Ncert Exercises & Solutions

Biot-Savart law indicates that the moving electrons (velocity \(v\)) produce a magnetic field \(B\) such that:

1.	\(B\perp v\).
2.	\(B\parallel v\).
3.	it obeys inverse cube law.
4.	it is along the line joining the electron and point of observation.

(a) Hint: Use the concept of Biot-Savart law.

Step 1: By Biot-Savart law,

dB = \frac{Idlsinθ}{r^{2}}

dB = \frac{I (d \vec{l} \times \vec{r})}{r^{3}}

According to Biot-Savart law, if the magnetic field is not perpendicular to the motion of charge, then it will not move in a helical path, which is not possible for the motion of a charge in the magnetic field.

Step 2: Interpret the direction of the magnetic field.

So the magnetic field is perpendicular to the direction of the flow of charge verifies answer 'a'.

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