Question

A velocity selector is built with a magnetic field of magnitude 5.2 T and an electric field of strength 4.6 × 10 ^ 4 N/C. The directions of the two fields are perpendicular to each other. At what speed will electrons pass through the selector without deflection? Let the charge of an electron q = −1.6 × 10 −19 C
Which characteristic is common to all permanent magnets?

Answer 1

Explanation :

Magnetic field, B = 5.2 T

Electric field, $E=4.6\times 10^4\ N/C$

The directions of the two fields are perpendicular to each other. Hence the force due to each field will equate each other.

Electrostatic force, F =qE.........(1)

Magnetic force, F = qvB........(2)

From equation (1) and (2)

   E = v B

$v=\dfrac{E}{B}$

$v=\dfrac{4.6\times 10^4\ N/C}{5.2\ T}$

$v=0.88\times 10^4\ m/s$

$v=8.8\times 10^3\ m/s$

Hence, the correct option is (a).

(2) A permanent magnet always has two poles as the North pole and south pole. The magnetic field lines start from north poles and terminate at the south pole of the magnet.

Hence, the correct option is (C).