AQA A Level Physics

Revision Notes

7.8.3 Magnetic Flux Density

Magnetic Flux Density Definition

  • The magnetic flux density B is defined as:

The force acting per unit current per unit length on a current-carrying conductor placed perpendicular to the magnetic field

  • Rearranging the equation for magnetic force on a wire, the magnetic flux density is defined by the equation:

Magnetic Flux Density Definition

  • Where:
    • B = magnetic flux density (T)
    • F = magnetic force (N)
    • I = current (A)
    • L = length of the wire (m)
  • Note: this equation is only relevant when the B-field is perpendicular to the current
  • Magnetic flux density is measured in units of tesla, which is defined as:

A wire carrying a current of 1A normal to a magnetic field of flux density of 1 T with force per unit length of the conductor of 1 N m-1

  • To put this into perspective, the Earth’s magnetic flux density is around 0.032 mT and an ordinary fridge magnet is around 5 mT
  • The magnetic flux density is sometimes referred to as the magnetic field strength

Worked Example

A 15 cm length of wire is placed vertically and at right angels to a magnetic field.

When a current of 3.0 A flows in the wire vertically upwards, a force of 0.04 N acts on it to the left.

Determine the flux density of the field and its direction.

Step 1: Write out the known quantities

    • Force on wire, F = 0.04 N
    • Current, I = 3.0 A
    • Length of wire, L = 15 cm = 15 × 10-2 m

Step 2: Write out the magnetic flux density B equation

Magnetic Flux Density Definition

Step 3: Substitute in values

Step 4: Determine the direction of the B field

    • Using Fleming’s left-hand rule :

F = to the left

I = vertically upwards

therefore, B = into the page



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