# Homework Help: Electromagnetic Waves

1. Mar 10, 2005

### Annie1011

I have a question given to me by my professor:

Which statement is true concerning a sinusoidal varying electromagnetic wave?

a) The energy in the wave is mostly associated with the electric field because the magnetic field has a very small value. The reason for this is that B=E/c and "c" is a large number (c=speed of light, 3*10^8 m/s)

b) The momentum in the wave is zero because the wave continuously oscillates between positive and negative values.

c) The average magnetic field in the wave is zero.

d) The total energy in the wave is proportional to the sum of amplitudes of the electric and magnetic fields.

e) The energy density in the wave is proportional to the amplitude of the magnetic field.

f) The wave's intensity is inversely proportional to the energy density in the wave.

So far I have ruled out b, e (it should be proportional to the square of the electric field), and f (it is directly proportional). Other than this progress I am stuck. So any advice on how to rule out any other answers, or if I have ruled out the incorrect answer would be greatly appreciated!

I also have a question with an inductor that is connected to a sinusoidal varying voltage source, and one question asks for the reactance of the inductor, and I am caught up on the verbiage and do not know where to start.

Thanks for your time and help!

2. Mar 11, 2005

### Galileo

For a) Apply the formula for the energy density of an electromagnetic field:

$$u=\frac{1}{2}\left(\epsilon_0E^2+\frac{1}{\mu_0}B^2\right)$$

and the relation between E and B for your wave.

3. Mar 11, 2005

### RoboSapien

An amature question

How to bend light using electrical fields or magnetic fields ?

Since Light are electromagnetic waves.

The answer to this question can be used to create most powerfull telescopes without refractions or mirrors.

4. Mar 12, 2005

### Annie1011

I found out that the answer to this is c. Thansk for the replys!

Annie