Question

A particle beam is made up of many protons each with a kinetic energy of 3.25times 10-15J. A proton has a mass of 1.673times 10-27kg and a charge of +1.602times 10-19C. What is the magnitude of a uniform electric field that will stop these protons in a distance of 2 m?

I am struggling with this problem. I don't understand where I use the KE or mass. I tried doing E=kq/r^2 but the answer I got was wrong.

Answer 1

Force = charge * electric field
or
F = Q E
if you do F = m a you can use the mass
but you can use
work done = change in Ke

work done = change in Ke= force * distance

so
Q E d = Ke

2 meters= 3.25*10^-15/(1.602^10^-19 E)

Answer 2

thanks!I also have a question about this one.

When a test charge q0 = 9 nC is placed at the origin, it experiences a force of 8 times 10-4 N in the positive y direction.

(c) If this force is due to a charge on the y axis at y = 3 cm, what is the value of that charge?

so in a I found E to be 88888.889
in b I found the F on a charge of -4nC to be -3.555e-4

I thought for C I would just use E=kq/(r^2) but it was wrong

Answer 3

Just use E = F/q