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

1. Lenz’s Law: B-fields form to produce 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 S A. a clockwise current. B. no current. C. a counterclockwise current.

2. The North pole of permanent magnet moves toward (into the page) the center of this loop of copper wire. N S A current is induced in the loop which travels clockwise counter-clockwise

3. The North pole of permanent magnet is pulled away (out of the page) the center of this loop of copper wire. N S The magnetic field created by the current induced in the loop creates a __ -pole facing us. North South

4. S N Moving a magnet closer to a conducting loop S N which lies flat (as pictured), its open loop NOT 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 current in the loop could build its own magnetic field to reduce the increasing field?

5. S S A B N N copper 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

6. A copper coil is set just behind the wire loop of this circuit. As the switch is closed, + - • 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.

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

8. A copper coil is set just behind the wire loop of this circuit. As the switch is opened, • 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.

9. s N As this rectangular loop rotates, the magnetic flux through its open face A) increases. B) decreases. C) remains constant.

10. s N To slow this decrease of field down, the loop must generate its own field lines A) pointing right. C) pointing up. B) pointing left. D) pointing down.

11. As this conducting coil is slowly rotated one quarter-turn (90o) in the direction shown, while within the B field pointing right, in what direction will current be induced? 1) clockwise 2) counter-clockwise 3) no induced current

12. b N S 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. 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.

13. b N S 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. 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.

14. Some Answers A. a clockwise current. 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. clockwise QUESTION 2 Current 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.. B. South QUESTION 3 Notice how a South pole would try to keep the magnet in place…resist its motion away! 1) A 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! QUESTION 4 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.

15. QUESTION 2 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 clockwise induced current. 1) a clockwise current The coil tries to replace the flux that starts to disappear as soon as the switch is opened. QUESTION 3 QUESTION 4 B) decreases. QUESTION 5 Turning away from the poles…and no longer facing them squarely, reduces the flux. A) pointing right. QUESTION 6 1) clockwise 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). B.is decreasing at this moment. QUESTION 8 A.Out of the screen, toward you. QUESTION 9 To replace the loss of flux (pointing right) our Right-Hand-Rule (with thumb to the right) shows our fingers curling up in back, out at top.