A-level Physics (Advancing Physics)/Electric Potential Energy/Worked Solutions

k = 8.99 x 109 Nm2C−2

1. Convert 5 x 10−13 J to MeV.

$$\frac{5 \times 10^{-13}}{1.6 \times 10^{-19}} = 3125000\mbox{ eV} = 3.125\mbox{ MeV}$$

2. Convert 0.9 GeV to J.

$$0.9 \times 10^9 \times 1.6 \times 10^{-19} = 1.44 \times 10^{-10}\mbox{ J}$$

3. What is the potential energy of an electron at the negatively charged plate of a uniform electric field when the potential difference between the two plates is 100V?

$$\epsilon_{elec} = V_{elec}q = 100 \times 1.6 \times 10^{-19} = 1.6 \times 10^{-17}\mbox{ J} = 0.1\mbox{ keV}$$

4. What is the potential energy of a 2C charge 2 cm from a 0.5C charge?

$$\epsilon_{elec} = \frac{kQq}{r} = \frac{8.99 \times 10^9 \times 2 \times 0.5}{0.02} = 4.5 \times 10^{11}\mbox{ J}$$

Remember, coulombs are big!

5. What is represented by the gradient of a graph of electric potential energy against distance from some charge?

Force exerted on the charge with that energy.