Force on a Wire: Magnetic Field and Direction of P and R - Homework Solution

[Cici2017]

Homework Statement

Homework Equations

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?

Please help~ I know this is soooo wrong :([/B]

 

[vela]

What's this left hand rule?

 

[Cici2017]

What's this left hand rule?

Fleming's left hand rule...

 

[vela]

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.

 

[Cici2017]

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

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

 

[vela]

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

 

[Cici2017]

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

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

 

[haruspex]

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

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.

 

[Cici2017]

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.

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?

 

[haruspex]

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?

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

 

[Cici2017]

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

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

 

[haruspex]

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

Yes.

 

[ElectricRay]

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

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

 

[Cici2017]

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

haha Yeah! It's like discovering a new world

 

[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 already pretty old