Question

An electron is moving with a speed of $10 ^ { 8 } \mathrm {~m} / \mathrm { sec }$ perpendicular to a uniform magnetic field of intensity B. Suddenly intensity of the magnetic field is reduced to B/2. The radius of the path becomes from the original value of r

• A. No change
• B. Reduces to r / 2
• C. Increases to 2r
• D. Stops moving

Answer 1

Answer: (C)

Increases to 2r

Answer 2

$r \propto \frac { 1 } { B }$ i.e. $\frac { r _ { 1 } } { r _ { 2 } } = \frac { B _ { 2 } } { B _ { 1 } } \Rightarrow r _ { 2 } = \frac { B _ { 1 } } { B _ { 1 } / 2 } \times r$= 2r