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Magnetic Fields

Magnetic Fields — American System Physics Revision Notes & Topical Questions

5 min read 6 parts Lesson 1 of 5 1 quiz

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Magnetic Fields: Magnets, Currents and Forces — AMERICAN SYSTEM PHYSICS notes: Describe magnetic fields, electromagnets and the force on currents and charges, including the d.c. motor and relay.Like poles repel and unlike poles attract.

  • hard magnetic material
  • Magnetic flux density
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4

Section 1 of 6Magnets and magnetic fields

Like poles repel and unlike poles attract.

Magnetic field
A magnetic field is a region where a magnetic pole, magnetic material or moving charge experiences a force.
To plot a field, place a bar magnet on paper and draw round it, put a small plotting compass near the N pole and mark the needle tip, move the compass so its tail is on the dot, mark the new tip, repeat until the S pole, join the dots with a smooth line and add an arrow from N to S.
Repeat from other starting points; closer lines mean a stronger field.
Compasses align with Earth's field and auroras result from charged particles in Earth's field.

A magnet attracts an unmagnetised iron nail as follows: the N pole is brought near, the field lines up domains in the nail, the nearer end becomes an S pole (induced magnetism), unlike poles attract and the nail is pulled to the magnet.

Removing the magnet, soft iron loses most of its magnetism.
Hard magnetic material
A hard magnetic material (steel) is harder to magnetise but keeps its magnetism, so is used for permanent magnets.
Heating or hammering can destroy magnetism.

Section 2 of 6Magnetic effect of a current and electromagnets

A current-carrying wire has circular field lines centred on the wire; field strength decreases with distance and reversing the current reverses the field.

Field strength increases with current and with the number of turns of a coil.
Magnetic flux density B is measured in tesla (T).
Solenoid
A solenoid is a long coil; with a current it produces a field like a bar magnet, uniform and strong inside (parallel, equally spaced lines).

To investigate an electromagnet, wind insulated wire (for example 20 turns) round a soft-iron nail and connect it in series with a switch, ammeter, variable resistor and d.c. supply.

Magnetic effect of a current and electromagnets
Set a current such as 0.5 A and count the paper clips lifted.
Increase the current in steps with the turns constant, then repeat with more turns at fixed current.
Switch off between readings to prevent overheating and plot clips against current.

A relay is an electromagnetic switch in which a small current in a coil attracts an iron armature, closing contacts in a second circuit that may carry a large current. Steps: the input switch closes and a small current flows in the coil; the coil and soft-iron core become an electromagnet; it attracts the armature, which pivots; the armature pushes the output contacts together, completing the high-current circuit; when the input stops the core loses its magnetism and a spring opens the contacts.

Section 3 of 6Forces on currents, charges and the d.c. motor

Magnetic flux density B is the force per unit current per unit length on a conductor at right angles to the field: F = BIL (tesla).

1
A moving charge feels F = qvB.
2
Using Fleming's left-hand rule: point the first finger along the field (N to S), the second finger along the conventional current (+ to −), keep the thumb and fingers mutually at right angles,
  • and the thumb gives the force (motion).
3
Reversing either field or current reverses the force.
4
For electrons, conventional current is opposite to the electron velocity.
5
A beam of charged particles is deflected by the field and follows a circular path if moving at right angles to a uniform field.
6
Loudspeakers use the force on a current-carrying coil and mass spectrometers use magnetic deflection.

In a d.c. motor current enters the coil through a brush and the commutator.

The two sides carry current in opposite directions across the field,
  • so by Fleming's rule one side is pushed up and the other down,
  • giving a turning effect.
Brushes
When the coil is vertical the brushes cross the gap in the split ring; the commutator reverses the current every half-turn so the forces keep turning the coil the same way and momentum carries it past the vertical. Brushes are carbon or metal contacts pressing on the commutator.
The turning effect is greatest when the coil plane is parallel to the field and zero when perpendicular.

Section 4 of 6Worked examples

  1. Describe how to show the force on a current-carrying conductor and use Fleming's left-hand rule to find its direction. [4]4 marks

  2. Describe how a magnet attracts an unmagnetised iron nail. [4]4 marks

  3. Describe how a relay switches a large current. [4]4 marks

  4. Explain why a d.c. motor needs a split-ring commutator. [3]3 marks

Section 5 of 6Key terms

TermMeaning
Magnetic fieldA region where a magnetic pole, magnetic material or moving charge experiences a force.
Hard magnetic materialA material such as steel that is hard to magnetise but keeps its magnetism.
SolenoidA long coil whose field is like a bar magnet's, uniform and strong inside.
RelayAn electromagnetic switch where a small coil current closes contacts in a second circuit.
Magnetic flux densityThe force per unit current per unit length on a conductor at right angles to the field; F = BIL, in tesla.
BrushesCarbon or metal contacts pressing on the commutator.
Beam of charged particlesA stream of charges deflected by a magnetic field, circular if moving at right angles to a uniform field.

Section 6 of 6Exam technique

1
Using electron direction in Fleming's rule — use conventional current.
2
Saying field strength increases with distance from a wire — it decreases.
3
Thinking the turning effect is greatest when the coil is perpendicular to the field — it is zero there.
4
Using steel for a temporary electromagnet core — soft iron loses its magnetism.

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