# Homework Help: Electric field

1. Jan 22, 2010

### aloshi

The electric field strength E is defined as the number of Newton force that each Coulomb charging is affected by where it ends up in the electric field.
my question:
How can E be constant? distance does not play any role?

is it right to say:
E is the same (constant) if and only if there is a homogeneous electric field, or only, and only if the various charges are added at the same point as E becomes constant???

is it right?

2. Jan 22, 2010

### Fightfish

Eh...I had some trouble understanding what you were trying to say, but anyways I think I've got the gist of it.

You are right in thinking that E is not constant generally, except in the case of a uniform electric field. What I'm not sure about is why were you led into your question of "how can E be constant" from the definition of the electric field strength E? The electric field strength at a point is the electric force acting per unit charge on a test charge placed at that point (ignore the nuances about small positive test charge blah blah blah for now). And yes, the electric force acting on a charge placed say a distance away from a point charge does depend on the distance of that charge from the point charge.

3. Jan 22, 2010

### aloshi

what is the difference between place a test charge or a "big" charge

4. Jan 22, 2010

### Fightfish

Usually a test charge is a charge assumed to be small enough such that it does not alter the electric field; "big" charges will have nonnegligible effects on the electric field in the region.

5. Jan 23, 2010

### aloshi

what is it determines the field direction?

6. Jan 23, 2010

### Fightfish

The direction of the electric field at a point is in the direction of the electric force experienced by a positive test charge placed at that point in the field.

7. Jan 23, 2010

### aloshi

why justett positive charge? why not a negative charge?

8. Jan 23, 2010

### Fightfish

It's just simply a matter of convention; the electric field has to point in a particular direction, so physicists historically chose to make it in the direction of the electric force experienced by a positive charge. The force experienced by a negative charge would be in the opposite direction.

9. Jan 23, 2010

### aloshi

$$E=\frac{F}{q}=\frac{k\frac{Qq}{r^2}}{q}=k\frac{Q}{r^2}$$
Electrical field strength is therefore only depending on the charge Q and the distance r. The field's direction is given according to the convention that q is positive.

http://www.pluggakuten.se/wiki/images/2/23/F%C3%A4.JPG [Broken]
1)
the picture shows three identical reports whose charge is +Q on everyone, the distance between them is equal. how comes the the electrical field strength to last in the point C? therefore which direction comes it to last there?
can one explain why it becomes so?

2)
another thing is homogeneous field strength?? there is of course one posetiv side and a negative side. so how can one know the field strength if it is for a posetiv charge or negative charge

Last edited by a moderator: May 4, 2017