# Magnetic Field

1. Jun 3, 2004

### DarkAnt

A few days ago my physics teacher told me that to find the magnetic field's direction in a wire you put up your right hand and whichever way your fingures bent was the magnetic field's direction, or at least that is what I understood. When I questioned him why this was the case he told me "because" and left it at that. Could someone tell me why this is the case?

2. Jun 3, 2004

### Integral

Staff Emeritus
I believe that you need to point you thumb in the direction of current flow. Your fingers then curl in the direction of the magnetic field lines. This is simply due to convention. We have defined the what positive current flow is, we have defined what the direction of the magnetic field is. This could just as well been a left hand rule. Perhaps the answer you are looking for then is "by definition"

3. Jun 3, 2004

### DarkAnt

yes, by definition

4. Jun 3, 2004

### honestrosewater

Isn't there one for EM waves with your thumb and two fingers; One for the direction of the wave and the other two for the electronic and magnetic disturbances? Wonder how you would combine the two...

What a nice teacher you have BTW

5. Jun 4, 2004

### Gza

If you're looking for the origin of this definition, it comes from the experimental work of Biot and Savart, of Biot-Savart law fame. The law is:

$$d\vec{B} = \frac{\mu_0}{4\pi} \frac{I d\vec{l} \times \hat{r}}{r^2}$$

yeah it's ugly, but what it tells you is that due to the properties of the vector cross product evident in:

$$I d\vec{l} \times \hat{r}$$

The magnetic field vector B, takes off in a direction perpendicular to both the current element dl, which points in the direction the current moves, and the r vector which points at the field point you are calculating the magnetic field at. I've attatched a picture which will hopefully help you visualize this.

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6. Jun 4, 2004

### DarkAnt

I kinda get it now. That whole equation is well...just plain scary. The picture helped alot. Thanks

7. Jun 4, 2004

### drag

If I got you right you wan'na know what the magnetic
field is. The magnetic field is basicly an electrical field
which is present when an electrical charge is moving
(relative to the device you measure the field with).
For example, if current flows in a wire en electrical
charge moving parallel to it will "see" more charges
with one sign and less of the other(that's a rough
approximation of course, and current in a wire
doesn't just flow that way). There may be more
on this on hyperphysics.

Live long and prosper.