# lenz's law

Discussion in 'MCAT Study Question Q&A' started by destroyMCAT, Aug 7, 2011.

1. ### destroyMCAT

Joined:
Aug 7, 2011
Messages:
21
0
Status:
Pre-Medical
If a loop enters a magnetic field that is going into the page, according to the lenz's law there would be a gain of magnetic field that is going out of the page.. and the current would be counterclockwise.

So, using right hand rule.
The velocity is going from left to right (thumb points that direction)
fingers = magnetic field so it points out of the page
and my force points down... which would cause an clockwise current. What am I doing wrong?

2. ### AugustMCAT 5+ Year Member

Joined:
Jul 3, 2011
Messages:
161
4
The new field opposes the change in flux. If you put a loop into a field, there is an increase in the magnetic field pointing into the page. So the new field must point out of the page. Don't use the 3 finger right hand rule. Just use the right hand rule of you grabbing the wire. If the field is coming out of the page, then your thumb must be pointing right, hence a counterclockwise current.
The 3 finger right hand rule also confirms this. Your velocity which you said is left to right is essentially the current. Be consistent with your analysis. I like to have the magnetic field use the perspective of WITHIN the loop.

3. OP

### destroyMCAT

Joined:
Aug 7, 2011
Messages:
21
0
Status:
Pre-Medical
Can you elaborate on this? thanks

edit: How do you know when to use the grabbing the wire right hand rule vs the 3 finger one?

#3
Last edited: Aug 7, 2011
4. ### AugustMCAT 5+ Year Member

Joined:
Jul 3, 2011
Messages:
161
4

You are asked for the magnetic field. Force is not in question here, so you don't need the 3 finger rule.

Generally, I always use the center of the loop for my perspective of the magnetic field. You were correct in saying velocity is from left to right. This velocity IS CURRENT. The movement of particles. So left to right (on the bottom half, using the center of the loop as perspective) creates a counterclockwise current.

We’ve been on the Internet for over 20 years doing just one thing: providing career information for free or at cost. We do this because we believe that the health education process is too expensive and too competitive.

We believe that all students deserve the same access to high quality information. We believe that providing high quality career advice and information ensures that everyone, regardless of income or privilege, has a chance to achieve their dream of being a doctor.