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4.5.3Electricity and Magnetism

Magnetic Effect OF A Current — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 11 parts Lesson 55 of 65 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

4.5.3 Magnetic Effect of a Current — IGCSE PHYSICS notes: Describe the magnetic field patterns around a straight wire, circular coil and solenoid and explain electromagnets, relays and loudspeakers.Source — a current in a conductor produces a magnetic field around it.Pattern — th…

  • Right-hand grip rule
  • Inside a solenoid
  • Grip rule for a coil
  • Reversing the current
  • Construction
  • Advantages over a permanent magnet

Part 1 of 9Field around a straight wire

Source
  • Source a current in a conductor produces a magnetic field around it.
Pattern
  • Pattern the field lines are concentric circles centred on the wire, in planes perpendicular to it, with no poles.
Direction
  • Direction the lines are clockwise or anticlockwise depending on the current, and reversing the current reverses the field.
Right-hand grip rule
  • Right-hand grip rule point the right thumb along the conventional current and the curled fingers give the field direction.

Part 2 of 9Field around a circular coil and a solenoid

Circular coil
  • Circular coil the field lines emerge through one face (north pole) and enter through the other (south pole).
Solenoid
  • Solenoid outside it the field is like that of a bar magnet, with lines leaving the north pole and returning to the south pole.
Inside a solenoid
  • Inside a solenoid the lines are parallel and evenly spaced, so the field is strong and uniform.
Grip rule for a coil
  • Grip rule for a coil curl the right-hand fingers in the direction of the current and the thumb points to the north pole.

Part 3 of 9Using the grip rule on a solenoid

1
Mark the direction of conventional current (+ to −) in the turns of the coil.
2
Curl the fingers of the right hand round the coil in the direction of the current.
3
The thumb points along the axis towards the north pole.
4
The opposite end is the south pole.

Part 4 of 9Strength of the field (Extended)

Straight wire
  • Straight wire the field is stronger for a larger current and weaker with distance, shown by lines closer together or further apart.
Solenoid
  • Solenoid the field is increased by a larger current, more turns and a soft-iron core.
Reversing the current
  • Reversing the current reverses the direction of the field and the poles.

Part 5 of 9Electromagnets

1Construction
  • Construction a solenoid wound on a soft-iron core, which becomes an induced magnet when current flows.
2Switching
  • Switching the magnet can be switched on and off, and soft iron is used because it magnetises and demagnetises easily.
3Advantages over a permanent magnet
  • Advantages over a permanent magnet its strength can be changed and its poles can be reversed.
4Uses
  • Uses scrapyard cranes, MRI scanners, bells, relays and speakers.

Part 6 of 9Relay

Construction
  • Construction an electrical circuit with an electromagnet and a second circuit with a switch near the electromagnet.
Current in circuit 1
  • Current in circuit 1 the coil magnetises the core, which attracts the switch and closes the contacts in circuit 2.
No current
  • No current the magnetic force stops and the current in circuit 2 stops.
Purpose
  • Purpose a small current safely switches a larger current.

Part 7 of 9Electric bell

1
Pressing the button K passes a current through the electromagnet E.
2
The magnetic field attracts the iron armature A, so the hammer strikes the bell B.
3
The movement of the armature breaks the circuit at T.
4
The current stops, the field collapses and the armature returns.
5
The circuit is re-established and the process repeats.

Part 8 of 9Loudspeaker

Loudspeaker
Structure
  • Structure a coil of wire wrapped around one pole of a permanent magnet.
Operation
  • Operation an alternating current in the coil gives a changing field that interacts with the magnet's field.
Force
  • Force it changes direction with the current, so the coil oscillates and moves the cone.
Result
  • Result the oscillating cone makes the air oscillate, producing sound waves.
Principle
  • Principle a loudspeaker works by the motor effect.

Part 9 of 9More from the specification

1
Current-carrying wire in a field
  • Current-carrying wire in a field a conductor experiences a force when its current is not parallel to the magnetic field
2
Electromagnetic induction
  • Electromagnetic induction moving a wire or magnet so that field lines are cut induces a voltage that a voltmeter can detect
3
Transformer
  • Transformer primary and secondary coils on an iron core change alternating voltage, with Vp/Vs = Np/Ns
4
Power equation
  • Power equation for an ideal transformer Vp x Ip = Vs x Is, so a stepped up voltage means a smaller current
5
Current-carrying wire in a field
  • Current-carrying wire in a field a conductor experiences a force when its current is not parallel to the magnetic field
6
Electromagnetic induction
  • Electromagnetic induction moving a wire or magnet so that field lines are cut induces a voltage that a voltmeter can detect
7
Transformer
  • Transformer primary and secondary coils on an iron core change alternating voltage, with Vp/Vs = Np/Ns
8
Power equation
  • Power equation for an ideal transformer Vp x Ip = Vs x Is, so a stepped up voltage means a smaller current
9
Potential difference
  • Work done by a unit charge passing through a component.
10
Step-up transformer
  • Transformer with more turns on the secondary, raising the voltage.
11
Step-down transformer
  • Transformer with fewer turns on the secondary, lowering the voltage.
12
Electromagnetic induction
  • Production of an e.m.f. by relative movement of a conductor and a magnetic field.
13
Transformer
  • Device that changes the size of an alternating voltage.
14
Primary
  • Input coil.
15
Voltmeter
  • Instrument that measures p.d., connected in parallel.
16
Generator
  • Device that turns kinetic energy into electrical energy.
17
Brushes
  • Carbon contacts pressing on the slip rings.
18
IpVp
IsVs
What the symbols mean: V = voltage (V) · N = number of turns · I = current (A) · p = primary coil · s = secondary coil

Key terms

TermMeaning
SolenoidLong coil of wire that acts like a bar magnet when carrying current.
ElectromagnetSolenoid with a soft-iron core.
Right-hand grip ruleRule giving the field direction around a current-carrying wire or coil.
RelaySwitch operated by an electromagnet.
LoudspeakerDevice that converts an electrical signal into sound by the motor effect.

Avoid these mistakes

1
Placing a compass inside a solenoid and expecting a circular pattern around it.
2
Using the left hand for the grip rule — it is the right hand.
3
Using steel instead of soft iron for an electromagnet core — steel keeps its magnetism.
4
Forgetting that reversing the current reverses the poles of an electromagnet.

Last updated:

Transformer Golden Equations Physics Standard

Transformer Turns Ratio & Efficiency

Ratio: Vp / Vs = Np / Ns = Is / Ip

  • Step-up reduces grid thermal loss (P = I2R).
  • Laminated core prevents eddy currents.
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