# Homework Help: Uniform electric field

1. Apr 21, 2012

### domyy

1. The problem statement, all variables and given/known data

A uniform electric field between two parallel plates has magnitude of 20N/C and is directed downward.

1) A charge brought into this field experiences a force of 10.0 N downward. The charge must be....?
2) The potential energy of this charge when moving downward is...?

2. Relevant equations

e = f/q

3. The attempt at a solution

By using the equation, I found 0.5C. However, I am trying to get a picture of it. If the plate above is positive and the one below is negative...and the force is downward, would that mean the charge is positive?
Then the answer for the second question means the charge is increasing.

Would that be correct?

2. Apr 21, 2012

### tiny-tim

hi domyy!

forget the picture … it'll only confuse you

use the equations

force = charge x electric field

potential energy = minus work done (= minus force "dot" distance)

3. Apr 21, 2012

### truesearch

Electric field direction is defined as the direction of the force that a positive charge would experience.

4. Apr 21, 2012

### domyy

The potential energy is decreasing because at the top it has maximum eletric potential energy but as it goes downward toward the negative plate it decreases!

In this case, my answer for q = negative 0.5 ?

5. Apr 21, 2012

### tiny-tim

the force is downward, and the field is downward, so the charge must be … ?

6. Apr 21, 2012

### domyy

I am afraid this is a trick question =/ The way you asked I was very tempted to think: if it´s all downward, then it must be negative. But this is what I have on my book:

I have a drawing with nothing written on it but some arrows. It seems that if the charge is positive, the force will go toward the negative plate and if the charge is negative, the force will go toward the positive plate.

So, If the force is downward, toward the negative plate, that means the charge must be positive.

7. Apr 21, 2012

### tiny-tim

physics is euqations !

in this case, the lorentz force equation

F = qE

so if F and E are in the same direction, q must be … ?

8. Apr 21, 2012

### domyy

negative.

9. Apr 22, 2012

### tiny-tim

how do you get that?

this is primary school arithmetic …

if F = qE, and if F and E are in the same direction,

then q must be positive, mustn't it?​

10. May 5, 2012

### domyy

I have a question concerning this problem. Here we are talking about a downward force. And if I am supposed to portray this, then I would draw a positive plate and a negative plate with arrows coming down toward the negative plate. Now, how to portray a force that is upward?
Should it start from the negative plate going toward the positive?
With an upward force, would the charge be negative and the potential energy increase?

11. May 5, 2012

### tiny-tim

do the arrows represent the field?

if so, yes
i don't understand the question

an upward force is represented by an upward arrow

but that has nothing to do with the direction of the arrow representing the field

12. May 5, 2012

### domyy

Well, when I was asked the question about the downward force, I was supposed to draw the E-field indicating +/- signs. Now, I am just curious how that would look like if the force were upward.

13. May 5, 2012

### domyy

The question was:

A uniform electric field between two parallel plates has magnitude of 20N/C and is directed downward. Draw the E-field indicating +/- signs, which plate is positive and which is negative?

I am just trying to understand if the force were upward, my drawing would have been anyhow different.

This is what I drew for the downward force: http://www.regentsprep.org/Regents/physics/phys03/aparplate/plate3.gif

I am just curious to know if the force were upward, the arrows would be coming from the negative plate toward the positive and ,therefore, upward.

Last edited: May 5, 2012
14. May 5, 2012

### tiny-tim

the direction (and strength) of the force depends on the charge it's acting on

the direction (and strength) of the field is the same whatever charge it's acting on