Paper 4 Magnetic Force Drills

Q1 Current Balance

A horizontal wire segment of length $L=0.0600\mathrm{m}$ lies perpendicular to a uniform magnetic field. A balance records the change in apparent mass $\Delta m$ when current $I$ flows. The current direction is fixed throughout.

$I$/A	0.50	1.00	1.50	2.00	2.50	3.00
$\Delta m$/g	1.1	2.1	3.2	4.3	5.3	6.5

1. Plot a graph of $\Delta m$ against $I$ and draw a best-fit straight line. [5]
2. Determine the gradient of the graph, in $\mathrm{kg}{\mathrm{A}}^{-1}$. [2]
3. Hence calculate the magnetic flux density $B$. Take $g=9.81\mathrm{m}{\mathrm{s}}^{-2}$. [3]
4. Explain why the wire must be perpendicular to the field for the relation used in part 3. [2]
5. State one precaution that reduces heating of the wire. [1]

Q2 Beam Deflection Comparison

A student compares electric and magnetic deflection of an electron beam.

1. State one observation that distinguishes electric from magnetic deflection. [2]
2. Explain why reversing magnetic field direction reverses deflection. [2]
3. State one safety precaution when using an electron-beam tube. [1]