Question

In this problem, you will use Lenz's law to explore what happenswhen an electromagnet is activated a short distance from a wireloop.

You will need to use the right-hand rule to find the directionof the induced current.

Part A

When the switch is open, which of thefollowing statements about the magnetic flux through the wire loopis true? Assume that the direction of the vector area of the wireloop is to the right.

There is nomagnetic flux through the wire loop. There is apositive flux through the wire loop. There is anegative flux through the wire loop.

Part B

What is the direction of the induced currentin the wire loop (as seen from the left) when the switch isopen?

There is no inducedcurrent.

Correct

Part C

Now the switch on the electromagnet isclosed. What is the direction of the induced current in the wireloop immediately after the switch is closed (as seen from theleft)?

There is noinduced current. The inducedcurrent is clockwise. The inducedcurrent is counterclockwise.

Now consider the new arrangement shownin the figure. Note that the orientation of thebattery is reversed with respect to the first arrangement youconsidered. Answer the following questions related to thearrangment with the new battery orientation.

Part E

The switch on the electromagnet, initiallyopen, is closed. What is the direction of the induced current inthe wire loop (as seen from the left)?

The induced current isclockwise.

Correct

Part F

Now the switch on the electromagnet isreopened. The magnitude of the external magnetic flux through thewire loop ______ (A. increases, B. decreases, C. remains constant),and there is _______ (A. zero, B. a clockwise, C. acounterclockwise) current induced in the loop (as seen from theleft.

Enter the letters corresponding tothe responses that correctly complete the statement above. Forexample: A,C

Answer 1

Concepts and reason

The concept required to solve this problem is Lenz’s law.

Use the Lenz’s law and right hand thumb rule to find the induced current direction and magnetic flux direction.

Fundamentals

Lenz’s law stats that the current induced in a conducting material by changing magnetic flux will create a magnetic field exactly opposite to the change in the applied field.

The right-hand thumb rule gives the direction of magnetic flux due to a current loop is given by the thumb if fingers curl in the direction of magnetic flux or vice versa.

(A)

When the switch is open no current flows through the electromagnet coil. There is no magnetic field produced by the electromagnet. So, there is no applied field which implies there is no magnetic flux or change. So, the magnetic flux is not positive or negative but zero. Thus, there is no magnetic flux through the wire loop.

(B)

When the switch is open no current flows through the electromagnet coil. There is no magnetic field produced by the electromagnet. So, there is no applied field which implies there is no magnetic flux or change in magnetic flux. The current is induced only if the magnetic flux through the loop changes. Thus, there is no induced current.

(C)

When the switch is closed, the current flows or increases through the electromagnet from positive terminal to negative terminal of the battery. The direction of flow of current in the electromagnet is counterclockwise from the left view.

The magnetic flux from the electromagnet therefore increase is towards right as thumb points towards right when fingers curl counterclockwise.

From the Lenz’s law, the induced magnetic flux opposes the increase in the magnetic flux. So, the magnetic flux is induced towards left. From the right-hand rule, again the thumb point towards left so the fingers in counterclockwise direction or the induced current is in counterclockwise direction.

(E)

When the switch is closed, the current flows or increases through the electromagnet from positive terminal to negative terminal of the battery. The direction of flow of current in the electromagnet is clockwise from the left view. The magnetic flux from the electromagnet therefore increases towards left as thumb points towards left, when fingers curl clockwise. From the Lenz’s law, the induced magnetic flux opposes the increase in the magnetic flux. So, the magnetic flux is induced towards right. From the right hand rule again, the thumb point towards right so the fingers in clockwise direction or the induced current is in clockwise direction.

(F)

When the switch is reopened, the current decreases through the electromagnet till it becomes zero from positive terminal to negative terminal of the battery. The direction of flow of current in the electromagnet is clockwise from the left view. The magnetic flux from the electromagnet therefore decreases towards left as thumb points towards left, when fingers curl clockwise.

From the Lenz’s law, the induced magnetic flux opposes the decrease in the magnetic flux. So, the magnetic flux is induced towards left. From the right hand rule again, the thumb point towards left so the fingers in counterclockwise direction or the induced current is in counterclockwise direction.

Ans: Part A

There is no magnetic flux through the wire loop.

Part B

There is no induced current.

Part C

The induced current is counterclockwise.

Part E

The induced current is clockwise.

Part F

Decreases, Counterclockwise