# Can anyone PLEASE tell me how a current balance works?

wukunlin
Gold Member
bingo! :)

Wonderful! :)

Now let me try this with (d).

The current is going to the right and I use the right hand rule, and swap both the current and field against each other, and I see that the magnetic field is pointed to the left, while the direction of the current is to the right.

I tried applying the left hand rule now, but I can't get anything because these two aren't making a 90 degree angle with each other :( What am I doing wrong?

I mean the answer is 'No movement.' Is that BECAUSE of the current and magnetic field not being perpendicular to each other?

wukunlin
Gold Member
have you learned cross product yet?

each of the vector F, B and I are cross products of the other two. The magnitude of the resulting vector is always the greatest when the other 2 are perpendicular to each other, zero magnitude when the other two are parallel, and in between there is a trigonometric relationship, for example F = BIL sin t, where t is the angle between B and I.

so that means your explanation to the lack of movement is correct ;)

I haven't but I think you're talking about the next topic I'm about to start as soon as I'm done with these questions. It's right there on the second pdf file I put up. 'Currents crossing fields', there's 'at right angles', and 'at an angle other than 90 degrees.' And there's all these components over there, so I'll come back to this after I've done that I suppose. Thank you IMMENSELY for your help! You can see I was quite confused, and now I'm not.

Just one more question,

When we were talking about finding the magnetic field strength through this set-up, you said to measure the length of the wire directly opposite the weights right? Well in the question right after the one we just did [it's there in the scanned file] the length they're telling me to consider is the sideways length. Can I use that in the regular flux density equation? Would it make any difference?

wukunlin
Gold Member
ummm, that's a bit weird....

considering it also says "the section of the conductor in the field is 5.0cm", I think the sideways lengths are just a distraction.

That's just the length of the wire inside the magnet right?

wukunlin
Gold Member
looks right doesn't it?

If B = 0.0078 T, then L has to equal 0.503 m, and I really don't know how that's supposed to come about. Unless of course the decimal in 5.0 is a misprint.

wukunlin
Gold Member
came out write when I did it, just check over your arithmetics

Using 1.962x10-3 N for force, 0.50 A for current, and 5x10-2 m for length, I get 7.85x10-2 T, which isn't the right answer. :( How'd you get it right?

wukunlin
Gold Member
the mass is 0.02g

Haha, I took the 0.02 g correctly, I just didn't convert it correctly. Used x10-2, must've pressed the wrong button. Got the correct answer!

But does this mean that I'm supposed to use the length of the wire that only passes through the magnet? [because only that can obviously be 5 cm]

wukunlin
Gold Member
yes, this force is produce only when the current is under the influence of a magnetic field, so outside the magnetic field there isn't this type of force (sorry for not clarifying this earlier)

Right, right, and right. Thank you SO much for your help. Life saver!
Good day. :)

wukunlin
Gold Member
no worries :)