BR physics- electrostatics

[anbuitachi]

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Hi, in BR physics they said
if test charge is positive, then forces associated with the test charge and electric field would point in the same direction. If negative, then opposite direction. Shown with E=F/q' where q' is test charge.

Can someone please clarify this? Why is positive test charge opposite direction and negative vice versa? shouldn't it also depend on the other charge (i'm assuming there is another charge...?) Or is it assuming the test charge isn't alone and is in a field ?

Thanks

 

[PhilIvey]

Hi, in BR physics they said
if test charge is positive, then forces associated with the test charge and electric field would point in the same direction. If negative, then opposite direction. Shown with E=F/q' where q' is test charge.

Can someone please clarify this? Why is positive test charge opposite direction and negative vice versa? shouldn't it also depend on the other charge (i'm assuming there is another charge...?) Or is it assuming the test charge isn't alone and is in a field ?

Thanks

What you stated is correct. The reason is that an electric field is defined as beginning at a positive charge and ending on a negative charge. Now, a positive test charge would be REPELLED by a field ORIGINATING from a positive charge. In order for this to be the case, the FORCE AND FIELD DIRECTION must be the same AWAY from the POSITIVE SOURCE CHARGE and TOWARDS the POSITIVE TEST CHARGE.

Now, a negative test charge would be ATTRACTED. So, the force must be opposite the DIRECTION OF THE FIELD. Remember, the field always leaves a positive charge so its direction is ALWAYS THE SAME. HTH

Now if there are multiple charges then it would depend on the magnitude and I see how you could be confused.

P1 P P2

Let's assume that P1 and P2 are both positive charges and we have a test charge. Well, P2 will puse P towards P1 and P1 push P towards P2.

What determines is the magnitude and distance. So, you're right that in this case a positive test charge could actually move towards another test charge. That's good that you think of novel situations so it makes you better able to make inferences.

 

[dingyibvs]

Hi, in BR physics they said
if test charge is positive, then forces associated with the test charge and electric field would point in the same direction. If negative, then opposite direction. Shown with E=F/q' where q' is test charge.

Can someone please clarify this? Why is positive test charge opposite direction and negative vice versa? shouldn't it also depend on the other charge (i'm assuming there is another charge...?) Or is it assuming the test charge isn't alone and is in a field ?

Thanks

It's due to the definition of field lines. A field line is DEFINED as going away from a positive charge and heading toward a negative charge. A positive charge would be repelled away from another positive charge and attracted toward a negative charge, just like field lines. A negative charge, OTOH, would be attracted toward a positive charge and repelled away from a negative charge, just opposite of field lines.