AQA A-Level Physics/Density

Density at A-Level is no different to density at GCSE. You should recall that density is defined as measure of a material's mass per unit volume. Using this definition we can work out that density, $\rho ={\frac {m}{V}}$, where $m$ is mass, in kg, and $V$ is volume in m³. Once again, using the formula, it can be found that density has the units kgˑm⁻³, since it is a fraction of kg over m³.

A more dense material will be heavier than a less dense material of the same volume, since a smaller numerator in the fraction reduces the value of $\rho$

Experiment to measure density

You are required to be able to state a method in order to measure the density of a given object.

1. The first step is to measure the objects mass. This should be done using a set of scientific balances of sufficient precision (e.g. ±0.001 grams). Record this value.
2. Measure the volume. The way you should go about this is dependent on the object:
   1. If the object is regular, for example, cuboid, find the cross-sectional area and multiply by the depth of the object.
   2. If the object is irregular, fill a beaker or measuring cylinder enough to cover the object once it is submerged and measure this initial volume. Place the object into the container and measure the new volume. The volume of the object itself is $\Delta V$, found by subtracting the initial volume from the final volume.
3. Use the density using your values and the formula above.

Practice Questions

Give all answers to an appropriate number of significant figures.

1. State the density of air at room temperature and pressure.
2. Given that the density of strontium is 2630kgm^-3, what is the mass of strontium in a sphere of radius 0.560m?

Answers

1. 1.3 kgm^-3
2. 1930 kg