# Direction of magnetic field

1. Aug 2, 2017

### Cici2017

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
using the left hand rule, second finger points the direction of current, thumb points direction of motion and first finger the direction of magnetic field. But If direction of current and motion is the same, what do I do? Thanks~

2. Aug 2, 2017

### Merlin3189

In the LHR if C and M are parallel, there is zero force.

But you are getting a bit confused about how LHR is applied. If a conductor is in a magnetic field LHR tells you which way the force (or Motion of the conductor) will be. But the electrons, constrained within the conductor, are moving in the direction of the current (though in the opposite sense for conventional current.)

Here the (original) motion of the electrons is not the "thumb" direction : the (original) motion of the electrons is in the current direction (but opposite sense.) It is perpendicular to the field from the wire current, so you will see a force on the electron represented by the "thumb" direction (and sense.)

Maybe you should consider the electron as if it were travelling along a conductor/wire. What force would that conductor experience?

3. Aug 2, 2017

### Cici2017

If the electron is travelling along a wire, in the same direction as the existing wire. The 2 wires attract. So the force is A? but the answer is D. Sorry, can you please explain a bit more, I am not really good with these ideas.

4. Aug 2, 2017

### Merlin3189

You have to be careful, as I hinted before, about the direction of current.
In the wire shown, the current is clearly shown going in direction B. So which direction are electrons flowing in that wire?
Now the electron is also moving in direction B. So if it were an electron moving along a wire, what direction would we say the current was going?

The LHR applies to the conventional direction and sense of current flow.

5. Aug 2, 2017

### Cici2017

So the conventional current is the opposite, so direction of C. So the 2 repel each other, thus force is in direction of D?

6. Aug 2, 2017

Yes.

7. Aug 2, 2017

Thank you!

8. Aug 2, 2017

### Merlin3189

As a BTW, I always forget which is LHR and which is RHR.
I think of the SI definition of the Ampere. There is a force between parallel conductors (hence parallel currents or charge flows.)
The snag is remembering whether it is attractive or repulsive, and for that I go back to Michael Faraday with his magnetic lines of force analogy. He imagined them a bit like elastic bands trying to be as short as possible, but being exclusive, unable to cross and repelling each other sideways. Once I sketch these in, the direction of force is easy. Maybe not Physics as we know it, but it helped him to have great insights in a field that was not well understood at the time.

9. Aug 2, 2017

### Cici2017

Yes, my teacher also mentioned this way of visualising it.

10. Aug 2, 2017

### Merlin3189

I see you have another Q. Have you tried sketching in the lines there?

11. Aug 2, 2017

### Cici2017

Yes, I have. And I think the force is to the right. Am I right?