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4.14) Two concentric circular coils X and Y of radii 16 cm and 10 cm, respectively, lie in the same vertical plane containing the north-to-south direction. Coil X has 20 turns and carries a current of 16 A; coil Y has 25 turns and carries a current of 18 A. The sense of the current in X is anticlockwise, and clockwise in Y, for an observer looking at the coils facing west. Give the magnitude and direction of the net magnetic field due to the coils at their centre.

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Using the right-hand thumb rule the direction of the magnetic field due to coil X and Y is shown in the figure below as Bx and By.

So, the magnetic field due to coil X at the centre is given by

$$
\begin{aligned}
B_x & =\frac{\mu_0 n_x i_1}{2 r_x} \\
B_x & =\frac{\left(4 \pi \times 10^{-7}\right) \times 20 \times 16}{2 \times\left(16 \times 10^{-2}\right)} \\
B_x & =\left(4 \pi \times 10^{-4}\right)(\text { Towards East })
\end{aligned}
$$


Similarly, the magnetic field at the common centre due to coil Y ,

$$
\begin{aligned}
& B_y=\frac{\mu_0 n_y i_2}{2 r_y n j} \\
& B_y=\frac{\left(4 \pi \times 10^{-7}\right) \times 25 \times 18}{2 \times\left(10 \times 10^{-2}\right)} \\
& B_y=9 \pi \times 10^{-4} \mathrm{~T} \\
& \text { (TowardsWest) }
\end{aligned}
$$


So, the net magnetic field at the common centre is

$$
\begin{aligned}
& B_{\text {net }}=B_y-B_x \quad\left(\text { as } B_y>B_x\right) \\
& B_{\text {net }}=\left(9 \pi \times 10^{-4}\right)-\left(4 \pi \times 10^{-4}\right) \\
& B_{\text {net }}=5 \pi \times 10^{-4} \mathrm{~T} \text { (TowardsWest) }
\end{aligned}
$$


Hence, the magnitude of the net magnetic field is due to the coils at their centre towards the west, is $B_{\text {net }}=5 \pi \times 10^{-4} \mathrm{~T}$.

 

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