Expressing Quantities & SI Units Practice Questions

Which of these is the SI unit of power?

Put a cross ( ☒ ) in the box next to your answer.

☐ A J

☐ B N

☐ C W

☐ D Hz

State the SI unit of frequency.

In the student's list is the statement:

"The mass of the trolley is 850 N."

Explain why this statement cannot be correct.

Complete the table by converting each measurement into the base SI unit shown.

Measurement	Value with prefix	Value in base SI unit
length of a screw	$45\ \mathrm{mm}$	.................... $\mathrm{m}$
power output of a power station	$2.5\ \mathrm{GW}$	.................... $\mathrm{W}$
mass of an apple	$320\ \mathrm{g}$	.................... $\mathrm{kg}$

A smoke detector responds to smoke in a time of $25\ \mathrm{\mu s}$.

Write this time in seconds, in standard form.

A scientist works at the control centre for a shift lasting 2 hours and 30 minutes.

Convert this time into seconds.

The satellite completes one orbit of the Earth in $5400\ \mathrm{s}$.

Calculate this time in hours.

Write the time $5400\ \mathrm{s}$ in standard form.

The display on the boat shows a speed of $30\ \mathrm{km/h}$.

Show that the dolphin is swimming faster than the boat is travelling.

Later the boat's display shows a speed of $50\ \mathrm{km/h}$.

Calculate this speed in m/s.

Give your answer to 2 significant figures.

speed = .............................................................. $\mathrm{m/s}$

Calculate the mean diameter of the fishing line.

Give your answer to an appropriate number of significant figures.

mean diameter = .............................................................. $\mathrm{mm}$

Write the mean diameter from part (a) in metres, in standard form.

Write $7\ \mathrm{nm}$ in metres, in standard form.

Estimate how many of these transistor widths would fit across the chip.

number of transistor widths = ..............................................................

Which of these is the unit of current?

Put a cross ( ☒ ) in the box next to your answer.

☐ A ampere

☐ B coulomb

☐ C ohm

☐ D watt

The energy transferred when a charge moves through a potential difference is given by

$$E = Q \times V$$

where $E$ is measured in joules (J) and $Q$ is measured in coulombs (C).

Determine a unit for potential difference in terms of joules and coulombs.

Use the equation $F = m \times a$ to show that the newton (N) is equivalent to $\mathrm{kg\ m/s^2}$.

Show that the constant $C$ has no unit.