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.

Hint: \(d\vec {B}=\frac{{I}({d} \vec {{l}} \times \vec{{r}})}{{r}^3}\)

Explanation: Biot-Savart law is given by; \(d\vec {B}=\frac{{I}({d} \vec {{l}} \times \vec{{r}})}{{r}^3}\)

According to Biot-Savart law, the direction of the magnetic field \(\vec B\) can be determined using the right-hand rule. According to this rule, if you point your thumb in the direction of the velocity vector \(\vec v\) and curl your fingers toward the position vector \(\vec r,\) your palm will face the direction of the magnetic field \(\vec B.\)
Therefore, this shows that \(\vec B\) is perpendicular to both \(\vec v\) and \(\vec r.\)
Hence, option (1) is the correct answer.

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