Units of Energy

Units of Energy

When dealing with calculations at a small scale (like at the level of electrons) it is more convenient to use different units for energy rather than the joule (J). We define a unit called the electron-volt (eV) as the kinetic energy gained by an electron passing through a potential difference of one volt.

\(E=q\times V\)

where q is the charge of the electron and V is the potential difference applied. The charge of 1 electron is \(\text{1.6} \times \text{10}^{-\text{19}}\) \(\text{C}\), so \(\text{1}\) \(\text{eV}\) is calculated to be:

\(1 \mathrm{eV}=\left(\text{1.6} \times \text{10}^{-\text{19}}\text{ C} \times \text{1}\text{ V} \right)= \text{1.6} \times \text{10}^{-\text{19}}\text{ J}\)

You can see that \(\text{1.6} \times \text{10}^{-\text{19}}\) \(\text{J}\) is a very small amount of energy and so using electron-volts (eV) at this level is easier.

Hence, \(\text{1}\) \(\text{eV}\) = \(\text{1.6} \times \text{10}^{-\text{19}}\) \(\text{J}\) which means that \(\text{1}\) \(\text{J}\) = \(\text{6.241} \times \text{10}^{\text{18}}\) \(\text{eV}\)

Optional Activity: Demonstration of the photoelectric effect

We can set up an experiment similar to the one used originally to study the photoelectric effect. The experiment allows us to measure the number of electrons emitted and the maximum kinetic energy of the ejected electrons.

Photoelectric effect apparatus

In the diagram of the Photoelectric Effect Apparatus, an ammeter allows for a current to be measured. Measuring the current allows us to deduce information about the number of electrons emitted and the kinetic energy of the ejected electrons.

In the diagram, notice that the potential difference supplied by the battery is zero and yet a current is still measured on the ammeter. This is due to the incoming photons having sufficient frequency and hence energy greater than the work function to eject electrons. The ejected electrons travel across the evacuated space and allow for a current to be measured in the circuit.

Remember, photon energy is related to frequency while intensity is related to the number of photons.

Optional Video: Photoelectric Simulation

PhET Photoelectric effect simulation