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1.9.2Electricity, Energy and Waves

Magnetic Force on a Current Carrying Conductor

5 min read 5 parts Lesson 48 of 54 Unit quiz

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Section 1 of 5The Motor Effect

1.9.2 Magnetic Force on a Current Carrying Conductor

A wire carrying a current has its own magnetic field, and this interacts with an external magnetic field.

Two fields interact
  • The field around the wire and the field of the magnets combine, so the wire experiences a force.
Where the force acts
  • The force pushes the wire at right angles to both the current and the magnetic field.
No current, no force
  • If the current is switched off there is no field around the wire, so the wire feels nothing.
Parallel wire
  • A wire lying parallel to the field lines experiences no force, but one at 90° to the field feels the maximum force.

Section 2 of 5Fleming's Left-Hand Rule

The rule gives the direction of the force when you know the field and the current.

1
Hold your left hand with the thumb, first finger and second finger all at right angles to one another.
2
Point the first finger in the direction of the magnetic field, from the north pole to the south pole.
3
Point the second finger in the direction of the conventional current, from the positive to the negative terminal.
4
The thumb now points in the direction of the force, which is the direction the wire will move.
5
Remember it as: thumb for thrust (motion), first finger for field, second finger for current.

Section 3 of 5Changing the Force

Changing the Force
Larger current
  • A bigger current produces a stronger field around the wire, so the force on the wire increases.
Stronger magnets
  • Using stronger magnets increases the field the wire sits in, which also increases the force.
Reversing current
  • Reversing the current reverses the direction of the force, so the wire moves the opposite way.
Reversing field
  • Swapping the poles of the magnets reverses the field and so reverses the direction of the force.

Section 4 of 5Using the Effect

Electric motors
  • A current-carrying coil in a field is pushed round by the force, which makes it spin.
Loudspeakers
  • A varying current in a coil attached to a cone moves it back and forth in a magnetic field and produces sound.
Experiments
  • A swinging wire between magnets, or a wire on a balance, can be used to show how the force changes.
Safety
  • Large currents heat up wires quickly, so apparatus must not be left switched on for a long time.

Section 5 of 5Exam Checklist & Tips

1.9.2 – Key Points for the Exam

  • Motor effect The force on a current-carrying conductor placed in a magnetic field.
  • Fleming's left-hand rule A rule linking the directions of force, field and current in the motor effect.
  • Conventional current The direction of current flow from the positive to the negative terminal of a cell.
  • Magnetic field The region around a magnet or current where a magnetic force is felt.

Common Mistakes

  • Using the right hand instead of the left hand for the motor effect.
  • Using the direction of electron flow instead of conventional current when applying the rule.
  • Thinking a wire parallel to the magnetic field feels the largest force; the force is zero in that case.
  • Forgetting that reversing both the current and the field leaves the direction of the force unchanged.
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.