Paper 4 Magnetic Fields Drills

Q1 Plotting Field Strength Around a Wire

A student investigates how the magnetic flux density $B$ at a fixed point near a long straight wire depends on current $I$. The sensor has been zeroed with no current in the wire. The results are shown below.

$I$/A	0.40	0.80	1.20	1.60	2.00	2.40
$B$/$\mu$T	4.2	8.1	11.8	16.2	19.9	24.1

1. Plot a graph of $B$ against $I$ and draw a best-fit straight line. [5]
2. Use the graph to determine the constant of proportionality between $B$ and $I$, giving its unit. [2]
3. State whether the results support $B\propto I$ and justify your answer from the graph. [2]
4. State two variables, other than current, that must be controlled. [2]
5. Explain how the student could reduce the effect of the Earth’s magnetic field on the result. [2]

Q2 Electromagnet Investigation

A solenoid is used as an electromagnet.

1. Describe an experiment to test how field strength depends on number of turns. [3]
2. Explain why the current and solenoid length should be kept constant. [2]
3. State one safety precaution. [1]