# 2 Q's: EM radiation produced by positive charge

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1. Feb 24, 2015

### abro

I have 2 questions about EM radiation produced by an occilating electric charge.
Q1: With graphs of electromagnetic fields, you can see that it contains an electric and magnetic component. What does these mean or give information about?
Q2: By occilating the charge, you will get such a graph. How will it look like?

I have tried drawing, doing research on the Poyting vector and I keep finding that the magnetic field will be in the opposite direction on the graph, in the case of the positive charge. And I sure know how to find the direction of a magnetic field...

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2. Feb 25, 2015

### Simon Bridge

Q1. these graphs give information about the electric and magnetic fields... what don't you understand?
Q2. How the graph looks depends on how you oscillate the charge ... if you oscilate the charge sinusoidally, for example, you will get sinusoidal graphs.
See: http://www.cco.caltech.edu/~phys1/java/phys1/MovingCharge/MovingCharge.html

You are given a position-time graph for the charge; what does the acceleration-time graph look like?

3. Feb 25, 2015

### abro

With the picture I attached, you can find the EM graph aswell. What I don't understand about that graph is wether it's the direction or magnitude of the the field. I have tried understanding what the graph of a charge moving sinusoidally look like, but it's inconsistent with for example the Poyting vector.

4. Feb 25, 2015

### Simon Bridge

The example EM graph on what you supplied shows both the magnitude (the line) and the direction (the arrow). That is why the E and the B curves are drawn perpendicular to each other.

The graph of the sinusoidally moving charge is given to you in thr same picture - top right - as y vs t. This is a position-time graph. There is nothing wrong with it.

5. Feb 25, 2015

### abro

So when the charge moves up, the electric field moves up?
The magnetic field move in a circle when it moves up, but it's in the other direction when moving up, that's the problem I run into.

6. Feb 25, 2015

### Simon Bridge

The electric field moves along, not up and down.
The magnitude may increase and decrease with time, and it may change direction.

How is the electric field related to the motion of the charge?
Don't guess - if you don;t know, look it up. I gave you a link even.

7. Feb 25, 2015

### abro

Took me some time to get the applet running, but now I can grasp it. Thanks!