Question

An electron is moving with a velocity $2 \times 10^6 \, m/s$ along positive x-direction in the uniform electric field of $8 \times 10^7 \, V/m$ applied along positive y-direction. The magnitude and direction of a uniform magnetic field (in Tesla) that will cause the electrons to move undeviated along its original path is

• A. 40 in -ve z-direction
• B. 40 in +ve z-direction
• C. 4 in +ve z-direction
• D. 4 in -ve z-direction
• E. 8 in +ve z-direction

Answer 1

Answer: (A)

For electrons to move undeviated along its original path,
$\Rightarrow $ Electric force $=$ Magnetic force
or $q E =q v B$
or $B =\frac{E}{v}=\frac{8 \times 10^{7}}{2 \times 10^{6}}$
$B=40\, T$
Direction of magnetic field will be $-$ ve $z$ -direction because of negative charge.