Electromagnetism - Lenz's Law

Questions 54-55.

A rectangular loop of wire with dimensions shown above is coplanar with a long wire carrying current I. The distance between the wire and the left side of the loop is r. The loop is pulled to the right as indicated. 

What are the directions of the induced current in the loop and the magnetic forces on the left and the right sides of the loop as the loop is pulled? 

	Induced Current	Force on Left Side	Force on Right Side
A.	Counterclockwise	To the left	To the right
B.	Counterclockwise	To the left	To the left
C.	Counterclockwise	To the right	To the left
D.	Clockwise	To the right	To the left
E.	Clockwise	To the left	To the right

(GR9277 #54)

Solution:

Lenz Law: The tendency of nature to resist any change in magnetic flux passing through a loop of wire.

When the loop is pulled away, it feels a decreasing current and resists the change of magnetic flux by inducing an increasing current in the same direction as I. 

→ The induced current in the loop is clockwise. A, B, C are FALSE.

Magnetic force between two currents

Since the current in the loop is clockwise:

• current in the left side of the loop is parallel to the long wire, they are attracted to each other → the force on the left is to the left.
• current in the right side of the loop is antiparallel to the long wire, they are repelled to each other → the force  on the left is to the right.

Answer: E

Electromagnetism- Len'z Law

Three wire loops and an observer are positioned as shown in the figure. From the observer’s point of view, a current I flows counterclockwise in the middle loop, which is moving towards the observer with a velocity v. Loops A and B are stationary. This same observer would notice that

1. clockwise currents are induced in loop A and B
2. counterclockwise currents are induced in loop A and B
3. a clockwise current is induced in loop A, but a counterclockwise current is induced in loop B
4. a counterclockwise current is induced in loop A, but a clockwise current is induced in loop B
5. a counterclockwise current is induced in loop A, but no current is induced in loop B

(GR0177 #42)

Solution:

Lenz Law:  The tendency of nature to resist any change in magnetic flux passing through a loop of wire.

When the middle loop moving toward the observer A:

• Loop A feels an increasing current and resists the change of magnetic flux by inducing a decreasing current in the opposite direction (clockwise).
• Loop B feels a decreasing current and resists the change of magnetic flux by inducing an increasing current in the same direction (counter-clockwise).

Answer: C