# Homework Help: Magnetic circuits

1. Mar 24, 2014

### Mutaja

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

Given the attached figure.

Draw the missing wires in order to connect the turns in series. Add a battery to get the polarity as shown. Lastly, show the flux and its direction with a dotted line.

2. Relevant equations

3. The attempt at a solution

I'm not sure I understand the question too well.

In our book, and lecture notes, the only magnetic series circuit we have is the attached fig. 2.

Surely to make the fig. 1 a series circuit, I would think that the magnet need a connection like the one in fig. 2 does:

But they aren't asking for that, so right from the start I'm probably wrong here. They ask me to draw any missing wire connections to connect the turns in a series connection. The only thing I can think of in regards to the turns is this:

Does anyone know what I'm on about here? Any help what so ever will be greatly appreciated.

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2. Mar 24, 2014

### Staff: Mentor

It would appear that they want you to add a battery and wiring, connecting to the existing turns, in order to produce the given magnet polarity (N and S poles). Look up the Right Hand Rule for solenoid polarity, it'll apply to the individual sets of turns.

3. Mar 24, 2014

### Mutaja

The Right Hand Rule for solenoid polarity will give me the direction of the current in my case, as the polarity is given?

My problem says this: Draw the missing wires to connect the two sets of turns in series. Then attach a battery so that the polarities are as shown. Lastly, show the flux and its direction with a dotted line.

I would assume the magnet itself connected the two sets of turns in series, but that's obviously wrong.

4. Mar 24, 2014

### Staff: Mentor

There are two circuits involved. One is the magnetic circuit comprising the flux, the other is the electrical circuit which runs through the wiring and produces the magnetic field.

You want to begin by creating the electrical circuit which will produce the required magnetic polarities. The right hand rule will guide you for choosing the correct current directions in the coils.

5. Mar 24, 2014

### Mutaja

My book doesn't cover such topics, not does my lecture notes. The right hand rule, yes, but there's not an example even remotely close to my problem here - that frustrates me. I really want to show some sign of progress, but I'm sorry. I can't figure this out.

Is it that simple, that I'm supposed to just know it? Or is it heavily implied by some of the earlier work I've done? I don't know, but I will keep looking.

Thanks for helping me out, I really appreciate it. I'm just sorry I'm not capable of figuring out what to do next.

6. Mar 24, 2014

### Staff: Mentor

It is odd that your book doesn't cover the magnetic polarity of a solenoid. Usually there's an illustration of a coil with a current direction and resulting magnetic polarity shown. Often the applicable right hand rule is described at the same time.

If your text is failing you, then you might turn to the web and do a search on the related terms: solenoid magnetic polarity right hand rule. Should be loads of info there.

7. Mar 24, 2014

### Mutaja

It covers the magnetic polarity of a solenoid, in terms of the right hand rule you mentioned. You know, direction of current -> thumb points towards the northern polarity. It probably covers everything I need in order to solve these problems, but I can't seem to understand it.

Lets get back to scratch. I'm looking to create the electrical circuit which will produce the required magnetic polarities. Do I want to just complete the wiring for the two sets or turns, and make it one set of turns? If I do that and apply the right hand rule, I will have the direction of the current, going in at the northern pole, and out on the southern pole. Will this help me in any way?

If not, and I should treat the two sets of turns as that - two sets - I get this:

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8. Mar 24, 2014

### Staff: Mentor

Your second diagram is the right start. You don't want to add more coils to the magnet, merely wire up the existing coils with external wiring and a battery in order to produce the required current flows. So take your arrows that indicate the currents and give them a wiring path (circuit) to get the job done.

9. Mar 24, 2014

### Mutaja

Thanks so much.

Just a quick question first.

The problem statement states "draw the missing wiring so that the sets of turns are connected in series". I think that's a understandable translation. What's meant by that? That's the reason I thought the first one was correct.

Edit: I use one battery, and then wire the two sets of turns in series through the battery?

10. Mar 24, 2014

### Staff: Mentor

The "missing wiring" refers to the external wiring you want to connect in order to form the electrical circuit that drives the coils. By "connected in series" they mean that the coils will be wired up as a series circuit (the alternative being a parallel connection) with the voltage source.

While your first diagram did indeed connect the coils in series, it did so by adding additional coil turns. You want the added wiring to be external to the magnet, not part of it.

11. Mar 24, 2014

### Mutaja

Alright, I think I understand. My reasoning behind this is current flows from positive to negative. Therefore, the current going into the sets of turns has to be positive, and the current going out of the turns has to be negative. That gave me this picture. And I obviously apologize for the poor drawing, but the concept should be sound I hope.

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12. Mar 24, 2014

### Staff: Mentor

Ah. While that would work to produce the correct current directions in the individual coils, you've connected the coils in parallel with the battery, not in series. See below:

Note how in a series connection there is one continuous current that flows through both coils, while the parallel connection supports two separate currents, one for each coil.

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13. Mar 24, 2014

### Mutaja

Hmm, I overlooked that fact.

If I connect them like this, I pretty much get the first option in post #7.

My other option is to connect both points on the northern pole to +, and both points on the southern pole to -. Like this:

To be fair, I can't tell which of them would work, but if neither will work, I'm really lost hah.

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14. Mar 24, 2014

### Staff: Mentor

Yes
That's correct!

That won't work because there is no closed electrical path (circuit) between the battery terminals; no current at all can flow.

You got it right in the first diagram.

15. Mar 24, 2014

### Mutaja

I was confused by this because it was technically what I did first, excluding the battery, but I now understand what the problem asked for, and I understand why I did what I did. Which is good!

I was, again, thinking that somehow the magnet itself would conduct the current. I now see how it works, though.

Thanks so much again for helping me out. I really appreciate it.

By the way, the flux is going from south to north, right? The opposite direction of the coil?

16. Mar 24, 2014

### Staff: Mentor

"Inside" the magnet (though the metal of the horseshoe shaped core) the field lines run from the south pole to the north pole. Outside the core the field lines run from north to south.

Do an image search on "horseshoe magnet" to see images of the magnetic field lines surrounding a horseshoe-shaped magnet.

17. Mar 24, 2014

### Mutaja

Ah, yes, I understand.

Thanks again. I will probably post more problems like this one (or slightly more advanced) throughout the day, so chances are we'll talk again :surprised