7.5.9

# Magnetic Flux Density

Test yourself

## Magnetic Force on a Wire Experiment

We can investigate how the force on a wire varies with flux density, current and length of wire in the magnetic field, by using a top pan balance.

### Apparatus

• Set up the apparatus as shown in the diagram.
• For this investigation, aim to make the angle of the wire with the field as close as possible to 90°.
• Caution: the current should not exceed 4A (as the variable resistor is likely to smoke).

### Variables to investigate

• Using the arrangement on the previous slide, we can investigate the magnitude of the force as a function of:
• Flux density of the magnetic field
• Current in the wire
• Length of wire in the field
• Angle between wire and field lines

### Formula

• The formula for the magnetic force is:
• $F = BIL\sin \theta$
• Where F is the force, B is the magnetic flux density, L is the length of wire in the field and θ is the angle between the field lines and the wire.

### Example analysis

• An example of the analysis possible:
• Plot a graph of force against current.
• The line of best fit should be a straight line that passes through the origin.
• By comparison with “y = mx + c”, the gradient of the line should be equal to BLsinθ.

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