Lenz’s Law: B-fields form toproduce a frictional resistance against

the changes that create them.

Generated current creates B-fields which OPPOSE the forces that create the current!

Inertia for electric and magnetic fields!

A conducting loop is pulled away from the South pole of a permanent magnet. As viewed

by the observer shown, the loop develops

A. a clockwise current. B. no current.C. a counterclockwise current.

The North pole of permanent magnetmoves toward (into the page) thecenter of this loop of copper wire.

A current is induced in the loop which travels

A. clockwiseB. counter-clockwise

The North pole of permanent magnetis pulled away (out of the page) thecenter of this loop of copper wire.

The magnetic field created by the current induced in the loop creates a __ -pole facing us.

A. NorthB. South

S N

Moving a magnet closer to a conducting loop

which lies flat (as pictured), its open loopNOT facing the moving magnet.

Although the magnet field near the loop, is in fact increasing,

none of the increasingly dense field lines

(the sign of increasing field strength)passes through the loop’s enclosed area.

Is there ANY direction an induced currentin the loop could build its own magnetic

field to reduce the increasing field?

copper

N

S

N

S

A bar magnet is held above the floor and dropped. Nothing is between magnet A and the floor. Magnet B falls through a copper loop. Which magnet falls faster?

A) A

B) B

C) same speed for both

A copper coil is set just behind the wire loop of this circuit.

As the switch is closed,

1) a clockwise current is induced in the copper loop.2) a counter clockwise current is induced in the copper loop.3) no current appears in the copper loop.

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A copper coil rests flat on a tabletop next to the wire loop of this simple circuit. As the switch is opened,

+

1) a clockwise current is induced in the copper loop.2) a counter clockwise current is induced in the copper loop.3) no current appears in the copper loop.

A copper coil is set just behind the wire loop of this circuit.

As the switch is opened,

1) a clockwise current is induced in the copper loop.2) a counter clockwise current is induced in the copper loop.3) no current appears in the copper loop.

A) increases.B) decreases.C) remains constant.

As this rectangular loop rotates, the magnetic flux through its open face

A) pointing right. C) pointing up. B) pointing left. D) pointing

down.

To slow this decrease of field down, theloop must generate its own field lines

As this conducting coil is slowly rotated one quarter-As this conducting coil is slowly rotated one quarter-

turn (90turn (90oo) in the direction shown, while within the ) in the direction shown, while within the BB field field

pointing right, in what direction will current be induced? pointing right, in what direction will current be induced?

1) clockwise 1) clockwise

2) counter-clockwise 2) counter-clockwise

3) no induced current3) no induced current

As this copper loop rotates (counter-clockwise) it turns its square face down, away from the N-pole at left. Segment b

(highlighted in orange) rides down.

b

The external magnetic flux (number of field lines) passing through the open loop

A. is increasing at this moment.B. is decreasing at this moment.C. remains constant.

To resist this decrease in magnetic flux,current is established, flowing which

way through the segment b?

A. Out of the screen, toward you.B. Into the screen, away from you.C. Current cannot flow through b.

As this copper loop rotates (counter-clockwise) it turns its square face down, away from the N-pole at left. Segment b

(highlighted in orange) rides down.

b

QUESTION 4 The induced current in the loop represents ENERGY consumed. It must come from

the falling magnet, but that means some of the potential energy used in falling goes into current, NOT kinetic energy; it doesn’t build quite as much speed!

1) A

The current induced as the magnet falls toward it, turns the ring into a little electromagnet…with a N-pole at its top…repelling (and so slowing down) the magnet. Once it falls through, the induced current reverses, trying to replace the flux being lost, with an N-pole now at its bottom, attracting (and dragging back) the S-end of the magnet falling away…STILL slowing it down.

QUESTION 1

Moving away reduces the flux through the loop. In an effort to try and maintain it, current is induced. To replace field lines pointing to the right (into the South pole of the magnet) the current has to run clockwise through the ring.

A. a clockwise current.

A. clockwiseCurrent is induced in an effort to cancel out the increased intensity of the magnetic filed (in this case an increase in the number of flux lines pointing into the page. To reduce (or cancel) these field lines requires the production of new flux lines pointing out of the page..

QUESTION 2

B. SouthQUESTION 3

Notice how a South pole would try to keep the magnet in place…resist its motion away!

Some Answers

QUESTION 7

Current needs to offset the right-pointing field lines that enter the loop as it turns,By generating its own B-field pointing left (as the loop turns).

To replace the loss of flux (pointing right) our Right-Hand-Rule (with thumb tothe right) shows our fingers curling up in back, out at top.

QUESTION 8

QUESTION 9

1) clockwise1) clockwise

B. is decreasing at this moment.

A. Out of the screen, toward you.

QUESTION 5 B) decreases.Turning away from the poles…and no longer facing them squarely, reduces the flux.

QUESTION 6 A) pointing right.

QUESTION 2

QUESTION 3

QUESTION 4

2) a counter clockwise current A clockwise current is created in the black wire loop, establishing a magnetic field suddenly pointing down into the center of the copper coil. To counter this, the coil will try to create a field of its own pointing out. It does this with a counter clockwise current.

2) a counter clockwise current While current flows the wire loop’s field points into the screen within its center, but doubles back outside the loop. That means where the copper coil lies, the field initially points out. When the switch is opened, this field suddenly vanishes. To counter the disappearance of flux, the coil tried to create its own, which requires a clockwiseinduced current.

1) a clockwise current The coil tries to replace the flux that starts to disappear as soon as the switch is opened.