# Homework Help: Electric Field direction question

1. Oct 1, 2009

### Suy

1. The problem statement, all variables and given/known data
An electric field of 280000N/C points due south at a certain spot. what are the magnitude and direction of the force that acts on a charge of -4.0*10^-6C at this spot?

2. Relevant equations

3. The attempt at a solution
I don't know how to get the direction, is it south? plz explain it...ty
And for question like this, what assumption would you make?, because i don't know the source charge is negative or positive

2. Oct 1, 2009

### Andrew Mason

By convention, the electric field direction is the direction that a positive charge will move when placed in the field.

AM

3. Oct 1, 2009

### Suy

The answer for this question would be north?
ty

4. Oct 2, 2009

### Andrew Mason

What is the expression for the force,
$$\vec F$$,
on charge q when placed in a field
$$\vec E$$ ?

AM

5. Oct 2, 2009

### Suy

I know how to get the force, but i am not sure how to get the direction,
if the electric field direction is the direction that a positive charge will move,
electric field due south, the source charge would negative and the placed charge is negative , repel, it would north?
Please confirm if i am right ty...
btw, when an electron moving at constant velocity, from negative to positive plate(parallel), What is the voltage?
because F=Fe F=ma Fe=Eq
a=om/s2 Fe=0 0=Eq E=0N/C E=V/d V=0V
am i right?

6. Oct 2, 2009

### Andrew Mason

I am trying to get you to solve this. You can do it. If the electric field direction is south, which is the direction of the force on a positive charge placed in that field, then what is the direction of the force on a negative charge placed in that field?

If an electron moves at constant velocity, is there a net force acting on it? From that answer, what can you say about the electric field? Since E = V/d what can you say about V?

AM

7. Oct 2, 2009

### Suy

But i understand what you mean, it would go north and repel,because if the positive charge placed in that field, and due south(toward the source charge), the source charge will be negative because of attraction,but for the question the negative was placed there, so it would repel and to the north,
but are you trying to tell me actually the direction of negative charge is south? If it is , please tell me what am i wrong?

constant velocity ,acceleration is 0, no force is acting on it, there will be no electric field
V=0

8. Oct 3, 2009

### Andrew Mason

Your answer is right but you are struggling with the explanation. It is very simple. The force vector is given by:

$$\vec F = q\vec E$$

If q is positive, how does the direction of the electric force F compare to the direction of the electric field E? If q is negative, what is the direction of F compared to E?

Correct.

AM

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