Define the term magnetic flux density.
A long, straight wire has a current into the page, as shown in Fig. 7.1. On Fig. 7.1, draw four field lines to show the magnetic field around the wire caused by the current in it.
Two identical wires X and Y are arranged parallel to each other. Both wires carry current into the page, as shown in Fig. 7.2.
Explain why a magnetic force acts between the two wires.
On Fig. 7.2, draw an arrow to show the direction of the magnetic force acting on wire X. Label the arrow $F$.
The current in X is double the current in Y. State how the magnetic force on wire Y compares with the magnetic force on wire X.
The direction of the current in both wires is now reversed. State, with a reason, the effect of this change on the direction of the force on wire X.