# Homework Help: Lenz's law(?)

1. Jan 18, 2006

### Solidmozza

Hi everyone.
At the moment im doing motors and generators in physics, in particular Lenz's Law.
I understand most of the stuff, but im really just stuck on 2 questions. If you could help me, I would really be appreciative (explanations, not just answers preferable).
Q1) A flexible metal loop is perpendicular to a magnetic field as shown. It is distorted to the shape shown in the other diagram. Is the direction of the induced current in the loop clockwise or anticlockwise?
X X X X X X <---circle inside there.... then turns into an ellipse.
X X X X X X
X X X X X X
X X X X X X
X X X X X X
Q2) In what direction, clockwise or anticlockwise, is the induced current in the loop of wire?
(circle) |
----------| current heading south, and increasing.
----------|
----------|
----------|
*note, the --- are space fillers*
Help MUCH appreciated!
Thanks again.

2. Jan 19, 2006

### Hootenanny

Staff Emeritus
Have you learnt Flemming's left hand rule?

3. Jan 19, 2006

### Solidmozza

Yep, I sure have.
I must be missing something big here, and I know it.
Still puzzled :(

4. Jan 19, 2006

### daniel_i_l

In these kinds of problems you can use Lenz's Law by first checking if the magnetic flux gets bigger or smaller, the inducted current will generate a magnetic field opposing the change. For example, in the first problem the area gets smaller so the flux does to. Now the current in the loop will generate a field in the direction of the current field in order to make the flux bigger. So what cirection is the current in the loop if it generates a field pointing inwards?

5. Jan 19, 2006

### Solidmozza

Daniel - I initially thought that the magnetic flux was decreasing in Q1 because the area was reducing, but I wasnt quite sure! Thus, the answer would be it flows clockwise for that one.

But the 2nd one im still a little stumped. How can you make it so that the current flows in the circle to oppose the increasing current? I, again, may be staring at the answer but not realising it!
Thanks for your efforts so far, much appreciated!

6. Jan 19, 2006

### James R

For the second one, what kind of field does the wire with the current produce inside the circular loop? Over time, does that field increase or decrease? What about the flux through the circular loop? Then, what does Lenz's law say about it?

7. Jan 21, 2006

### Solidmozza

:( I'm not sure what type of current the wire with the current produces in the circular loop... I need this explained, please =D. Once I understand this sort of thing I think I will be ok..
Here is my thinking - The wire with the current creates a magnetic field according to the right hand grip rule - then im lost as to which direction it creates in the loop, and why it creates in the loop.. ugh :(

8. Jan 22, 2006

### Solidmozza

Just bunting this up, because I really need help with this - just the answer and an explanation and I will be happily on my way...

Really appreciate it!

9. Jan 26, 2006

### James R

Sorry for the delay.

You have a current heading down the page in a wire, and a circular loop to the left of the wire. The current is increasing with time.

The current in the wire produces a magnetic field directed into the page on the left of the wire and out of the page on the right. So, the circular loop has magnetic field from the wire going through it, directed into the page. This field is increasing with time, because the current generating it is increasing.

Lenz's law says the loop wants to generate its own field which opposes the change in flux through the loop. Since the flux is increasing, producing more field into the page, the loop wants to generate its own field directed out of the page. An anticlockwise current is therefore induced around the loop to generate that field.

Does that help?