AQA A Level Physics

Revision Notes

7.9.1 Magnetic Flux

Magnetic Flux

  • Electromagnetic induction is when an e.m.f is induced in a closed circuit conductor due to it moving through a magnetic field
    • Examples are a flat coil or a solenoid
  • This happens when a conductor cuts through magnetic field lines
  • The amount of e.m.f induced is determined by the magnetic flux
  • The amount of magnetic flux varies as the coil rotates within the field
    • The flux is the total magnetic field that passes through a given area
    • It is a maximum when the magnetic field lines are perpendicular to the plane of the area
    • It is 0 when the magnetic field lines are parallel to the plane of the area

 

  • The magnetic flux is defined as:

The product of the magnetic flux density and the cross-sectional area perpendicular to the direction of the magnetic flux density

Magnetic flux diagram, downloadable AS & A Level Physics revision notes

The magnetic flux is maximum when the magnetic field lines and the area they are travelling through are perpendicular

  • In other words, magnetic flux is the number of magnetic field lines through a given area
  • Magnetic flux is defined by the symbol Φ (greek letter ‘phi’)
    • It is measured in units of Webers (Wb)
  • Magnetic flux can be calculated using the equation:

Φ = BA

  • Where:
    • Φ = magnetic flux (Wb)
    • B = magnetic flux density (T)
    • A = cross-sectional area (m2)

Worked Example

An aluminium window frame has a width of 40 cm and length of 73 cm shown in the diagram below.

WE Calculating Magnetic Flux question image, downloadable AS & A Level Physics revision notes

The frame is hinged along the vertical edge AC.

When the window is closed, the frame is normal to the Earth’s magnetic field with magnetic flux density 1.8 × 10-5 T.

a) Calculate the magnetic flux through the window when it is closed.

b) Sketch the graph of the magnetic flux against angle between the field lines and the normal when the window is opened and rotated by 180°

Part (a)

Step 1: Write out the known quantities

Cross-sectional area, A = 40 cm × 73 cm = (40 × 10-2) × (73 × 10-2) = 0.292 m2

Magnetic flux density, B = 1.8 × 10-5 T

Step 2: Write down the equation for magnetic flux

Φ = BA

Step 3: Substitute in values

Φ = (1.8 × 10-5) × 0.292 = 5.256 × 10-6 = 5.3 × 10-6 Wb

Part (b)

The magnetic flux will be at a minimum when the window is opened by 90o and a maximum when fully closed or opened to 180o

WE Calculating Magnetic Flux graph solution, downloadable AS & A Level Physics revision notes

Exam Tip

Consider carefully the value of θ, it is the angle between the field lines and the line normal (perpendicular) to the plane of the area the field lines are passing through. If it helps, drawing the normal on the area provided will help visualise the correct angle.

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