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

I looked for hours and hours and I haven't been able to find any video or link [there's one REALLY bad video on youtube] and I REALLY need to know how a current balance works because that specific chapter that I'm studying on electromagnetism has questions mostly coming up with the current balance apparatus and I don't get it at all.

All I know is that it's used to measure the magnetic flux density. That is ALL.

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wukunlin
Gold Member
by current balance do you mean ampere balance?

basically the current through the coils create magnetic force, the balance balances the magnetic force with adjustable weight force. From this weight force which is equal in magnitude to the magnetic force, allows us to work out the amount of current running through the coil.

Yeah but see I'm not trying to find out the amount of current, because in my case I've been allowed to have an ammeter. I have to find out the magnetic flux density.

So in this case:

Get current from the ammeter,
Measure the length of the wire you're using. [I'm not sure which fixed points I'm supposed to be measuring from btw. It'd be great if you could help me with that]
and get the value for force from F=mg, with m being the mass the balance shows and g being 9.81.

Use the B = F/Ixl and you'll get the magnetic flux density. Now I hope I won't have a lot more problems with this question.

[If this isn't helping I can scan the page from which I'm studying this topic.]

The real problem is when the book says, 'Another version of a current balance is shown in Figure 25.16. This consists of a wire frame which is balanced on two pivots. When a current flows through the frame, the magnetic field pushes the frame downwards. By adding small weights to the other side of the frame, you can restore it to a balanced position.'

If you want I can scan that page for you.

wukunlin
Gold Member
yeah, the page would be nice.

I don't really see where you have a problem though, aside from the undefined length of the wire.

Be back in a little while then [book's kinda tight, so it'll take a few tries to scan it correctly].

The problem is in the 'Another type of current balance.' Well, in any case, I'll put it up in a little while so wait a bit. Thanks a lot!

That question at the end. I need to know how to be able to solve questions like that.

I think I'll be able to do that once I understand this second type of balance. If you need to see those four arrangements as well, then ask for them and I shall deliver.

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wukunlin
Gold Member
I don't think these questions are as complicated as you think, it just looks like a typical question involving current carrying wire.

using F = BIL, L being the length of the wire opposite to the small weight in the diagram at the bottom of the page, because when you look at how the pivots are positions, forces generated by the other two lengths won't affect the position of the balance

i KNOW they aren't complicated man. Problem is 'I' just don't quite seem to get them :(

That question at the bottom, it says 'In the examples shown in the diagrams in Figure 25.17, which current balances will tilt? Will the side carrying the current tilt upwards
or downwards?'

Fig 25.17 has four balances exactly like the ones in Fig 25.16 [the diagram at the bottom of the page], and one turns one way, one doesn't turn, one moves sideways, and the other tilts down. I can't predict any of those movements. That's what I'm asking for. Help me predict which way the wire-frame will move.

I'll even put up the stupid Fig. 25.17. Back in a sec

wukunlin
Gold Member
well, for the tilting one, have you learned the left hand rule or equivalent to remember the direction relationship between F B and I?

Also, in case you missed it like I did, if you look closely at the magnet in 25.16, there are tiny blue arrows pointing from one side the of the magnet to another, indicating the direction of the magnetic field (or magnetic flux density as some people call them)

Wow that pdf really magnified the page didn't it? I looked at it in the book and it was still really hard to notice! Bad colour choice I guess, haha.

In any case, of course I know the left hand rule, and applying it here, the frame is going to go downwards. My question for you after this is (just tell me if I'm right or not),

The frame goes down, and we add the weights on the other side to tip the frame back into its original position. Using F=mg, we find out the force, and [let's just say we can use an ammeter, even if it's not there in the photo] using an ammeter we can find out the current, and also measure the length of the wire opposite the weights, we can find out the strength of the magnetic field. Correct?

And just to make sure I understand, could you do this one question with me? It's the same one. But I don't think I get these. I'll try again right now of course, but I'm sure I'm gonna need help.

This isn't going against board rules is it?

