A uniform electric field and a uniform magnetic field are acting along same direction in a certain r

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4.Moving Charges and Magnetism

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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

A

will turn towards right of direction of motion

B

will turn towards left of direction of motion

C

speed will increase

D

speed will decrease

(AIPMT-2011)

Solution

Force on electron due to electric field,

$\vec{F}_{E}=-e \vec{E}$

Force on electron due to magnetic field,

$\vec{F}_{B}=-e(\vec{v} \times \vec{B})=0$

since $\bar{v}$ and $\vec{B}$ are in the same direction.

Total force on the electron,

$\vec{F}=\vec{F}_{E}+\vec{F}_{B}=-e \vec{E}$

Electric field opposes the motion of the electron, hence speed of the electron will decrease.

Standard 12

Physics

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