# I Magnetic Field Direction

1. Apr 16, 2018

### Schfra

When can we use the right hand rule to find the direction of a magnetic field from current?

I know that it works for an infinite wire. Does it work for a finite wire? Where does it work and not work?

2. Apr 16, 2018

### Mister T

It works for both a finite and an infinite wire.

3. Apr 16, 2018

### Schfra

Does it apply just whenever we have a current?

And if it does work on an infinite wire does that mean that there is no magnetic field past either end of a finite wire with current running through it?

4. Apr 16, 2018

### ZapperZ

Staff Emeritus
Please note that the term "finite wire" needs to be taken with a grain of salt. You can't have current (at least, not steady current that are usually used in intro physics classes) in a finite wire. It needs to be closed somehow for there to be current flow.

The RHR is useful and can be easily used for a straight wire segment and also for a loop wire. You line up your thumb with something that you know is flowing straight, OR, you curl your fingers around something that you know is "curling".

This means that for a straight wire segment, current is flowing along the wire, so you line your thumb along it, and the curl of your fingers shows the direction of the magnetic field. For a loop of wire, you curl your fingers in the direction of the current flow in the loop of wire, and your thumb shows the direction of the magnetic field.

RHR is useful if you know how to use it properly.

Zz.

5. Apr 16, 2018

### Schfra

So would it be correct to say whenever you have current through a wire, you can use the RHR to find the direction of the magnetic field around any given point on that wire? A wire being any circular conductor in this case. I suspect that we couldn’t apply the RHR to a wire with a non-symmetrical shape like the in the attached image (assuming the current is flowing out of the image).

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6. Apr 16, 2018

### ZapperZ

Staff Emeritus
If the wire is twisted and in some weird geometry, then you can't simply use the RHR, because this is no longer a straight wire.

Zz.

7. Apr 16, 2018

### Schfra

But what classifies as a wire? Does it have to have circular cross sections, or does it have to be thin?

Also, how can I determine whether or not a wire is a loop or has weird geometry?

8. Apr 16, 2018

### ZapperZ

Staff Emeritus

Now c'mon. Are you telling me that you can't tell the difference between a simple loop and a weird geometry? Again, is this going to be relevant in your class?

If you intend to continue on studying physics, you will have plenty of opportunities to consider and look at many of these complicated situations. Unless you think you have already mastered the basic principle of this concept, I suggest you sharpen your knowledge and skills at solving and addressing the type of problems that are relevant at your level first.

Zz.

9. Apr 16, 2018

### Schfra

It’s not that these specific questions come up in class, but there are questions I’ve seen in class where I’m not sure if the RHR applies or not. I feel uncomfortable applying things like this if I don’t have at least a decent understanding of when they can be applied.

I’m not sure if simple loop means only a circular loop or if it’s more general than that.

10. Apr 16, 2018

### ZapperZ

Staff Emeritus
Then attempt them, see if you got it right, and if you don't understand them, post your question in the HW/Coursework forum. Don't start making things up beyond the scope of what your level of education will cover.

I don't mind going beyond what you need to know, but you must already establish that you have the knowledge for what you should already know. This is not the case here.

Zz.