**Stopping the Proton**

**Solution Below:**

Part A

Use 9.81 m/s

^{2}for the magnitude of the acceleration due to gravity.

For this problem, you just need to balance the force of gravity with the force from the magnetic field. The gravitational force is equal to (make sure to convert the mass of the particle to kilograms):

Fg = ma

Fg = 0.00143 kg * 9.81 m/s^{2}

Fg = 0.01403 N

The force from the magnetic field needs to be equal to the above, but in the opposite direction (so we need a force of -0.0140 N). The formula for the magnetic force is:

Fg = qE

We know the required force, and the magnitude of the field was already given:

-0.01403 N = q * 660

q = -2.125 * 10^-5 C

q = -2.13 * 10^-5 C

-2.13 * 10^-5 C

Part B

Use 1.67 * 10^−27 kg for the mass of a proton, 1.60 * 10^−19 C for the magnitude of the charge on an electron, and 9.81 m/s^2 for the magnitude of the acceleration due to gravity.

To solve this part, just use the same concept as in Part A:

Fg = qE

9.81 * m = q * E

9.81 * (1.67 * 10^−27) = (1.60 * 10^−19) * E

E = 1.0239 * 10^-7

E = 1.02 * 10^-7

E = 1.02 * 10^-7