# Find the electric potential difference ΔVe required to stop an electron (called a "stopping potential") moving with an initial speed of 1.93 107 m/s.

(a) Find the electric potential difference ΔVe required to stop an electron (called a "stopping potential") moving with an initial speed of 1.93 107 m/s.
kV

(b) Would a proton traveling at the same speed require a greater or lesser magnitude of electric potential difference?

Explain.

(c) Find a symbolic expression for the ratio of the proton stopping potential and the electron stopping potential, ΔVp/ΔVe. (Use the following as necessary: mp for the mass of proton and me for the mass of electron.)

 ΔVp ΔVe
=

asked Jan 20, 2012 in Physics

+1 vote

Thie problem can be solved using the conservation of energy.

Kf - Ki + Uf - Ui = 0

And so Kf - Ki = - (Uf - Ui)

Substitute in the proper equations now:

0 - (1/2)*m*v^2 = - (q*V)

where q = -1.602*10^-19 C

m = 9.109*10^31 kg

v = 1.93*10^7 m/s

Now just substitute in these values and solve for V.

V = - ((1/2)*(9.109*10^31 kg)*(1.93*10^7 m/s)^2) / (1.602*10^-19 C))

and so V = -1060 V  or  -1.06 kV

For Part B, the answer is greater because a proton has greater mass (mass proton = 1.673*10^-27 kg) and hence would require a greater change of electric potential.

For Part C, the ratio is simply =   - (mass of proton)/ (mass of electron)

or:  - mp / me

answered Jan 24, 2012 by ~Expert~ (3,020 points)
selected Jan 24, 2012 by Roar3215