Test Charge Explained: Why Positive & Can't Assume Negative

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Discussion Overview

The discussion revolves around the concept of a test charge in the context of electric fields, specifically addressing why a test charge is typically assumed to be positive and the implications of using a negative test charge. Participants explore the relationship between the direction of the electric field and the force experienced by different types of test charges.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants question whether assuming a negative test charge would result in a negative electric field, suggesting that the direction of the field may change based on the type of test charge used.
  • Others clarify that using a negative test charge would reverse the direction of the force acting on it, while the electric field itself remains unchanged.
  • A participant expresses confusion about the role of the test charge in determining the direction of the electric field, questioning if it can truly indicate the field's direction.
  • Some participants assert that the standard convention is to define the electric field direction as the force experienced by a positive test charge, leading to the conclusion that the field points from positive to negative charges.
  • There is a mention of a source that describes the electric field's direction as radially outward from positive charges and inward toward negative charges, but the implications of this are debated.

Areas of Agreement / Disagreement

Participants express differing views on the implications of using a negative test charge, with some agreeing that the electric field direction does not change, while others remain uncertain about how the test charge influences the understanding of electric fields.

Contextual Notes

Participants highlight the need for clarity regarding the definitions and roles of test charges in electric field analysis, indicating potential misunderstandings about the relationship between force and field direction.

JLABBER
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Why a test charge is always positive. WHy can't 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?

Additional Details
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?
 
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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.
 
But, i don't 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 , isn't 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?
 
JLABBER said:
But, i don't 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.
Correct.
The condition reverses if we use a negative test charge?
What changes is the direction of the force, not the field.
BUt , isn't 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?
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.
 
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?
 
JLABBER said:
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?

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
The direction of the field is taken to be the direction of the force it would exert on a positive test charge. The electric field is radially outward from a positive charge and radially in toward a negative point charge.

In the end it doesn't matter what we label as the direction as long as the produced effects are the same.
 
ok.well thanks a lot...
 
JLABBER said:
ok.well thanks a lot...

Anything else you are confused about?
 
Well, if i do, i will seek your help again.ThanKS a lot!
 

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