# Force on a wire

1. Jul 29, 2017

### Cici2017

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

2. Relevant equations

3. The attempt at a solution
Right hand rule: magnetic field of P on Q is out of the page, and magnetic field of R on Q is also out of the page.
Left hand rule: Force of P on Q is to the left and R on Q is to the right?

2. Jul 29, 2017

### vela

Staff Emeritus
What's this left hand rule?

3. Jul 29, 2017

### Cici2017

Fleming's left hand rule...

4. Jul 29, 2017

### vela

Staff Emeritus
Explain how you're applying the rule since you're apparently doing it wrong somehow.

Also, I've never heard of Fleming's left hand rule.

5. Jul 29, 2017

### Cici2017

So for wire P, magnetic field around it is clockwise, and for wire R, it is anti-clockwise.

6. Jul 29, 2017

### vela

Staff Emeritus
The field directions you deduced are fine. How did you reason out the direction of the forces?

7. Jul 30, 2017

### Cici2017

for the force of wire P on Q, magnetic field is out of the page, current is down, so force is to the right.

8. Jul 31, 2017

### haruspex

Rather than using handedness rules, I find it much easier to visualise the field lines.
If you have currents the same direction along two parallel wires, the field lines run the same way around them, so at a sufficient distance merge to become one system of enveloping lines. The lines like to shrink in length and spread apart from each other, so they pull the wires together.
With the currents in opposite directions, the two sets of lines run opposite ways around, leading to a bunching up between them. That pushes the wires apart.

9. Jul 31, 2017

### Cici2017

Yes, I understand that.
So how do I then work out the direction of force on wire Q due to P and R?
If one attract and the other repel, doesn't that mean the middle wire just stay where it is?

10. Jul 31, 2017

### haruspex

If you get pushed on one side and pulled on the other, do you stay where you are?

11. Jul 31, 2017

### Cici2017

No, I don't. OMG I got it. Thank you so much! So the answer is D right?

12. Jul 31, 2017

### haruspex

Yes.

13. Jul 31, 2017

### ElectricRay

Thats a nice feeling when you finally got it right? :)

14. Jul 31, 2017

### Cici2017

haha Yeah! It's like discovering a new world

15. Jul 31, 2017

### ElectricRay

It is fun to see that some people give some hints and finaly you get the answer. I agree it is like discovering a new world although it is allready pretty old