LearnovaAll Physics notes
OCR Gateway PhysicsMagnetism and electromagnetism

The motor effect

Explain forces on current-carrying conductors.

Start here

The key idea

A current-carrying conductor in a magnetic field experiences a force.

Equation to know

force = magnetic flux density x current x length

Motor Effect
NSIforcecurrent + magnetic field = force

Use the labels to explain the scientific relationship shown.

Revision notes

The bit that matters

Short notes first. Learn the idea, then use the worked example and questions to check it properly.

1

The motor effect

When a current-carrying conductor is placed in a magnetic field, the field of the wire interacts with the external field and the wire experiences a force.This is called the motor effect.The force is greatest when the wire is at 90 degrees to the magnetic field and is zero when the wire is parallel to the field.Reversing either the current or the field direction reverses the direction of the force.

2

Fleming's left-hand rule

Fleming's left-hand rule gives the direction of the force on the conductor.The thumb shows the direction of the Force (motion), the First finger the magnetic Field (north to south), and the seCond finger the Current (conventional, positive to negative).The three must be held at right angles to each other. This rule lets you predict which way the wire will move.

3

Calculating the force

For a conductor at right angles to a magnetic field, force = magnetic flux density x current x length (F = B x I x L).B is the magnetic flux density measured in tesla (T), I is the current in amps and L is the length of the conductor in the field in metres.Increasing the current, the field strength or the length all increase the force.

4

The electric motor

An electric motor uses the motor effect.A current flows through a coil in a magnetic field; the forces on the two sides of the coil act in opposite directions, creating a turning effect (a couple) that rotates the coil.A split-ring commutator reverses the current direction every half turn so the coil keeps rotating in the same direction.Loudspeakers also use the motor effect to convert electrical signals into sound.

Key terms

Definitions to learn

Motor effect

The force experienced by a current-carrying conductor in a magnetic field.

Magnetic flux density

A measure of magnetic field strength, symbol B, measured in tesla (T).

Fleming's left-hand rule

A rule giving the directions of force, field and current at right angles.

Split-ring commutator

A device that reverses the current every half turn to keep a motor spinning one way.

Couple

A pair of equal and opposite forces that produces a turning effect.

Worked example

A wire of length 0.20 m carries 4.0 A at right angles to a 0.50 T field. Calculate the force.

1

Use F = BIl.

2

Substitute the values.

Final answer

0.40 N

Exam habit

Write F = BIl, identify B (T), I (A) and l (m), then substitute.Use Fleming's left-hand rule: thumb = force, index = field, middle finger = current.State the equation is only valid when the conductor is perpendicular to the field.

Watch out

The equation applies when the conductor is perpendicular to the field.

Examiner tips

How to score full marks

  • 1For Fleming's left-hand rule, current is conventional current (positive to negative) and field runs N to S.
  • 2The force is maximum when the wire is at 90 degrees to the field and zero when parallel to it.
  • 3When using F = B x I x L, ensure B is in tesla, I in amps and L in metres.
Practice questions

Try these yourself

Start with the core skill, then open the answer only after you have attempted the full question.

