Any electric current has a magnetic field around it. If we have two currents, each will have its own magnetic field and we might expect these to interact.

Explaining the forces

There are two ways to understand the origin of the forces between current-carrying conductors. In the first, we draw the magnetic fields around two current-carrying conductors (Figure 24.22a). Figure 24.22a shows two unlike (anti-parallel) currents, one flowing into the page, the other flowing out of the page.
Their magnetic fields circle round, and in the space between the wires there is an extra-strong field. We imagine the field lines squashed together, and the result is that they push the wires apart. The diagram shows the resultant field, and the repulsive forces on the two wires.
Figure 24.22b shows the same idea, but for two like (parallel) currents. In the space between the two wires, the magnetic fields cancel out. The wires are pushed together.

Figure 24.22: The forces between current-carrying wires.

The other way to explain the forces between the currents is to use the idea of the motor effect. Figure 24.23 again shows two like currents, ${I_1}$ and ${I_2}$, but this time we only consider the magnetic field B of one of them, ${I_1}$. The second current ${I_2}$ is flowing across the magnetic field of ${I_1}$; from the diagram, you can see that B is at right angles to ${I_2}$. Hence, there will be a force on ${I_2}$ (the BIL force), and we can find its direction using Fleming’s left-hand rule. The arrow shows the direction of the force, which is towards ${I_1}$.
Similarly, there will be a BIL force on ${I_1}$, directed towards ${I_2}$.
These two forces are equal and opposite to one another. They are an example of an action and reaction pair, as described by Newton’s third law of motion.

PRACTICAL ACTIVITY 24.3

 

Observing the forces between currents

You can observe the attraction and repulsion between two parallel currents using the equipment shown in Figure 24.24.
Two long thin strips of aluminium foil are mounted so that they are parallel and a small distance apart.
By connecting them in series with a power supply, you can make a current occur in both of them. By changing the connections, you can make the current first in the same direction through both strips (parallel currents) and then in opposite directions (anti-parallel currents).

If you try this out, you will observe the strips of foil either bending towards each other or away from each other. (Foil is used because it is much more flexible than wire.)
You should find that parallel currents attract one another, while anti-parallel currents repel. This may seem surprising, since we are used to opposite charges attracting, and opposite magnetic poles attracting. Now we have found that opposite currents repel one another.

Question

 

Two flat circular coils of wire are set up side by side, as shown in Figure 24.25. They are connected in series so that the same current flows around each, and in the same direction. Will the coils attract or repel one another? Explain your answer, first by describing the coils as electromagnets, and second by considering the forces between parallel currents. What will happen if the current is reversed in both coils?