Lenz's Law Practice

The bar magnetic is at rest next to the loop. What direction is the induced current in the wire?

• Clockwise

• Counter Clockwise

• I = 0

Answer explanation

1. Existing field goes right.

2. Flux is constant.

3. Constant flux means no current!

The loop is being pulled to the right out of the magnetic field. What direction is the induced current in the wire?

• Clockwise

• Counter Clockwise

• I = 0

Answer explanation

1. Existing flux points out of the page.

2. Flux is DECREASING.

3. New flux must oppose this decrease and point out of page.

4. Using RHR #1, for B in loop to point out of page, current is CCW.

The bar magnet is moving down towards the loop from above the page. What direction is the induced current in the wire?

• Clockwise

• Counter Clockwise

• I = 0

Answer explanation

1. Existing flux points out of page (towards south pole).

2. Flux is INCREASING out of the page as magnet gets closer.

3. New flux must oppose this and point INTO the page.

4. Using RHR #1, to have field pointing into page in loop center, current is CW.

The loop shrinks in a constant magnetic field. What direction is the induced current in the wire?

• Clockwise

• Counter Clockwise

• I = 0

Answer explanation

1. Existing flux points out of the page.

2. Flux is DECREASING as area decreases.

3. New flux must oppose this and point out of page.

4. Using RHR #1, for B in loop to point out of page, current is CCW.

The magnet is moving towards the loop from below the page. What direction is the induced current in the wire?

• Clockwise

• Counter Clockwise

• I = 0

Answer explanation

1. Existing flux points out of the page (away from north pole).

2. Flux is INCREASING as magnet moves closer.

3. New flux must oppose this and point INTO the page.

4. Using RHR #1, for B in loop to point into page, current is CW.

The loop is expanding in a constant B-field. What direction is the induced current in the wire?

• Clockwise

• Counter Clockwise

• I = 0

Answer explanation

1. Existing flux points into the page.

2. Flux is INCREASING as area increases.

3. New flux must oppose this and point out of page.

4. Using RHR #1, for B in loop to point out of page, current is CCW.

A loop of wire (seen from the edge) is rotated away from a perpendicular B-field. What direction is the induced current in the wire ASSUMING we are looking from the left side?

• Clockwise

• Counter Clockwise

• I = 0

Answer explanation

1. Existing flux points left

2. Flux is DECREASING as the amount of area perpendicular to the field decreases.

3. New flux must oppose this and point left.

4. Using RHR #1, for B in loop to point left, current is CCW when looking from the left side towards the loop.

B-field decreases around the wire loop from what is shown What direction is the induced current in the wire?

• Clockwise

• Counter Clockwise

• I = 0

Answer explanation

1. Existing flux points out of the page.

2. Flux is DECREASING.

3. New flux must oppose this and point out of page.

4. Using RHR #1, for B in loop to point out of page, current is CCW.

A long, current carrying wire passes through a circular loop of wire. If the current is constant, what is the induced emf of the loop?

• Clockwise

• Counter Clockwise

• I = 0

Answer explanation

If the current is constant, the magnetic field is constant. If B and A are constant, flux is constant, therefore there is no induced current.

A long, current carrying wire passes through a circular loop of wire. If the current is INCREASES, what is the induced emf of the loop?

• Clockwise

• Counter Clockwise

• I = 0

Answer explanation

The generated B-field encircles the wire, just like the coil. Because it goes left on one side and right on the other, the flux ends up cancelling out. No flux means no induced current.