Moving Charges and Magnetism - Result Question 6
6. A uniform electric field and uniform magnetic field are acting along the same direction in a certain region. If an electron is projected in the region such that its velocity is pointed along the direction of fields, then the electron [2011]
(a) will turn towards right of direction of motion
(b) speed will decrease
(c) speed will increase
(d) will turn towards left direction of motion
Show Answer
Answer:
Correct Answer: 6. (b)
Solution:
- (b) $\vec{v}$ and $\vec{B}$ are in same direction so that magnetic force on electron becomes zero, only electric force acts. But force on electron due to electric field is opposite to the direction of velocity.
(i) If $\vec{v}, \vec{E}$ and $\vec{B}$ are all collinear. In this case the magnetic force acting on the electron is zero and only electric force will act. So, acceleration $\vec{a}=$ $\frac{\vec{F}}{m}$. The electron will undeflected in a straight line path with change in speed.
(ii) When $\vec{v}, \vec{E}$ and $\vec{B}$ are mutually perpendicular. In this case $\vec{F}= \vec{F} _e+ \vec{F} _m=0$
Here, $F_e=$ electric force
$F_m=$ magnetic force
Here, $\vec{a}=\frac{\vec{F}}{m}=0$
The particle (electron) will pass with same velocity without any deviation in path.
