Question

Determine the direction of the force that will act on the charge in each of the following situations.

A negative charge moving into the screen in a magnetic field that points downward.

A negative charge moving downward in an electric field that points out of the screen.

A positive charge moving to the left in a magnetic field that points to the left.

Answer Bank: To the left, to the right, zero force, out of the screen, into the screen, downward, inward

Answer 1

1. Negative charge is moving into the screen that is current due to it is directed out of the screen and magnetic field is downwards therefore by Fleming's left hand rule the force on it must be acting 'to the right'.

(Or we can use vector form of magnetic component of Lorentz Force

$\vec{F}_m=q(\vec{v}\times\vec{B})$

Where q is charge $\vec{v}$ Is velocity of charge and $\vec{B}$ Is magnetic field)

2. Electric field is directed out of the screen and charge is negative, therefore force acting on charge will be directed 'into the screen'.

( Force on charge in electric field is given by

$\vec{F}_E=q\vec{E}$

where $\vec{E}$ Is electric field)

3. Since the direction of motion of charge is parallel to the direction of magnetic field, hence no force will act on it. That is 'zero force'

( Force on moving charge in magnetic field is given by $\vec{F}_m=q(\vec{v}\times\vec{B})$

If charge is moving parallel to magnetic field than angle between velocity vector and magnetic field is zero, therefore $(\vec{v}\times\vec{B})$ will be zero hence force will be zero.)