# Confusing test charge

1. Oct 8, 2011

### JLABBER

Why a test charge is always positive. WHy cant we assume the test charge to be negative?
Just that if we assume it to be negative test charge, will the electric field turns to be negative?

reply: does it mean that if we put a negative charge as a test charge near a positive charge source, it shows that the positive charge actually exerts a field pointing inward? before, if we put a positive charge near a positive source, it means field radially outward, how if we change the test charge to a negative charge?

2. Oct 8, 2011

### Staff: Mentor

If you use a negative test charge, the direction of the field will be opposite to the direction of the force on that charge. With a positive test charge, the field and force are in the same direction.

3. Oct 8, 2011

### JLABBER

But, i dont really get your meaning. suppose a positive source located at origin. A positive test charge is put at distance x from the source. Then , the force on it is towards positive x-axis and the field also. The condition reverses if we use a negative test charge? BUt , isnt the test charge is used to determine the direction of the electric field alone? I thought that if a test charge is used, it can show only the direction of flow of electric field?

4. Oct 8, 2011

### Staff: Mentor

Correct.
What changes is the direction of the force, not the field.
A test charge is used to determine the direction of the field, but it doesn't change the field. If you use a positive test charge, the force points in the direction of the field; if you use a negative test charge, the force opposes the field. The direction of the field doesn't change.

5. Oct 9, 2011

### JLABBER

Ok, i know this statement. Then , what i confuse is then if the test charge is used to determine the direction of force, then it certainly cannot proof the direction of field. Therefore, what i want to know is what can show that electric field is from positive to negative?

6. Oct 9, 2011

### Drakkith

Staff Emeritus
I think it is simply a standard to consider the direction of the field from positive to negative.

From here: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elefie.html
In the end it doesn't matter what we label as the direction as long as the produced effects are the same.

7. Oct 9, 2011

### JLABBER

ok.well thanks a lot...

8. Oct 9, 2011

### Drakkith

Staff Emeritus
Anything else you are confused about?

9. Oct 9, 2011

### JLABBER

Well, if i do, i will seek your help again.ThanKS a lot!