The electron loses electric potential energy as it moves because it gains kinetic energy and the total energy is constant.

2. C

Use the formula EPE = q V but remember that q = - e, where e = 1.6 10^{-19} C.

3. B.

Use the formula E = - delta V / delta s.

4. A

The solution is obtained by using EPE(+) + KE(+) = EPE(-) + KE(-), where + or - refer to the plates, and KE is the electron's kinetic energy. We know that KE(-) = 0 since the electron starts at rest. Thus KE(+) = EPE(-) - EPE(+) = q V(-) - q V(+) = -q [V(+) - V(-)] = -q d E = - -1.6*10^{-19}*0.012*2.1*10^{6} = 4*10^{-15} J.