# Why does an electron create a magnetic field when it moves?

1. Feb 14, 2016

### Ruben sellman

i am just a student who wishes to learn more about physics and the way the universe works, but because i am 14 and am just starting to learn all of this on my own i have to start from scratch, please present me with any usable information

2. Feb 14, 2016

### Staff: Mentor

An electric field that changes in time generates magnetic fields. Physics cannot answer "why" question on a fundamental level. This is just an observation.

3. Feb 14, 2016

### Staff: Mentor

In general, the force that a charged particle A exerts on another charged particle B depends on the positions and velocities (i.e. speed and direction) of both A and B. Because of the way the subject developed historically, we say that the part of the force that depends only on the positions of A and B is due to an "electric field" produced by A, that acts on B; and the part of the force that depends on the velocities of A and B is due to a "magnetic field" produced by A, that acts on B. Nowadays we consider these two fields to be merely components of a single unified tensor field, the "electromagnetic field."

4. Feb 14, 2016

### Staff: Mentor

Thanks, JT, I never thought of it that way.

Unfortunately the question you ask isn't easy to answer without getting into some complicated and confusing concepts. For the time being it may be better just to accept that it happens and wait until you learn more about physics before asking again. (Seriously. I don't even understand it fully. )

5. Feb 15, 2016

### Sturk200

There is a deep reason that I don't fully understand having to do with special relativity. I'll give it a shot.

Maybe you have heard of something called length contraction. Essentially when an object moves very fast its length actually gets shorter. This was one of the consequences of Einstein's special relativity. Now think about a current flowing through a metal sheet. There is an equal amount of negative and positive charges (since atoms are neutral). The negative charges are moving, say, to the right. If you fasten yourself to one of those negative charges -- that is, consider the system from the reference frame of a negative charge -- it will look like the whole sheet is moving in the opposite direction while you are standing still. OK, the sheet is moving, that means, according to special relativity, that its length contracts. But the sheet has a net positive charge, so if its length contracts, then the charge density of the sheet must increase, and if the charge density of the sheet increases, then there is a higher concentration of positive charge per unit area than negative charge per unit area. Thus applying special relativity to this sheet of current implies that the sheet has a net positive charge, and what do net positive charges do? They repel other positive charges, of course. Remember, though, this was only from the perspective of the charges moving in the sheet. The upshot is that when you move along in the direction of the current, you observe a length contraction, and therefore an electric field perpendicular to the direction of the current. Prior to special relativity (and still today for practical purposes) this effect was called "magnetism."

Be patient, though, you'll get to this stuff.

6. Feb 15, 2016

### Isaac0427

Very interesting. I have never actually come across this. Despite it being a very interesting explanation, I must stress to the OP that he should save this answer somewhere, but not try to understand it until he gets a lot more under his belt, including Maxwell's equations as it appears. Trying to learn relativity before I knew a sufficient amount of classical mechanics and calculus just made it harder for me to learn relativity, as I had to forget all the bad physics I taught myself.

7. Feb 16, 2016

### Sturk200

If you're interested, Edward Purcell's Electricity and Magnetism is where I learned about this. But I definitely agree. These ideas are intricate and complex and the only way to really understand them is to build up from the basics.

8. Feb 16, 2016

### Isaac0427

Thank you very much. I am very interested.