# Electromagnetic signals questions

1. Feb 11, 2006

### dervast

Hi i have noticed that some times we can denote an electromagnetic signal using cos os some timew we can write it in exponential form? Is that true? How can we convert from one type to the other?
For example i think that the signal Acos(wt-kr) it can be written to Ae^i(wt-kr)

Thx a lot

2. Feb 11, 2006

### Meir Achuz

Usually, the physical result from complex notion is to take the real part,
so what you have written is correct. Keep the complex notation until you want to write down the predicted physical measuement.
The relation is $e^{ix}=\cos x +i\sin x$.

Last edited by a moderator: Feb 11, 2006
3. Feb 11, 2006

### dervast

so when i write Acos(wt-kr) i forget to write the imaginary part right? Dont we lose information in that way? What the real part means (is used for) and what the imaginary one..
Thx a lot

4. Feb 12, 2006

### Meir Achuz

The Im part is useful for intermediate calculations, but a final physical answer is always just the real part. For Quantum, the physical answer is always bilinear, like \psi^* \psi, which is real.

5. Feb 14, 2006

### dervast

Thx a lot and what intermediate calculations are?

6. Feb 15, 2006

### Meir Achuz

I just mean the steps between the beginning and the final answer.