2023 A/L Physics Past Paper · Question 01 (English Medium)

Answer: (1) energy.

What the question asks

We want: what kind of thing the electron volt measures.

What each letter means:

$e$ ($\text{C}$): the charge of one electron.

$V$ ($\text{V}$): the potential difference an electron goes across.

$\text{eV}$ ($\text{J}$): one electron volt, the unit we are asked about.

Relevant theory

The unit for measuring potential difference is the "volt" while another unit called the "electron volt" is used to measure very small amounts of work and energy, specially such as those in the measurement of radiant energy.

An electron volt is the work done in taking an electron across a potential difference of one volt.

This amount of energy is equal to 1.6022 × 10-19 J.

Reading the options

For example, think of a kilometre and a millimetre.

A kilometre is a big unit and a millimetre is a tiny unit, but both measure a length.

In the same way, the joule is the usual unit of energy, and the electron volt is a tiny unit of energy.

Both measure energy. The name has the word volt in it, but a unit is told by what it measures.

The quotes say that the volt is for potential difference, and that the electron volt is for small amounts of work and energy.

The third quote gives the size of one electron volt in joules, so it is an amount of energy.

Test each option against the quotes.

(1) Energy: the electron volt measures small amounts of work and energy. This matches.

(2) Power: there is no time in an electron volt.

(3) Charge: the electron volt is the work done on an electron, not its charge.

(4) Voltage: the volt is the unit of potential difference, and the electron volt is another unit.

(5) Force: the electron volt is an amount of energy.

Only option (1) matches.

Why the other options fail

Power has time in it. The electron volt is an amount of work and energy, with no time in it.

The electron volt is the work done in taking an electron across one volt. It is not the charge of the electron.

The volt measures potential difference. The electron volt is another unit, for small amounts of work and energy.

A force is a push or a pull. The electron volt is an amount of energy, given in joules.

Conclusion

The electron volt is a unit of energy.

The answer is (1).

A unit is told by what it measures, not by one word in its name.

The volt measures potential difference, but the electron volt, a different unit, measures energy.

Exam tip