Question

A coil of 1200 turns and mean area of 500$c{m}^2$ is held perpendicular to a uniform magnetic field of induction $4\times {10}^{-4}$T. The resistance of the coil is 20 ohms. When the coil is rotated through ${180}^0$ in the magnetic field in 0.1 s, the average electric current (in mA) induced is :

• A. 12
• B. 24
• C. 36
• D. 48

Answer 1

Answer: (B)

24

Magnetic flux through the loop is given by $\varphi =nB.A$

Change in flux when it is rotated by ${180}^o$ is given by:

$\Delta \varphi =nBAcos\left(\pi \right)-nBAcos\left(0\right)=-2nBA$

$Emf=\frac{-\Delta \varphi }{dt}$
$=\frac{1200\times {4\times 10}^{-4}\times 2\times 500\times {10}^{-4}}{0.1}$
$=12\times {4\times 10}^{-2}V$
$i=\frac{emf}{R}=\frac{12\times 4\times {10}^{-2}}{20}=12\times 2\times {10}^{-3}A=24mA$