# Homework Help: Solenoids and Lenz's Law

1. Jan 13, 2012

### wordgames

1. The problem statement, all variables and given/known data
Hi. My questions are on Lenz's Law.
1.
We have a bar magnet indicated by the [N | S]. It is moving to the left of a solenoid that is connected to an ammeter. Two points on the circuit are labelled X and Y. ( A ) is just an ammeter. The question is.. What direction does the current flow when the magnet moves?
(i.e. X to Y or Y to X)

[N | S] {||||||||||}
<----- |__( A )___|
..........X ............Y

(ignore the periods, they were necessary to keep the formatting for the diagram)

2. Relevant equations
Lenz's Law - coil acts to oppose change in B

3. The attempt at a solution
Here's my crack at it. As the magnet moves away, there is less flux cutting the solenoid. The magnet's field goes from N -> S so the solenoid is getting a reduction of flux going <-. Then, using the Right Hand Screw/Grip Rule, the direction of the field is <- so current will flow going in to the page. I don't quite understand which way the current is supposed to flow from here (X to Y or Y to X) and the diagram given to me was pretty shoddy.

Also, for these type of problems involving loops, wires, solenoids, magnets being moved around and Lenz's Law, what type of Right-Hand Rule is used? My textbook instructs me to use the Screw/Grip Rule, while some websites say different things. The Grip Rule is also changed many when working with solenoids etc. Thumb changes from direction of B to direction of current. It is quite confusing.

1. The problem statement, all variables and given/known data
2.
Same sort of problem. We have a cardboard tube, and on it are wound 2 circuits. One is labelled A, and consists of a solenoid in series with a switch labelled P. The secondary circuit is labelled B. Two points on B are chosen to be X and Y. When the switch P is closed, what direction will the current flow in B? (X to Y or Y to X)
............A...........................B
[...|||||||||||||............|||||||||....]<- The cardboard tube
....|_ \_(+/-)__|............|__(A)__|
....P.............................X..........Y

(Again, the periods have no significance, they are present for formatting)

2. Relevant equations
Same as above. Lenz's Law etc.

3. The attempt at a solution
Once the switch P is closed, the current in A will travel from the +ve terminal to the -ve terminal. Using the Grip Rule for solenoids, North is in -> for the solenoid in A. This means that the change in magnetic field for the solenoid in B is an increase in flux in ->. To oppose this, the induced magnetic field will have to be going <-. Using the Grip Rule again, this time thumb pointing <-, current will travel in to the page and once again, I'm stuck. Which way does current flow (X to Y or Y to X)?

Answers from the book are 1. X to Y and 2. Y to X, but these have little significance to me if I don't know the reason why.