# DC Electromagnetic Motor-small question

1. Jun 10, 2010

### moshrom

1. The problem statement, all variables and given/known data
[PLAIN]http://img695.imageshack.us/img695/206/95144436.jpg [Broken]

2. Relevant equations
Hand rules

3. The attempt at a solution
This is a practice question with the answer already given in the diagram. I know how to draw the coils and use the left hand rule to find the poles. I do not understand b), though. I know the two north poles repel, but why does A move forward instead of backwards?

Thanks.

Last edited by a moderator: May 4, 2017
2. Jun 10, 2010

### collinsmark

I think it has to do with manner in which the brushes are drawn with respect to the rest of the commutator. See the part of the diagram near where "Front" and "Back" are written.

Imagine what would happen if it moved in each direction (A moved forward, and A moved backwards). Consider the impact of the way the brushes/commutator are drawn.

3. Jun 10, 2010

### moshrom

Oh okay, I think I get it. Normally the direction can be determined if the armature is drawn slightly turned. I assumed the 'front' and 'back' labels were written as part of the answer, I guess they were originally given. Thanks.