Electromagnetic radiation

Oersted's experiment

It appears that moving charges (currents) can make Magnetic fields that magnets will react to.

When charges move in one direction through a wire, this causes an M-field.

Moving charges

When a current goes around in circles, the field looks a lot like the field of a magnet.

That is...

 

Moving charges are the cause of all magnetic forces / fields as far as we know. (We haven't found any monopoles yet.)

When charges move back and forth, this causes a changing M-field.


Faraday's experiment

Faraday experimentChanging Magnetic fields inside loops of wire can create Electric fields: An electric field is what caused charges to move (a current), and made the galvanometer needle twitch.

Faraday's law:

  • When a wire loop is placed in the vicinity of a magnet,
  • If either the magnet or the loop move,

...an electric current is created within the loop for as long as the motion continues.

Waves...

antennaAs the E-field changes, this causes an M-field, which is changing.

The changing M-field causes an E-field....

This just goes on and on!

Here's a slightly different looking animation of an E-M field wave.

AM antenna

Here is a very long wire with "charges going up and down". It is called an antenna.

In AM radio transmission the tower itself (it's made of conducting metal) is the antenna.

Your car uses a similarly oriented 'whip' antenna to receive the transmission: The changing "E"-field part of the E-M field wave causes electrons in your whip antenna to move up and down, which changes the voltage at the bottom of the antenna.

Radio waves are E-M field waves.

CC13

A proton is placed at rest in the middle of a vacuum chamber. Neglecting everything but the proton, at another point X in the chamber there is...

  1. an electric field,
  2. an electric force,
  3. a magnetic field,
  4. a magnetic force,
  5. none of the above, since after all, there is nothing at X.

CC14

As before, a proton is placed at in the middle of a vacuum chamber, but this time it is shaken back and forth. Now at another point X in the chamber there is...

  1. an electric field,
  2. an electric force,
  3. a magnetic field,
  4. a magnetic force,
  5. none of the above, since after all, there is nothing at X.

Microwaves

If microwaves are E-M field waves, then the changing E- field ought to make the loose electrons in wires move back and forth. Does this happen???

As the electrons move through a wire the friction of the electrons running into other atoms makes things heat up, eventually hot enough to glow.

Microwaves are also E-M field waves.

Why don't microwaves heat things like plates / cups?

If electrons are tightly bound, you're pushing and pulling on them with the field, but they aren't free, so can't be dragged past the other atoms to heat them up.

Why *do* microwaves heat water?

What happens when you put a dipole in an electric field? ...and then make that field slosh right and left?

Death rays?!

Could you shoot a "death ray" or "heat ray" at someone? Maybe a concentrated microwave to heat somebody up remotely?

Actually, the U.S. military is developing so-called "Active Denial Systems" that do just that. These could be used for crowd control. But critics say that water cannons already work well. It might be simple to protect yourself from this...

Make a shield of aluminum foil...

E-M disturbance

Every wave is a disturbance in some...medium.

An E-M wave is apparently a disturbance in the electromagnetic field of a charge, because of its motion.

This is not anything material--Weird!

Experimentally, it is found that things like AM radio waves and microwaves travel at 300,000 km / sec...

...the same as the speed of light.