# Electromagnetic Wave Problem

1. Sep 14, 2006

### avb203796

I have yet another problem that I need help with. An elctromagnetic wave with a frequency of 7.0 MHz forms a standing wave pattern on a transmission cable. The velocity of the wave in the cable is 0.95 of the speed of light in a vacuum. the length of the cable is such that the third harmonic has three nodes along the length of the cable, one at each end, and one in the middle. how long is the cable? what is teh distance between the nodes of the 7th harmonic on the cable?

I believe that the speed of light in a vacuum is equal to to 3.0 x 10^8 m/s therefore, the velocity of the wave would be 2.85 x 10^8 right?

But now where do I go from there? I am not sire what my next step should be.

2. Sep 14, 2006

### Kurdt

Staff Emeritus
What is the equation relating the speed of light, its frequency and the wavelength?

3. Sep 14, 2006

### avb203796

velocity=(frequency)(wavelength)?

4. Sep 14, 2006

### Kurdt

Staff Emeritus
yep! Now you know how many wavelengths are on the wire so you can work out how long it is.

5. Sep 14, 2006

### avb203796

Do I need to convert my 7.0 MHz to Hz and if so what is the conversion factor? Also, how do I know how many wavelengths are on a wire? the above formula gives me the wavelength but then I am not sure how many wavelengths are on the wire.

6. Sep 14, 2006

### Kurdt

Staff Emeritus
1Mhz is 1million Hz. What do you know about the definition of a complete wave?

7. Sep 14, 2006

### avb203796

Half of the wavelength would be the length of the cable correct?

8. Sep 14, 2006

### Kurdt

Staff Emeritus
The cable should be a whole wavelength. Remember a wavelength is measured crest to crest.

9. Sep 14, 2006

### avb203796

Ok so then my cable length would be equal to my wavelength and the distance between nodes on the 7th harmonic would be 1/7 of the wavelength?