# Some questions about electromegntic & EM force

1. Jun 21, 2014

### baby_1

Hello
1-here is a problem that want to specify which current loop has max electromagnetic intensity?(the correct choice is d)

(Loop radius: small loop (r) big loop(2r))
so i think choice C is correct it because of this result

and

do i think correct or not?

2-for obtain electromagnetic filed we assume that current is from electron or proton?

Thanks

2. Jun 21, 2014

### PhysicoRaj

If you observe in c, the bigger loops cancel each other, and the smaller loops also. in+in+out+out=0.

3. Jun 21, 2014

### PhysicoRaj

Try option a..

4. Jun 21, 2014

### baby_1

Thanks dear PhysicoRaj
But as I do the right hand rule for each big loop i found the MF of each of them is out of the page and they add to each other not cancel.I show each MF direction via hollow circles and the Bold.

Last edited: Jun 21, 2014
5. Jun 21, 2014

### PhysicoRaj

Do that for small loops also and you will find that they cancel. c can't be the answer. What do you think about a?

6. Jun 21, 2014

### baby_1

Hello
dear PhysicoRaj
yes in choice a two small loop cancel their MF.

but as you see the choice 'c' the current direction of each loop is different and has different MF direction.

7. Jun 21, 2014

### PhysicoRaj

How? Aren't the currents in the same direction?

8. Jun 21, 2014

### PhysicoRaj

So shouldn't the MF in different directions cancel?

9. Jun 21, 2014

### PhysicoRaj

baby_1, magnetic field is a vector field and has to be added vectorially. To put that, fields in the same direction add up and fields in the opposite directions tend to cancel out.
When you have currents in the same direction, then the produced magnetic fields are also in the same direction and add up.
When currents flow in opposite directions, the produced magnetic fields are also opposite in direction and they cancel out.

10. Jun 21, 2014

### baby_1

Hello
I try to explain more about choice 'c'
we separate choice 'c' and do Spperpostition.
the MF of two small loop is going into page(am I right?)

so it increase amount of MF intensity(B1+B2)
and the MF of two large loop is going out of page(am I right?)

(B3+B4)

as we know the amount of B3 and B4 is smaller than B1 and B2 it because of their loop radius

(radius of big loops is 2r and radius of small loops is r) so B1+B2 can remove the effect of large loops and remain some MF into page direction(am I right).

if we assume r=1 and 2r=2 we have(we ignore constants(m0/2))
B1=1/1
B2=1/1
B'=B1+B2=2(into page)

B3=1/2
B4=1/2

B''=B3+B4=1(out of page)

BT=B'-B''=1(into page)

now for choice 'd' we can do same approach

B1=1/1(into page)
B2=1/1(out of page)
B'=0

B3(large loop)=1/2

Bt=1/2

so choice 'c' is bigger than choice 'd'

11. Jun 21, 2014

### PhysicoRaj

Rather than thinking in terms of into or out of the page, think like, up or down the axis. Every picture shows an axis right? And you are asked to probably find in which choice the field has greatest intensity at the point indicated as a thick dot on the axis.

See (a): both currents are turning clockwise. So the field is added.

See (b): the upper current flows clockwise whereas the bottom one anticlockwise. So the magnetic fields are opposite and cancel out.

See (c): the two small currents are opposite and their magnetic fields cancel. Similarily the two bigger currents are opposite and their magnetic fields cancel.

See (d): both of the small currents are in anticlockwise direction and their fields add up. Plus the large current flows in same direction too! Hence all the three add up!

12. Jun 21, 2014

### baby_1

Thank you so much PhysicoRaj for your approach
is it possible to tell me about my approach?and my mistakes?

As I try to send you a private message that what reference and book you suggest to me about your approach and easiest ways to obtain electromagnetic force and filed effect , it seems you locked your account.If there is no problem could you introduce me some book and reference that you think it is suitable for me?

13. Jun 21, 2014

### PhysicoRaj

There was no major difference in the methods used by you and me. You just needed some refinement in the method.
I am no expert in suggesting study materials, so I advice you to post it in the 'physics learning materials' forum: https://www.physicsforums.com/forumdisplay.php?f=160 [Broken].
Good luck.

Last edited by a moderator: May 6, 2017
14. Jun 21, 2014

### baby_1

Hello
Dear PhysicoRaj