A combination of an electric field and magnetic field can be used as a speed sel

Question

A combination of an electric field and magnetic field can be used as a speed selector for the electrons. Electrons are emitted at different speeds from an anode. The electrons travel through a capacitor and magnetic field simultaneously, and only the electrons with speed v are not deflected. The magnitude of the field is 83mT, and the voltage across the capacitor is 500V, and the capacitor plates are 5.0cm apart.

c) What is the net force on a charge if it is travelling at constant speed?

d) What is the magnitude of the electric field?

e) What is the v of the electron? Show all working.

Explanation / Answer

c)

since speed is constant , acceleration = a = 0

hence net force = ma = 0

d)

E = electric field = Voltage / plate saperation = 500 / 0.05 = 10000 volts/m

e)

magnetic force= electric force

qVB = qE

v = E/B = 10000/0.083 = 1.2 x 105

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