A physics technician sets up a demonstration to investigate the magnetic field surrounding a vertical current-carrying brass rod. The rod passes through a flat horizontal sheet of clear perspex, which is supported by a clamp stand, as shown in the diagram below.
Describe how the technician can use a single small plotting compass to plot the pattern of a magnetic field line on the perspex sheet.
The current in the brass rod flows vertically downwards (in the direction of the arrow III). State the shape of the magnetic field lines on the sheet and describe their direction (clockwise or anticlockwise) when viewed from above. Explain how you determined this direction.
An orienteer uses a liquid-filled magnetic compass to navigate through a dense forest. Explain how the behaviour of the compass needle provides evidence that the Earth has its own magnetic field.