Magnetism & Electromagnetism - 7.2.2 Fleming’s Left Hand Rule (HT only) (GCSE Physics AQA)

Fleming’s Left Hand Rule (HT only)

The Motor Effect

1. A current-carrying wire creates a magnetic field. We can place this wire into another magnetic field. This causes the two magnetic fields to interact.
2. When they interact, the fields exert forces on each other. The wire exerts a force on the field and the field exerts a force on the wire. This is known as the motor effect.
3. As it exerts a force, the motor effect can cause the wire to move. The force experienced by the wire is strongest when it is placed at 90 degrees to the existing magnetic force.

Using Fleming’s Left Hand Rule

You can predict the direction of the force-motion effect from Fleming’s left-hand rule.

• The thumb represents the direction the force acts.
• The first finger represents the direction of the magnetic field.
• The second finger represents the direction of the convention current.

Factors Affecting Force on Conductor

There are three main factors that affect the size of a force on a conductor:

1. Current: The larger the current, the larger the force.
2. Length: The longer the length of the conductor, the larger the force.
3. Magnetic Flux Density: The higher the density, the more magnetic field lines and the larger the force on the conductor.

Linking Force and Flux Density

The force experienced by a wire in a magnetic field can be calculated using the equation below.

For AQA exams, you need to be able to remember and use the following equation.

Where:

• force, F, in newtons, N
• magnetic flux density, B, in tesla, T
• current, I, in amperes, A (amp is acceptable for ampere)
• length, l, in metres, m

Question: Calculate the force generated when the magnetic flux density is 5T, the current is 30 000 milliamps and the length of wire is 100cm.

1. Get the correct units.

If you don’t convert correctly, the answer may be out by a factor of 100+
Current should be in amps so 30 000 milliamps = 30 amps
Length should be in metres so 100 cm = 1m

2. Substitute in the numbers.

F = B I l
F = 5 x 30 x 1
F = 150 Newtons