### Re: What is the difference between an electric and magnetic field?

Date: Fri Jun 23 11:32:14 2000
Posted By: Michael Richmond, Faculty, Physics, Rochester Institute of Technology
Area of science: Physics
ID: 961562698.Ph
Message:

```> What is the difference between an electric and magnetic field?
```

We say that there is an electric field in some region of space if electrically charged objects (such as electrons and protons) accelerate in a particular way, but electrically neutral objects (such as neutrons or neutral atoms) do not. In the simple case of a uniform electric field, electrically charged particles will accelerate uniformly along the direction of the field, while neutral particles will not.

Likewise, we say that there is a magnetic field in some region of space if objects with inherent magnetism (such as bar magnets) rotate in a particular way, but objects without any inherent magnetism (such as pencils or cats) do not. In the simple case of a linear magnetic field, a bar magnet will rotate so that its long axis is along the direction of the field, while a pencil will not.

Things are somewhat complicated by the fact that a MOVING object with an electric charge will be affected by a magnetic field; in addition, electric fields which vary with time can give rise to magnetic fields, and vice versa. It may sound confusing, and sometimes it is; but it is possible to describe the behavior of electric and magnetic fields completely with just four (pretty complex) rules. These rules were first figured out by Scottish physicist James Clerk Maxwell, and they are known as Maxwell's Equations. You can find them in most introductory university-level textbooks on physics, but do be aware that they involve calculus and some abstract geometric ideas.

Electric and magnetic phenomena are so closely related that we sometimes describe them as a single entity, "electromagnetism."

```> How do photons use this to travel at the velocity of light?
```

I'm afraid I don't understand what you are asking. Photons don't "use" electric or magnetic fields; it's more accurate to say that they ARE travelling electric and magnetic fields. Disturbances in an electric or magnetic field travel at the speed of light, and so do photons.

```> Do other force particles such as the graviton and gluon do
> this also to travel at the velocity of light?
```

Gluons -- I don't know, sorry.

Gravitons -- do they travel at the speed of light? A very good question. Most physicists do believe that gravitons (disturbances in gravitational fields), just like photons (disturbances in electromagnetic fields), do travel at the speed of light. However, it is very difficult to detect gravitons, and as far as I know, there are no direct laboratory experiments which can confirm that they travel at the speed of light. There is some indirect evidence for it: general relativity predicts that gravitons move at the speed of light, and general relativity does a good job at predicting the motions of several observed pairs of neutron stars.

A more detailed answer may be found in sci.astro's Frequently Asked Quetions, item D.04. See (for example) http://www.faqs.org/faqs/astronomy/faq/part4/

The LIGO (Laser Interferometry Gravitational Observatory) experiment, which is just starting to enter its test phase, may provide some evidence of the speed of gravitons. See http://www.ligo.caltech.edu/.

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