How do I know magnetic force is pointing downward from this information? WOuld l

Question

How do I know magnetic force is pointing downward from this information? WOuld love an explanation. THank you.

An electron moves at speed 8.0 Times 10^6 m/s toward the right between two parallel plates, such as shown in Figure PI7.41. A 0.12-T magnetic field points into the paper parallel to the plate surfaces. Determine the magnitude and direction of the magnetic force that the magnetic field exerts on the electron. What should be the magnitude and direction of an field caused by oppositely charged plates to produce an electric force that just balances the magnetic force?

Explanation / Answer

Here,

speed of electron,v = 8 *10^6 m/s

This is given that the magnetic field points into the paper

as magnetic force is given as

F = q * ( v X B)

electron is negative charged

Using Right hand rule , we find that the magnetic force on electron is downwards.

a)

magnitude of magnetic force = e *v * B

magnitude of magnetic force = 1.602 *10^-19 * 8 *10^6 * 0.12

magnitude of magnetic force = 1.54 *10^-13 N

direction of magnetic force is downwards

b)

let the electric field is E

magnetic force = electric force

e * v * B = e * E

E = 8 *10^6 * 0.12 N/C

E = 9.6 *10^5 N/C

the electric field is 9.6 *10^5 N/C

as the electric force must be upwards , the direction of field is downwards

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