CIE A Level Physics (9702) exams from 2022

Revision Notes

20.1.4 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 equation 1

  • 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 straight conductor 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 5mT

Worked example: Calculating flux density

Magnetic_Flux_Density_Definition_Worked_example_-_Calculating_Flux_Density_Question, downloadable AS & A Level Physics revision notes

Step 1:            Write out the known quantities

Force on wire, F = 0.04 N

Current, I = 3.0 A

Length of wire = 15 cm = 15 × 10-2 m

Step 2:            Magnetic flux density B equation

Magnetic Flux Density Definition equation 1

Step 3:            Substitute in values

Magnetic Flux Density Definition Worked Example equation 2

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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