# Explain me what is a electromagnetic wave?

#### benzun_1999

can any on explain me what is a electromagnetic wave?????

-benzun

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#### chroot

Staff Emeritus
Gold Member
It is a self-reinforcing oscillation of the electric and magnetic fields that propagates through a vacuum at the speed of light, c.

A changing electric field creates a magnetic field, and a changing magnetic field creates an electric field -- therefore, when both fields are changing, they are self-reinforcing. Specifically, the two fields oscillate in two planes orthogonal to each other and to the direction of the wave's propagation.

- Warren

#### FrankMak

The basic descriptor for this class of wave is "transverse wave". As stated by Warren, EM waves have two components that are at 90 degrees to each other and the action of these components are transverse to the direction of the wave, as opposed to "longitudinal waves". See the discussion at the following URL:

http://www.physicsclassroom.com/Class/waves/U10L2a.html

#### Arc_Central

Whats the difference between an electric and a magnetic field?

Please explain in detail, or show the difference if that is possible.

#### Claude Bile

Electric fields are caused by charge. Electric fields apply force to a positive charge in the direction of the field. Electric fields are conservative (If you move a charge around and get back to where you started from, no work is done) and thus have a scalar field associated with it called the electric potential (Usually denoted V).

Magnetic fields are caused by moving charge. Magnetic fields apply force to moving charges perpendicular to the direction of the field and the motion of the charge. Thus Magnetic fields are non conservative and do not have a scalar potential associated with them. Magnetic fields have a vector potential instead, however it has no physical significance, it is more a mathematical conveniance.

Changing electric fields generate magentic fields and vice versa. Thus an electromagnetic field can be self propagating as explained by chroot.

That's all I can rattle off on the top of my head, for more information, do a web search or look up 'Introduction to Electrodynamics' by Griffiths.

Claude.

#### Integral

Staff Emeritus
Gold Member
I believe that the only true answer to this is simply, none. A person stationary with respect to a electric charge sees an electric field, a person moving wrt to the same charge sees a magnetic field. All that changes is the velocity of the observer, same charge, same field.

#### Claude Bile

Integral is correct, the distinction we make between Electric and Magnetic fields is purely man made.

This is an important point, one that I probably should have made in my previous post.

Claude.

#### Albrecht

Integral is correct, the distinction we make between Electric and Magnetic fields is purely man made.
True! But it is even stronger. A magnetic field simply does not exist. Magnetic forces are seeming forces. In a weak analogy it is like the coriolis force: There is a force, but in fact something different than assumed at the first glance.

The origin of magnetism is: If you observe a collection of moving electric charges from a moving position, you will - by relativistic causes - see those charges contracted; but some of the charges more contracted than the others. This causes a resulting force even if the total summary of all charges is zero. This is magnetism.

You will find a short explanation of in in P. French: Special Relativity. You will find it in detail in Rosser: Electromagnetism by Special Relativity (Butterworth, London)

#### GENIERE

Albrecht

by relativistic causes - see those charges contracted;
Thanks

#### Albrecht

magnetism

Assume you have a wire with an electric current in it. The electric current can be understood as a chain of positive electric charges into one direction and a chain of negative charges into the other one.

Both chains shall have the same density and move at the same velocity.

A test charge (assume a negative one) at rest will see a neutral electric field. It will see both chains a bit contracted due to relativity. (Special relativity states that every object in motion is contracted.) Contraction is the same for both.

If now the negative test charge is moved in the same direction as the negative chain, i.e. parallel to it, this test charge will see the negative charges less contracted than before and the positive charges more contracted than before. So it will, in a summary, see a positive charge in the wire, and it will be attracted to the wire.

An observer at rest, who does not see this non-balance, will assume a new force which by tradition has the name: "magnetic force".

#### AndersHermansson

On most illustrations you see the electric wave and magnetic wave are "in sync". Aren't they really 90 degrees phase dislocated to each other?

#### Integral

Staff Emeritus
Gold Member
Originally posted by AndersHermansson
On most illustrations you see the electric wave and magnetic wave are "in sync". Aren't they really 90 degrees phase dislocated to each other?
Look up at the corner of your room. The Electric field varies on one wall, the Magnetic field on the other. In this case the direction of motion is toward the floor or ceiling.

When the Electric field is at an absolute maximum value on its wall the Magnetic field is at an absolute minimum value (at zero) on the other.

When viewed Relativistically it becomes clear that the Magnetic and Electric fields are different manifestations of the same phenomena.

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