# Homework Help: Electric Fields: Oscillating point charge

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1. May 6, 2015

### abrianna

1. The problem statement, all variables and given/known data
Reading a journal from that crazy old retired physics professor on the hill, you
stumble upon a scheme to generate high frequency (HF) radio waves (λ = 10 m). It
requires generating an electric field that diverges from a point and increases in strength
linearly with respect to distance from that point. (a) What type of common charge would create
an oscillation if placed in this field? Explain why the others
would not. (b) Could the charge be placed at rest at the di-
vergence point and still oscillate? (c) What must the E-field
strength coefficient be to generate the HF radio waves?
(hint: does this really need to be a three dimensional prob-
lem?) Extra credit (and possibly greater accolades) - How
could such a field be generated?
Known: the electric field is diverging from a single positive point charge. The lines all spread out and get further apart as they move from the center dot.
2. Relevant equations
This is more of a conceptual problem so I don't know if there ARE any pertinent equations.

3. The attempt at a solution
My first thought was to Google all the vocab that I don't understand such as "common charge" and what causes an oscillating point charge and write it down. That just left me even more confused. I also don't know what an E-field coefficient is. I have perfect attendance and take good notes I just feel like my prof. enjoys making everyone feel stupid. The averaged in my class is a 48%. :( Could some one please walk me through this or maybe set me in the right direction?

2. May 6, 2015

### Simon Bridge

You homework should always be done in the context of the classwork you have already covered.

I don't think the wording would make a lot of sense without your class notes anyway.

(a) what sort of common charge would create an oscillation if placed in this field?
... there are only two "sorts" of charge - positive and negative. But I dont think this is what he's talking about. What sorts of charges are listed in your classwork?
... it is unclear what would oscillate - the wording would suggest the charge itself is not. However, you probably have a bit of classwork in your notes that talks about oscillation in connection with electric fields.

The electric field is described as increasing linearly in magnitude from a point source, and the source is positively charged.
The source being positively charged would suggest something about the direction of the electric field vector.
The increasing strength suggests something about what happens to the field lines... how is the strength of the electric field related to the lines?
Basically you are expected t think around the problem like that until you build up a picture you can understand.
Until you have that you are unlikely to figure out what words like "strength coefficient" mean.

But there are relevant equations - they describe the common behaviours of fields. You don't need to use them as maths, but it may help to use them as language ... either that or just describe what they say in words. i.e. there is an equation for "linear".

3. May 6, 2015

### abrianna

Unfortunitely the point of this homework is that my professor gives us assignments related to what we are about to cover so outside of the textbook (which mentions nothing about oscillation anywhere in the chapter at hand) is the internet. I honestly don't know what to do. Non of the physics tutors in campus could figure it out either.

4. May 7, 2015

### Simon Bridge

Sounds familiar somehow, I think I've met this professor before in another's question...
OK then - you will still be expected to work things out from what you already know.
When you work stuff out you should use your existing knowledge - i.e. do you know about charges and electric fields?
What sort of charges do you know about? What sort of electric fields? Do you know how to draw field lines? Is the electric field described something you can get from a point charge? Stuff like that - compare the description with what you know.

However: I'd like you to check that the description of the electric field is correct when it says that it "increases in strength linearly ...". Should it perhaps say that the strength decreases linearly? If you have copied the description correctly, then double check by asking the prof. Also check how other students are coping.