Faraday's Law (Edexcel A Level Physics): Revision Note

A small rectangular coil contains 350 turns of wire. The longer sides are 3.5 cm and the shorter sides are 1.4 cm.

The coil is held between the poles of a large magnet so that it can rotate about an axis through its centre.

The magnet produces a uniform magnetic field of flux density 80 mT between its poles. The coil is positioned horizontally and then turned through an angle of 90° in a time of 0.18 s.

Calculate the magnitude of the average e.m.f induced in the coil.

Answer:

Step 1: Write down the known quantities

• Magnetic flux density, B = 80 mT = 80 × 10^-3 T

• Area, A = 3.5 × 1.4 = (3.5 × 10^-2) × (1.4 × 10^-2) = 4.9 × 10^-4 m²

• Number of turns, N = 350

• Time interval, Δt = 0.18 s

Step 2: Write out the equation for Faraday’s law:

$\epsilon =\frac{∆\left(N\Phi \right)}{∆t}$

Step 3: Write out the equation for the change in flux linkage:

• The number of turns N and the coil area A stay constant

• The flux through the coil changes as it rotates

• Therefore, the change in flux linkage can be written as:

$∆\left(N\Phi \right)=NA\left(∆B\right)$

Step 4: Determine the change in magnetic flux linkage

• The initial flux through the coil is zero (flux lines are parallel to the coil face) 

• The final flux through the coil is 80 mT (flux lines are perpendicular to the coil face)

• This is because the coil begins horizontally in the field and is rotated 90°

• Therefore, the change in flux linkage is:

$∆\left(N\Phi \right)$ = 350 × (4.9 × 10^-4) × (80 × 10^-3) = 0.014 Wb turns

Step 5: Substitute change in flux linkage and time into Faraday’s law equation:

$\epsilon =\frac{0.014}{0.18}$ = 0.076 V

The 'magnitude' of the e.m.f. just means it's size, rather than its direction. This is often what is required in exam questions, so the minus sign in Lenz's law is not necessarily required in calculations. However, you may be expected to explain the significance of the negative sign in the equation, so be prepared to interpret it as an expression of Lenz's Law! 

Remember that 'd' and the greek letter delta, 'Δ', simply mean 'change in'.