## Mastering Physics Solutions: Understanding Changing Flux

Understanding Changing Flux

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 D = B, B

Solutions Below:

In this problem, you will use Lenz’s law to explore what happens when an electromagnet is activated a short distance from a wire loop. You will need to use the right-hand rule to find the direction of the induced current.

Consider the arrangement shown in the figure.

Part A

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

Since the switch is open, there is no current moving through the circuit, and no magnetic field will be induced.

• There is no magnetic flux through the wire loop.
• There is a positive flux through the wire loop.
• There is a negative flux through the wire loop.

There is no magnetic flux through the wire loop.

Part B

What is the direction of the induced current in the wire loop (as seen from the left) when the switch is open?

• There is no induced current.
• The induced current is clockwise.
• The induced current is counterclockwise.

Just like in Part A.

There is no induced current.

Part C

Now the switch on the electromagnet is closed. What is the direction of the induced current in the wire loop immediately after the switch is closed (as seen from the left)?

• There is no induced current.
• The induced current is clockwise.
• The induced current is counterclockwise.

Since the switch is closed and now current is moving through the electromagnet, just follow the right hand rule.

The induced current is counterclockwise.

Part D

Finally, the switch on the electromagnet is reopened. The magnitude of the external magnetic flux through the wire loop ______ (A. increases, B. decreases, C. remains constant), and there is _______ (A. zero, B. a clockwise, C. a counterclockwise) current induced in the loop (as seen from the left).

Enter the letters corresponding to the responses that correctly complete the statement above. For example, if the
correct answers are A and C, type A,C

The answer is B, B. For the first one, it should be fairly obvious, but the second part might be a little counterintuitive. Lenz’s law states that the current induced in the wire loop opposes the change in the external magnetic flux that caused the current. So if the external flux (magnetic field) has disappeared, the induced current has to now be opposite to what it was when the field was present. Strange, but true.

B, B

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