# Homework Help: Direction of a positive charge's velocity in an electric field.

1. Apr 18, 2012

### theBEAST

1. The problem statement, all variables and given/known data
Here is the question with the answer:
http://dl.dropbox.com/u/64325990/phys153Q/21.5.PNG [Broken]

3. The attempt at a solution
I initially thought it would be A but it isn't and does anyone know why?

Last edited by a moderator: May 5, 2017
2. Apr 18, 2012

### tiny-tim

hi theBEAST!
wouldn't it only be A if the electric field was uniform?

3. Apr 18, 2012

### theBEAST

Ohhhh thanks that makes sense.

I have another one:
http://dl.dropbox.com/u/64325990/phys153Q/23.9.PNG [Broken]

I thought this was A, if you put a point charge there... How could it feel a force?

Last edited by a moderator: May 5, 2017
4. Apr 18, 2012

### tiny-tim

what is the equation relating electric potential and electric field?

5. Apr 18, 2012

### theBEAST

Hmmmm one is 1/r and one is 1/r^2 so would this mean E ≠ 0? Thus F is not zero? Not really sure.

6. Apr 18, 2012

### tiny-tim

you should know this definition

the electric field is minus the gradient of the (scalar) potential …

E = -φ ​

(what did you think the electric potential was for? )

7. Apr 18, 2012

### theBEAST

Thanks Tim, I kind of know the definition... The concept was potential gradients was not taught in our physics class but I did read some of it on my own online. They are related in that the derivative of potential with respect to x is the electric field. So I am not sure how I can relate it to the multiple choice question :S.

I hope I'm not asking too many question at once but it is getting late and I would love to get a hint at what I am doing wrong for this question as well (once I wake up :P):
http://dl.dropbox.com/u/64325990/phys153Q/26.9.PNG [Broken]

I thought R would affect the maximum charge stored as well because it affects the V of the battery and so the equation C=Q/V?

Last edited by a moderator: May 5, 2017
8. Apr 18, 2012

### tiny-tim

hint: when the capacitor C is fully charged, what is the voltage drop across the resistor R ?