Define the term magnetic flux density.
A long, straight wire carries a current into the page, as shown in Fig. 7.1. On Fig. 7.1, draw four field lines to show the magnetic field produced by the current in the wire.
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.
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 the two wires exert a magnetic force on each other.
On Fig. 7.2, draw an arrow to show the direction of the magnetic force exerted on wire X. Label your arrow $F$.
The current in X is double the current in Y. State how the magnetic force exerted on wire Y compares with the magnetic force exerted on wire X.
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.