KCET · Physics · Magnetic Effects of Current

In the given figure, the magnetic field at \(O\).

A \(\frac{3}{4} \frac{\mu_{0} I}{r}+\frac{\mu_{0} I}{4 \pi r}\)
B \(\frac{3}{10} \frac{\mu_{0} I}{r}-\frac{\mu_{0} I}{4 \pi r}\)
C \(\frac{3}{8} \frac{\mu_{0} I}{r}+\frac{\mu_{0} I}{4 \pi r}\)
D \(\frac{3}{8} \frac{\mu_{0} I}{r}-\frac{\mu_{0} I}{4 \pi r}\)

Verified Solution

Answer & Solution

Correct Answer

(C) \(\frac{3}{8} \frac{\mu_{0} I}{r}+\frac{\mu_{0} I}{4 \pi r}\)

Step-by-step Solution

Detailed explanation

According to the figure, magnetic field at point is given as

\(B_{\text {net }}=\) magnetic field due to straight wire \(A B+\)
magnetic field due to straight wire \(C D+\) magnetic field due circular wire \(B C\)
Since, point \(O\) is along the length of the wire \(B A\). So, \(B_{A B}=0\)
\(\begin{aligned}
B_{\text {net }} &=0+\frac{\mu_{0}}{4 \pi} \cdot \frac{I}{r}+\frac{270}{360}\left(\frac{\mu_{0} I}{2 r}\right) \\
&=\frac{\mu_{0} I}{4 \pi r}+\frac{3}{4} \cdot \frac{\mu_{0} I}{2 r} \\
&=\frac{\mu_{0} I}{4 \pi r}+\frac{3 \mu_{0} I}{8 r}(\text { downward })
\end{aligned}\)

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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