Question

The parallel plate capacitor has potential difference of 100 V and separation between the plate is 1 mm. An electron is projected along x axis in between the plates. If the electron comes out of the plates along the x axis undeviated then the magnitude and direction of magnetic field that must be applied between the plates : (particle is projected with a velocity of 10⁵ m/s along x axis)

• A. B = 1$\widehat{k}$ Tesla
• B. B = 1$\widehat{j}$ Tesla
• C. B = 1(–$\widehat{j}$) Tesla
• D. B = 10 (–$\widehat{j}$) Tesla

Answer 1

Answer: (B)

B = 1$\widehat{j}$ Tesla

Answer 2

F_electric = F_mag

qE = q(V × B)

B = E/V = = $\frac { 100 \mathrm {~V} } { 1 \times 10 ^ { - 3 } \times 10 ^ { 5 } \mathrm {~m} / \mathrm { s } }$

= 1T

Electric force is along direction then magnetic force must be in $- \hat { k }$ direction.

Therefore B must be in + $\hat { \mathrm { j } }$ direction.