A simple and common technique for accelerating electrons is shown in Figure 17.3

Question

A simple and common technique for accelerating electrons is shown in Figure 17.34, where a uniform electric field is created between two plates. Electrons are released, usually from a hot filament, near the negative plate, and there is a small hole in the positive plate that allows the electrons to continue moving.

(a) Calculate the acceleration of the electron if the field strength is 7.30 ? 104 N/C.
m/s2
(b) Why will the electron not be pulled back to the positive plate once it moves through the hole?
---Select--- There is no field outisde the plates. The other side of the positive plate also has a negative charge. The force of gravity is too strong.

Explanation / Answer

A simple and common technique for an accelerating electrons is shown in figure, where a uniform electric field is created between two plates.

(a) The acceleration of the electron if the field strength is 7.3 x 104 N/C which will be given as :

for an electric force,    Fe = q E                                                          { eq.1 }

from newton's second law of motion, F = m a                                                     { eq.2 }

equating eq.1 & 2, we get

m a = q E

a = q E / m                                                                                                           { eq.3 }

where, q = charge on electron = 1.6 x 10-19 C

m = mass of electron = 9.1 x 10-31 kg

inserting these values in eq.3.

a = (1.6 x 10-19 C) (7.3 x 104 N/C) / (9.1 x 10-31 kg)

a = (11.68 x 10-15 N) / (9.1 x 10-31 kg)

a = 1.28 x 1016 m/s2

(b) The electron will not be pulled back to the positive plate once it moves through the hole because :

The other side of the positive plate also has a negative charge.

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