwise around it; current out of the page, shown as a dot, has anticlockwise field lines. The circles are further apart further from the wire.
Right-hand grip rule: thumb along the current, fingers curl the way the field goes.

Show the pattern by passing the wire vertically through a horizontal card, sprinkling iron filings and tapping the card; use plotting compasses to find the direction. EXTENDED The field is strongest close to the wire and weakens with distance; a larger current makes it stronger, and reversing the current reverses the field.

A cross-section of a solenoid: the wires along the top carry current out of the page and those along the bottom into the page. Inside, the field lines are straight, parallel and evenly spaced, pointing left to right; outside, they loop round from the right-hand end, the N pole, back to the left-hand end, the S pole, like the field of a bar magnet.
A solenoid behaves like a bar magnet; a soft-iron core makes it a stronger electromagnet.

Uses. A relay uses an electromagnet to close a switch in another circuit (topic 4c). A loudspeaker passes an alternating current through a coil in the field of a permanent magnet: the force on the coil keeps reversing, so the coil and the paper cone vibrate and produce sound.

A loudspeaker in cross-section: a coil attached to a paper cone sits in the gap of a permanent magnet. An alternating current from the amplifier flows in the coil, so the force on it keeps reversing and the coil and cone vibrate, making sound.
The coil moves in and out at the frequency of the current.

Force on a current-carrying conductor

A wire carrying a current in a magnetic field feels a force (the motor effect). Show it by placing a loose copper rod on two rails between the poles of a magnet and switching on the current: the rod rolls along the rails. Reversing the current or reversing the field reverses the force; reversing both leaves it unchanged.

Left: a wire carrying current into the page between the N pole on the left and the S pole on the right; the field runs from N to S and the force on the wire is downwards. Right: Fleming's left-hand rule: thumb for motion or force, first finger for field, second finger for current, all at right angles.
EXTENDED Fleming’s left-hand rule gives the direction of the force.

EXTENDED A beam of charged particles is a current too. For electrons, point your second finger opposite to their motion (conventional current flows the other way).

A beam of electrons travelling to the right enters a region of magnetic field directed into the page and curves downwards.
EXTENDED The force is always at right angles to the beam, so it bends into a curve.

The d.c. motor

A current-carrying coil in a magnetic field experiences a turning effect, which is increased by more turns on the coil, a larger current, or a stronger magnetic field.

A rectangular coil lies horizontally between the N and S poles of a magnet, its ends connected to a split-ring commutator touching two carbon brushes joined to a cell. The force on one side of the coil is upwards and on the other side downwards, so the coil turns.
EXTENDED The split-ring commutator reverses the current in the coil every half turn.

EXTENDED The currents in the two sides of the coil are in opposite directions, so the forces on them are opposite: one side is pushed up, the other down, giving a turning effect. As the coil passes the vertical, the gaps in the split ring swap the connections to the brushes, reversing the current in the coil, so the forces keep turning it the same way.

✏️Worked example

A copper rod rests on two horizontal rails in a vertical magnetic field. When the current is switched on, the rod rolls to the right. (a) State two ways to make it roll to the left. [2] (b) State what happens if both the current and the field are reversed. [1] (c) State two ways to make a d.c. motor turn faster. [2] (d) EXTENDED Explain the purpose of the split-ring commutator. [2]

(a) Reverse the current (swap the supply connections); reverse the magnetic field (turn the magnet over).

(b) It still rolls to the right: two reversals cancel.

(c) Any two: increase the current; use a stronger magnet; use more turns on the coil.

(d) It reverses the current in the coil every half turn, so the force on each side always acts to turn the coil in the same direction.

Check it. With Fleming’s left-hand rule, swapping one of field or current turns the thumb round; swapping both leaves it pointing the same way.
“The commutator makes the coil spin.” The forces on the wires make it turn; the commutator only keeps the turning in one direction.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) A long straight wire carries a current out of the page. Which describes the magnetic field around it? A: straight lines towards the wire. B: straight lines away from the wire. C: circles, clockwise. D: circles, anticlockwise.
D. Right-hand grip: thumb out of the page, fingers curl anticlockwise.
2. (Theory.) Describe an experiment to show the magnetic field pattern around a straight wire carrying a current. [3]
Pass the wire vertically through a hole in a horizontal card. Switch on a large current, sprinkle iron filings on the card and tap it: the filings form concentric circles. Place plotting compasses on the card to show the direction of the field.
3. (Multiple choice.) EXTENDED A beam of electrons moves from left to right across the page through a magnetic field directed into the page. In which direction is it deflected? A: up the page. B: down the page. C: into the page. D: out of the page.
B. Conventional current is right to left; field into the page; the left-hand rule gives a force down the page (as in the diagram above).
4. (Theory.) EXTENDED State how the strength of the magnetic field around a straight wire changes (a) further from the wire, (b) when the current is doubled. [2]
(a) It decreases. (b) It increases (becomes stronger).
5. (Theory.) Explain why a solenoid with a soft-iron core is used in a relay rather than a permanent magnet. [2]
The electromagnet is magnetised only while current flows, so the relay can be switched on and off; soft iron loses its magnetism as soon as the current stops.
6. (Theory.) EXTENDED In the d.c. motor diagram above, explain why the two sides of the coil experience forces in opposite directions. [2]
The current flows in opposite directions in the two sides of the coil (along one side and back along the other), and the field is the same, so the forces are opposite.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • PhET “Magnets and Electromagnets” — solenoid fields
  • Walter Fendt — electric motor (d.c.) simulation with commutator