A uniform electric field and a 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

• Correct answer
  A

  Speed will decrease
• B

  Speed will increase
• C

  Will turn towards right of direction of motion
• D

  Will turn towards left of direction of motion

Solution

Given condition is shown in figure
Force due to magnetic field is given by $F_m=q v B \sin \theta$ Here $\theta=0 \Rightarrow F_m=0$ Force due to electric field is $\vec{F}_e=q \vec{E}$ Here, $\vec{F}_e=-e \vec{E}$ $\therefore$ Force due to electric field will be opposite to the direction of motion. Hence its speed will decrease.

Hence, the correct answer is option: 'A'.