Define the tesla in terms of magnetic flux density.
As shown in Fig. 5.1, two long straight vertical wires $X$ and $Y$ are $4.5\,\text{cm}$ apart. They pass through a horizontal card $PQRS$. Wire $X$ carries a current of $6.3\,\text{A}$ upwards. At first, wire $Y$ carries no current. On Fig. 5.1, sketch, in the plane $PQRS$, the magnetic flux pattern due to the current in wire $X$. Include at least four flux lines.
As shown in Fig. $5.1$, two long straight vertical wires X and Y are $4.5\,\text{cm}$ apart. They pass through a horizontal card PQRS. At the start, wire Y has no current. Wire X carries a current of $6.3\,\text{A}$ upwards.
The magnetic flux density $B$ at a distance $x$ from a long straight current-carrying wire is given by $B = \frac{\mu_0 I}{2\pi x}$, where $I$ is the current in the wire and $\mu_0$ is the permeability of free space. Calculate the magnetic flux density at wire Y due to the current in wire X.
A current of $9.3\,\text{A}$ is then switched on in wire Y. Use your answer in (ii) to calculate the force per unit length on wire Y.
The currents in the two wires in (b)(iii) are different. Explain whether the force per unit length on the two wires will be the same, or not.