Speeds of electrons on plates?

Question

What is the speed of the electron placed at a plate with potential 25V to the plate of 140V. What if this were to be changed to a proton? What speed would it be traveling?

How do I do this?

I don't understand how to set up the conservation of energy problem.

Answer 1

You need to equate the electrical potential-:

qV
where-: q is charge on the particle, V is the accelerating potential.

to the Kinetic Energy-: 1/2 m v^2
(m is mass of particle, v is velocity)

Therefore-:
q V = 1/2 m v^2

You require velocity, so re-arrange for v....

v^2 = 2 q V / m

v = sqrt (2 q V / m)

Just plug in your different two sets of numbers. (Remeber, v is velocity and V is potential difference)

If the particle were changed to a proton the speed will be reduced, as a proton has about 1860 times more mass than an electron. (Make sure this is true with your answers.)