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wukunlin
Gold Member
yup, what you described in post #10 is exactly right. :)

That is one HUGE burden off my back! THANK you.

Now I'm gonna try these questions and come up with my thinking and answers and then you just point out where I'm wrong. I think that's quite fair :)

wukunlin
Gold Member

Okay so the question.

For (a) [in the scan0003 file], according to the left hand rule, the direction of current is towards the right, and, I don't understand exactly how the magnetic field is going to pan out. I mean they're supposed to be repelling each other but the field lines are going to be such that there's an empty zone [I forgot but there was a name for this specific thing, where there's no magnetic forces inside a magnetic field]. So I can't predict what exactly happens. The book says, 'Section of wire in field tilts down.'

For (b), all I can deduce is, that the direction of the current is from the start of the solenoid to the end of the solenoid [in that direction I mean], and using the right hand rule, the magnetic field is going to be clockwise. So [as far as I know], the start of the solenoid will be the south pole and the other end the north pole. I don't know what to do after that :( The book says, 'Tilts down.'

For (c), okay this is quite simple. Using the left hand rule, the current is going as before, to the right, while the magnetic field is going from north to south [obviously], and applying the rule to that, the wire will move in TOWARDS the magnet. The book says, 'Will try to move horizontally, into horseshoe.'

and lastly for (d), the current is moving towards the right, and using the right hand rule, you get that the magnetic field is going to be clockwise, meaning the start of the solenoid will be south and the end of it will be north. And again, as with the other example involving the solenoid [part (b)], I don't know what to do with this.

wukunlin
Gold Member
a) I believe they mean for both of the magnets' south side is pointing to the top of the page

b) use the right hand grip rule to figure out the magnetic field coming out of the solenoid

c) you got this right

d) see b) and remember the left hand rule is derived from cross product of vectors

(a), I don't understand. If the both the fields are pointing upwards, then why does the wire tilt downwards?

(b) and (d), I DID use the right hand rule as I stated in the post. But the only things I know that I can tell using that rule is the direction of the current, the poles [which is north which is south], and whether the magnetic field is going in a clockwise or anti-clockwise direction. And I've done all that. What do I do next with this?

wukunlin
Gold Member
a) well...

Code:
magnet        field          magnet
____________               ____________
|S        N|  -------->    |S        N|
____________               ____________
no?

b) and d), you can actually swap the current and magnetic field around so your fingers represent current in the solenoid and thumb represent magnetic field direction

(a) Whoops! Wow I'm on a roll with these small mistakes here! In any case, it's from north to south, and using the left hand rule, with the magnetic field coming towards you and your second finger going to the right, the thumb is in a downwards position, and so there we have the correct answer! Easy!

(b) and (d), Okay I've never tried doing that, neither have I ever heard of it. So, if I swap the current and the field, which way is each supposed to go? Let's just concentrate on (b). In that case, what's supposed to happen? The current's supposed to go upwards/forward and the field goes to the left? [because that's what I get when I use my curled fingers to represent the current]

wukunlin
Gold Member
in the case of b) the current is flowing anticlockwise in the solenoid

When I do that the magnetic field/my thumb is pointing right back at me. I don't know how that happened. :(
I mean I don't get what's happening.

wukunlin
Gold Member
it means its pointing at the gap of the balance

I THINK I understand but, umm, why does the wire tilt down then? I mean the field is simply pointing backwards. Shouldn't the wire be trying to go in that direction then?

wukunlin
Gold Member
well, with the solenoid involved it becomes a 2 step questions.

-use the grip rule to figure out the direction of the magnetic field
-now that we now where the magnetic field is heading, use the left hand rule to find the direction of the force

don't mixed up the current direction of the balance and the solenoid

Okay you are an awesome teacher! [at least with respect to this]

So using the left hand rule now [second step], the first finger/magnetic field, is pointing towards me, and the second finger/direction of current, is to the right, with that arrangement, my thumb points downwards. And we get the correct answer. Is this correct?