# What is the purpose of having a magnetic field?

1. Oct 9, 2014

### k9b4

What does a magnetic field do? I understand that we create the idea of an electric field to more easily explain why electric charges attract and repel each other, but what is the purpose of having a magnetic field?

2. Oct 9, 2014

### Matterwave

The magnetic field is as necessary to explain the motion of charges just as much as the electric field is.

3. Oct 9, 2014

Why?

4. Oct 9, 2014

### Matterwave

The electric field only gives you the forces between charges which are static. Once the charges start to move, they will see magnetic fields come into play and require both the electric field and magnetic field to figure out their motion.

This is a pretty broad field, so maybe look here first: http://en.wikipedia.org/wiki/Magnetic_field

5. Oct 9, 2014

### k9b4

Does a magnetic field cause charged particles to move?

6. Oct 9, 2014

### Matterwave

A static magnetic field won't cause a static charge to move, but it will change its direction of motion once it does start moving.

The full force law considering both electric and magnetic fields is the Lorentz force law:

$$\vec{F}=q(\vec{E}+\vec{v}\times\vec{B})$$

7. Oct 9, 2014

### k9b4

Why does a magnetic field only affect charged particles when the magnetic field is moving? A static electric field created by a charged particle will attract or repel other charged particles.

8. Oct 9, 2014

### Staff: Mentor

A charged particle moving in a magnetic field experiences a force that is perpendicular to the direction of motion and proportional to its speed. If the particle is not moving, the speed and therefore the magnetic force are zero. If anything starts the particle moving there will be a non-zero force from the magnetic field, as well as the force from the electrical field.

9. Oct 9, 2014

### Matterwave

That's just the way things are. The magnetic field has to do with charges in motion. It is different from the electric field, because it's not the electric field. There are a whole lot of intricacies in this, and the motion of charges in electric and magnetic fields is a very wide area of study (basically all of electro-magnetic theory), so I would suggest you read up on it, perhaps first through the wikipedia page, or from some online resources, or from your E&M textbook.

10. Oct 9, 2014

### Staff: Mentor

This is just a little nitpick, but the magnetic field only affects charged particles when the charged particles are moving. The magnetic field itself doesn't have a velocity, although changes in the electromagnetic field do.

11. Oct 9, 2014

### k9b4

But what is a magnetic field? I understand that electric fields do not physically exist, and that you can imagine how charges will interact with each other without thinking of an electric field at all. The negative electron repels other negative electrons and when you oscillate the electron up and down it causes other electrons to move up and down because the electron is changing position and pushing other electrons around. What does a magnetic field do and how can I imagine what it does without imagining that a magnetic field is there at all?

Unless my way of viewing charge interaction without an electric field is wrong?

12. Oct 9, 2014

### Staff: Mentor

Unfortunately there really isn't an easy way to visualize a magnetic field like you can the electric field since the magnetic field acts sort of indirectly on charged particles. It is only when they are moving or when the magnetic field is changing that a particle will respond to it, and even then the particle doesn't respond in a direct sort of way, instead responding by moving in directions perpendicular to the direction of the field lines.

You may have to settle for simply not being able to visualize charge interaction without using a magnetic field.

13. Oct 9, 2014

### Staff: Mentor

Also, you need to understand that the entire concept of a "field" is a mathematical and conceptual way to make sense of how particles interact. Fields are used extensively in physics because they offer what is probably the simplest way of understanding much of the fundamental interactions of nature. They are so useful that entire theories have been developed to describe even particles in terms of an underlying field. (See the various Quantum Field Theories)

Trying to understand how particles interact without understanding the field that describes their interaction is not going to get you anywhere.

14. Oct 9, 2014

### k9b4

But I like to think about what is actually there. A field is not actually there. An electron however, has a mass and exists in reality.

15. Oct 9, 2014

### Staff: Mentor

Last edited: Oct 9, 2014
16. Oct 9, 2014

### k9b4

17. Oct 9, 2014

### Staff: Mentor

Feynman's point is that the person asking "why" might not have enough background knowledge to understand the answer.

18. Oct 9, 2014

### k9b4

But I'm asking for an intuitive answer, not a mathematical one. "I don't know" is a perfectly acceptable answer.

19. Oct 9, 2014

### Staff: Mentor

How do you know the field isn't actually there? Keep in mind that a particle is no less mathematical or conceptual than a field is. And if the field isn't there, what happens to the energy lost from radiating particles before the radiation is absorbed by another particle? Is that energy simply gone? If the field doesn't exist, then why do field theories have such success over purely particle theories?

A field allows us to answer questions like this by treating the field as an actual entity and not just clever math. (of course one could argue that all math is "clever math" if it enables us to accurately describe reality, whether or not that math is for particles or fields)

20. Oct 9, 2014

### Staff: Mentor

There aren't always intuitive answers. Sometimes reality simply doesn't match up with our intuition. The key is to realize that intuition isn't something that is set in stone, but is something that can be molded as you learn more. Many things that don't make sense to you now will probably be absolutely obvious in the future.