1State two ways to increase the force on a current-carrying wire.
Mark scheme
  1. 1.Use the quantities in F = BIl.
Increase the current, field strength or length in the field.
2What happens to the force direction if the current is reversed?
Mark scheme
  1. 1.Use Fleming's left-hand rule.
The force reverses direction.
3Explain how the motor effect makes a coil rotate.
Mark scheme
  1. 1.Consider opposite current directions on opposite sides.
Opposite sides of the coil experience forces in opposite directions, producing a turning effect.
4State the directions represented by the thumb, first finger and second finger in Fleming's left-hand rule.[2 marks]
Mark scheme
  1. 1.Thumb = force.
  2. 2.First finger = field.
  3. 3.Second finger = current.
Thumb = force/motion (1); first finger = magnetic field (1); second finger = current (1) — award 2 for all three correct, 1 for two correct.
5A wire of length 0.2 m carries a current of 3 A in a magnetic field of flux density 0.5 T. Calculate the force on the wire.[2 marks]
Mark scheme
  1. 1.Use F = B x I x L.
  2. 2.Substitute.
F = B x I x L = 0.5 × 3 × 0.2 (1) = 0.3 N (1).
6State two ways to increase the size of the force on a current-carrying wire in a magnetic field.[2 marks]
Mark scheme
  1. 1.Increase current.
  2. 2.Increase field/length.
Increase the current (1); increase the magnetic flux density or the length of wire in the field (1).
7A 0.15 m wire experiences a force of 0.9 N when carrying a current of 4 A. Calculate the magnetic flux density.[3 marks]
Mark scheme
  1. 1.Rearrange F = B x I x L to B = F / (I x L).
  2. 2.Substitute.
  3. 3.Evaluate.
B = F / (I x L) = 0.9 / (4 × 0.15) (1) = 0.9 / 0.6 (1) = 1.5 T (1).
8Explain how a simple direct current electric motor produces continuous rotation in one direction, referring to the forces on the coil and the role of the commutator.[4 marks]
Mark scheme
  1. 1.Current in coil in field experiences forces.
  2. 2.Opposite sides have opposite forces.
  3. 3.Creates turning effect.
  4. 4.Commutator reverses current each half turn.
Current flows through the coil sitting in a magnetic field, so each side of the coil experiences a force due to the motor effect (1). The forces on opposite sides act in opposite directions, creating a turning effect (couple) that rotates the coil (1). As the coil turns, the split-ring commutator reverses the direction of the current through the coil every half turn (1), so the forces keep acting in the same rotational sense and the coil continues to rotate in one direction (1).
9A horizontal wire of length 0.50 m carrying a current of 2.0 A is placed at right angles to a horizontal magnetic field of flux density 0.30 T. Calculate the force on the wire and state its direction if the field points north and the current flows east.[2 marks]
Mark scheme
  1. 1.F = B x I x L = 0.30 × 2.0 × 0.50.
  2. 2.Use Fleming's left-hand rule: field north (first finger N), current east (second finger E).
  3. 3.Thumb gives direction of force: upward.
F = 0.30 × 2.0 × 0.50 = 0.30 N (1); using Fleming's left-hand rule with field pointing north and current flowing east, the force on the wire acts vertically upward (1)
10Explain why a loudspeaker uses the motor effect to produce sound. Describe the role of the coil, permanent magnet and cone.[3 marks]
Mark scheme
  1. 1.Alternating current in coil creates varying force.
  2. 2.Force direction reverses with current direction.
  3. 3.Coil moves back and forth.
  4. 4.Cone attached to coil vibrates, creating sound waves.
The loudspeaker has a coil of wire (voice coil) attached to a paper cone, sitting in the field of a permanent magnet (1); when an alternating electrical signal (audio current) flows through the coil, the current direction changes many times per second; the motor effect causes the coil to experience a force that reverses with each current reversal (1); the coil therefore moves back and forth rapidly, carrying the cone with it; the oscillating cone pushes and pulls the air in front of it, creating compressions and rarefactions that travel as sound waves (1)
11A rectangular coil with 50 turns has sides of length 0.04 m in the field and 0.06 m perpendicular to the field. It carries a current of 2.0 A in a magnetic field of flux density 0.25 T. Calculate the force on each side that is in the field and the total torque (turning moment) on the coil.[2 marks]
Mark scheme
  1. 1.Force per side = B x I x L x N (for N turns).
  2. 2.F = 0.25 × 2.0 × 0.04 × 50.
  3. 3.Torque = F x width of coil (distance between the two sides) = F x 0.06.
  4. 4.But torque from two sides: total torque = 2F x half the width = F x 0.06 for the couple.
Force on one side = B x I x L x N = 0.25 × 2.0 × 0.04 × 50 = 1.0 N (1); the two forces on opposite sides form a couple; torque = F x perpendicular distance = 1.0 × 0.06 = 0.06 Nm (1)
12Explain why the force on the coil of a motor is zero when the coil is in the plane perpendicular to the magnetic field (i.e. the coil plane contains the field), and explain how the commutator ensures the coil passes through this position and keeps rotating rather than stopping.[3 marks]
Mark scheme
  1. 1.When coil is perpendicular to field, current is parallel to field on those sides.
  2. 2.F = B x I x L x sin(0) = 0 for sides parallel to field.
  3. 3.Commutator at this point reverses current.
  4. 4.Reversal means force returns and keeps rotation going.
When the plane of the coil is perpendicular to the field, the two sides that were experiencing forces are now moving parallel to the field lines; the current in them is parallel to the field, so F = BIL sin(0 degrees) = 0 and there is no force to continue rotating the coil (1); however, the coil has rotational inertia (momentum) from its previous motion and carries through this position; at this exact point the split-ring commutator reverses the connections so the current direction in the coil reverses (1); this ensures that when the coil continues past the dead spot, the forces on its sides act in the same rotational direction as before, so the coil keeps spinning continuously in one direction rather than oscillating back and forth (1)
Official exam-board sources
Specification Past papers
Browse all Physics topics