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7.2.2 Fleming's left-hand rule (HT only)

7.2.2 The motor effect and Fleming's left-hand rule (HT)

The motor effect

Definition

The motor effect

The motor effect is the force produced when a conductor carrying a current is placed in a magnetic field.

  1. A current-carrying conductor produces its own magnetic field, which interacts with the field of a magnet.
  2. Because the two fields interact, the magnet and the conductor exert forces on each other.
  3. The force on the conductor can make it move, which is the effect used in electric motors.
Key Idea
  • The motor effect needs a magnetic field.
  • It also needs a current flowing through a conductor in that field.

Fleming's left-hand rule

  1. Fleming's left-hand rule shows the relative directions of the magnetic field, the conventional current and the force on the conductor.
  2. Hold the thumb, first finger and second finger of your left hand so they are mutually perpendicular.
  3. First finger points along the magnetic field, from north to south.
  4. Second finger points along the conventional current.
  5. Thumb points along the force on the conductor.
  6. A useful memory aid is First finger = Field, seCond finger = Current, thuMb = Motion (force).
  7. Reversing either the current or the field reverses the force, but reversing both leaves the force unchanged.

A diagram of a left hand showing Fleming's left-hand rule. The thumb, first finger, and second finger are held at right angles to each other. The thumb represents motion or force (F), the first finger represents the magnetic field (B), and the second finger represents the current (I).

Common Mistake
  • The magnetic field is directed from north to south.
  • Use conventional current, not the direction electrons move.
  • The thumb gives the force on the conductor; the force on the magnet is in the opposite direction.
  • Use your left hand for the motor effect.

Factors affecting the size of the force

Definition

Magnetic flux density

Magnetic flux density is a measure of the strength of a magnetic field, measured in tesla, T\text{T}T.

  1. The force is increased by increasing the magnetic flux density.
  2. The force is increased by increasing the current in the conductor.
  3. The force is increased by increasing the length of conductor inside the field.
  4. The force is greatest when the current is at right angles to the field, and there is no force when the current is parallel to the field.

Calculating the force

  1. For a conductor at right angles to the field, the force is given by F=BIlF = BIlF=BIl.
  2. FFF is the force in newtons, N\text{N}N.
  3. BBB is the magnetic flux density in tesla, T\text{T}T.
  4. III is the current in amperes, A\text{A}A.
  5. lll is the length of conductor inside the field in metres, m\text{m}m.
  6. The length lll is only the length of conductor actually inside the field, and the equation applies when the conductor is at right angles to the field.
  7. To find another quantity, rearrange the equation to B=FIlB = \dfrac{F}{Il}B=IlF​, I=FBlI = \dfrac{F}{Bl}I=BlF​ or l=FBIl = \dfrac{F}{BI}l=BIF​.
Example

Question: A wire carries a current of 4.0 A4.0\ \text{A}4.0 A at right angles to a magnetic field of flux density 0.35 T0.35\ \text{T}0.35 T, with 0.080 m0.080\ \text{m}0.080 m of the wire inside the field. Calculate the force on the wire.

  1. Write the equation: F=BIlF = BIlF=BIl.
  2. Substitute the values: F=0.35×4.0×0.080F = 0.35 \times 4.0 \times 0.080F=0.35×4.0×0.080.
  3. Calculate: F=0.112 NF = 0.112\ \text{N}F=0.112 N, which is 0.11 N0.11\ \text{N}0.11 N to two significant figures.
Exam technique
  • For a direction question, mark the field from north to south and the conventional current before applying Fleming's left-hand rule.
  • For a calculation, check the conductor is at right angles to the field, convert the length to metres, then use F=BIlF = BIlF=BIl and give the answer in newtons.
Self review
  • What is the motor effect?
  • In Fleming's left-hand rule, what do the first finger, second finger and thumb represent?
  • Name three factors that increase the size of the force on a conductor.
  • When is the force on a current-carrying conductor greatest?
  • State the equation linking force, magnetic flux density, current and length.
  • What are the units of magnetic flux density?
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The motor effect is the force produced when a conductor carrying a current is placed in a magnetic field. It occurs because the magnetic field around the current-carrying conductor interacts with the magnetic field of a magnet.

The magnet and the conductor exert forces on each other. The force on the conductor can make it move, which is the effect used in electric motors.

For the motor effect to occur, there must be both a magnetic field and a current flowing through a conductor within that field.

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7.2.2 Fleming's left-hand rule (HT only) Revision Guide

  1. GCSE
  2. /Physics
  3. /7.2.2 Fleming's left-hand rule (HT only)

Revision notes for AQA GCSE Physics 7.2.2 Fleming's left-hand rule (HT only). Open the guide for explanations and worked examples. Written against the AQA GCSE Physics (8463) specification, so the content matches what's examinable rather than general Physics background.

Revision guides