Paper 4 Magnetic Fields Answers

Q1 Plotting Field Strength Around a Wire

1. Axes correctly oriented and labelled with units [1]; sensible scales using most of the grid [1]; six points plotted accurately [2]; best-fit straight line [1].

2. The gradient is approximately

   $$\frac{\Delta B}{\Delta I}\approx 10.0\mu {\text{T A}}^{-1}.$$

   Accept a value consistent with the candidate’s best-fit line. [2]

3. Yes: the data are close to a straight line whose intercept is consistent with zero within the experimental scatter. Both conditions are needed to support direct proportionality. [2]

4. Any two of: perpendicular distance from the sensor to the wire; sensor orientation; wire geometry/orientation; nearby magnetic materials or currents; temperature where it affects sensor response. [2]

5. Measure the sensor reading with the same current magnitude in both directions and take half the difference, or measure the background at $I=0$ and subtract it, while keeping the sensor fixed. [2]

Q2 Electromagnet Investigation

1. Wind solenoids with different turns on the same core/length, keep current fixed, measure field at the same point using a sensor, and repeat readings. [3]
2. For a long solenoid, $B$ depends on turns per unit length $N/L$ and current. Keeping both $I$ and $L$ fixed isolates the effect of changing $N$. [2]
3. Use low current or switch off between readings to avoid overheating. [1]