# Can someone better explain how motion affects an electric field?

1. Mar 21, 2014

### nhmllr

I am reading through the Feynman lectures in my spare times (the parts that interest me, anyway). The book and it's explanations are usually high quality, although unfortunately I have come across a very poorly explained part that I cannot understand.

http://www.feynmanlectures.caltech.edu/I_28.html#Ch28-S1

In section 28-2 "Radiation," the book talks about how for an accelerating charge, the field drops off as 1/r instead of 1/r^2. I find the book's explanation confusing though, especially the following sentence (and everything after it):

"Of course, the end of er′ goes on a slight curve, so that its acceleration has two components. One is the transverse piece, because the end of it goes up and down, and the other is a radial piece because it stays on a sphere. It is easy to demonstrate that the latter is much smaller and varies as the inverse square of r when r is very great."

What does he mean by er' curving? I cannot understand this. I also couldn't find anything else online to clarify it. Do you understand what is meant by this?

Thanks

2. Mar 23, 2014

### Simon Bridge

Feynman tended to be somewhat casual in his presentation, so you get passages like the one you are puzzled about.

$\hat e_{r'}$ is the unit vector pointing at the apparent position of the charge - so it cannot "go in a curve".
I think he's talking about a line he is drawing on a blackboard at this point, which will be a trajectory for the charge he is considering for the example.

What's important to this discussion is the acceleration of the charge. If the charge changes direction things can get more complicated because changing direction is also an acceleration.
That's really all that's on his mind.

Note:
You should not use the Feynman lectures by themselves for learning physics.
Use them in conjunction with other materials, other texts.

They are not famous for being good for learning from - they are famous for being presented by Richard Feynman. In their day, the lecture series was a failure. These days they are best used after a basic course of physics has been completed - as a way of solidifying your knowledge.

3. Mar 23, 2014

### HomogenousCow

They were??

4. Mar 23, 2014

### nhmllr

I've heard that before, but I've found them to be pretty helpful and waaaayyy better (and more centralized) at explaining things than most sources. I'm open to suggestion, though. Do you have any better books in mind that would be better for this sort of thing?

5. Mar 23, 2014

### Cosmobrain

Haha. Definately they were

cb

6. Mar 23, 2014

### Simon Bridge

Failures?

For teaching freshman physics - yes. There were all sorts of contributing factors though.

My copy of the Feynman Lectures had an introduction that included a description of the original reception. Basically, beginners are best taught by people who did not find physics easy when they were starting out.

If you come from a strong physics background they can be great.
Feynman's "take" on teaching physics is something of a "magical mystery tour" and he fills in a lot of the gaps that the regular style of teaching tends to leave (leaving other problems behind - there are no easy answers).

Practically any other Freshman physics text is better for learning from. Pick one and use it as an accompaniment. I've used everything from Tipler to Eisberg & Resnick.

But bear in mind - you may just happen to be one of those whose learning style happens to mesh well with Feynman's teaching style. The way to tell is to do exercises: can you apply what he is teaching?

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