Magnetic Flux (AQA A Level Physics): Revision Note

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

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 [2]

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° [3]

Answer:

Part (a)

Step 1: Write out the known quantities

• Cross-sectional area, $A=40\mathrm{cm}\times 73\mathrm{cm}=0.40\times 0.73=0.292{\mathrm{m}}^2$

• Magnetic flux density, $B=1.8\times {10}^{-5}\mathrm{T}$

Step 2: Write down the equation for magnetic flux

$\Phi =BA$

Step 3: Substitute in values

$\Phi =\left(1.8\times {10}^{-5}\right)\times 0.292$ [1 mark]

$\Phi =5.256\times {10}^{-6}\mathrm{Wb}=5.3\times {10}^{-6}\mathrm{Wb}\left(2\mathrm{s}.\mathrm{f}.\right)$ [1 mark]

Part (b)

• The magnetic flux will be at a maximum when fully closed or opened to 180° [1 mark]

• The magnetic flux will be at a minimum when the window is opened by 90° [1 mark]

[1 mark]

Consider carefully the value of $\theta$, 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.