# Detailed Physics of the D.C. Motor

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1. Oct 23, 2014

### Zachary Markham

So I have been curious as to the exact workings of a simple D.C. electric motor, and the transformation of electric energy into motor power. I think I have a basic understanding of the topic but there are numerous gaps in my understanding. Firstly, I think I am correct in saying that as an electric current goes through, say, a wire, it creates an electromagnetic field around said wire, a field that rotates around the wire clock-wise if you are looking at the negative end from the positive. What about electrons travelling through the wire makes it create that field? Why is the field oriented the way it is? Lastly, how does that field cause the wire to physically move as it does when it is put inside a larger magnetic field, I don't see how the larger field could repulse the wire's field that is wholly inside of it. It seems like the positive and negative parts of the smaller field would cancel out for the larger field and make it inert?

2. Oct 23, 2014

### Simon Bridge

The field does not "rotate" like, say, a top rotates. The arrow that gives the direction of the field at a point is an imaginary compass needle ... the "rotation" is just how a compass needle gets deflected when it is close to a wire.

The external fixed magnetic field in your example is usually thought of as interacting with the electric field of the electrons moving in the wire. When a charge $q$ moves with velocity $\vec v$ in a magnetic field $\vec B$ it experiences a force $\vec F = q\vec v\times\vec B$ ... since the charges, in this case, are confined to the wire, the whole wire gets a shove.