# Video: Physics Past Exam • 2017/2018 • Pack 1 • Question 11B

Physics Past Exam • 2017/2018 • Pack 1 • Question 11B

03:15

### Video Transcript

A primary coil is connected to a DC voltage source and placed inside a secondary coil. When the circuit of the primary coil is opened, which of the following options best describes the current in the secondary coil? a) Forward induced current, b) backward induced current, c) alternating current, d) direct current.

To start out, let’s make a sketch of this primary coil. In this circuit loop, we know we have a DC power supply and a coil of wire. And we’ve drawn in a switch that we can close or open at will. We know that if we close the switch, the circuit will be complete and current will begin to flow through this loop.

This current-carrying loop though isn’t the only part of our setup. We also have a secondary coil which surrounds the primary coil. That secondary coil could look like this. And if we saw the coils end on, we would see the primary coil inside the loops of the secondary coil.

When the switch to our initial current loop is closed, we know that steady current will flow through this circuit. And we can say that the steady-state current amount is capital 𝐼. And we can say that the direction of 𝐼, the direction of current in this loop, is to be considered the forward direction. This is the steady-state condition of our circuit.

But we’re told that a change is made, that the primary coil is opened up. When this happens, when our switch opens and the current suddenly stops flowing, a drastic change occurs. For one thing, the current in the circuit changes very rapidly. If we call that change in current Δ𝐼 divided by Δ𝑡, we know the direction of the change would be opposite the direction of the original current flow.

We could say then that a changing current is running counterclockwise now through our primary coil loop. That changing current has an effect on the magnetic field in the loops of the primary coil. Thanks to the change in current, a change in the magnetic field is created. We can call it Δ𝐵 sub 𝑝 to say that it’s the change in the magnetic field created by the primary coil.

If our secondary coil wasn’t present, this might be the end of the effect. But because of the secondary coil, more phenomena occur. And that’s because this changing magnetic field, Δ𝐵 sub 𝑝, is experienced through the loops of the secondary coil. And it’s opposed by that coil thanks to Lenz’s law. That means an opposing magnetic field will be induced in the secondary coil loops. We’ll name it 𝐵 sub 𝑠 to specify that it’s created by the secondary coil.

This new magnetic field, 𝐵 sub 𝑠, will induce an emf in the secondary coil which will drive current. And the question is, which way through the loop will the current go? If we use our right-hand screw rule, we can tell that the induced current in the secondary coil will flow in this direction, clockwise from our perspective.

Notice that this current, which we can call 𝐼 sub 𝑠, is flowing in the direction we defined as forward in our primary coil loop. Furthermore, this current is an induced current. And therefore, as we look over our four answer choices, we see that the choice that matches best is choice a). When the steady-state current flow in the primary coil is interrupted, then the current in the secondary coil will be forward induced current.