Electric Potential Q

[axp107]

The concept of electric potential and the formula for electric potential seem contradictary sometimes..

For example:

Picture this (Grav PE is negligible):

1) A positive charge placed +5cm away from a negative charge at 0 cm (y axis)

Or the positive charge placed +3 cm away from a negative charge at 0 cm

Which has the higher potential? The positive charge placed 5cm away.. b/c its farther away right? Since elec field is +ive to negative.. how would you use the formula V= kq/r ... the only way it would work is if you make q NEGATIVE.. but how do you decide to use the negative charge. Or you could use EPE = kqq/r .. then EPE would be negative.. so do you use EPE or V formula? Whats the difference!?

2) NOW, what if you had a positive charge at 0 cm

and a negative charge placed either at 5cm or at 3 cm above it..

At which point would the negative charge have a higher potential?

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[EECStoMed]

The concept of electric potential and the formula for electric potential seem contradictary sometimes..

For example:

Picture this (Grav PE is negligible):

1) A positive charge placed +5cm away from a negative charge at 0 cm (y axis)

Or the positive charge placed +3 cm away from a negative charge at 0 cm

Which has the higher potential? The positive charge placed 5cm away.. b/c its farther away right? Since elec field is +ive to negative.. how would you use the formula V= kq/r ... the only way it would work is if you make q NEGATIVE.. but how do you decide to use the negative charge. Or you could use EPE = kqq/r .. then EPE would be negative.. so do you use EPE or V formula? Whats the difference!?

2) NOW, what if you had a positive charge at 0 cm

and a negative charge placed either at 5cm or at 3 cm above it..

At which point would the negative charge have a higher potential?

The formula for U is equal to kq1q2/r, so the one that is closer has the higher potential, the ones that are farther away have less. You have to be careful with this because the electric field which is analogous to "g" on earth is actually not constant, it's a lot stronger when the charge is closer to each other. It's quite tricky. The only place where a charge would have a higher potential is if you were in a constant electric field, like between a capacitor. Then you would use the formula U = qEd where d is the difference btween two places.

 

[axp107]

Thats not true. It depends on the charge. Conceptually if you think about it .. a positive charge wants to move towards to the stationary negative charge.

EDIT: Are you talking about #2?

The farther away it is from the negative charge, the greater the potential.

I'm just trying to reconcile theory with formula.

 

[EECStoMed]

Thats not true. It depends on the charge. Conceptually if you think about it .. a positive charge wants to move towards to the stationary negative charge.

EDIT: Are you talking about #2?

The farther away it is from the negative charge, the greater the potential.

I'm just trying to reconcile theory with formula.

Well of course it depends on charge, I was referring to two charges with the same charge. But according to the formula at that single place in time, the one that is closer has a higher state of potential energy because you are dividing by a smaller #? Am I missing something here? And the reason for that is BECAUSE it has a much stronger E.

 

[axp107]

Thats true.. for positive charges, the one closer has a higher potential.

So are EPE and V directly proportional. I'm having difficulty understanding the difference.



2) NOW, what if you had a positive charge at 0 cm

and a negative charge placed either at 5cm or at 3 cm above it..

At which point would the negative charge have a higher potential?

I'm guessing the one at 5 cm (farthest away) b/c EPE = kqq/r and one of the qs is negative..

 

[BerkReviewTeach]

Thats true.. for positive charges, the one closer has a higher potential.

You should have held to your guns, because you were right before.

The problem with this topic is similar to electron affinities versus ionization energy, in that the sign convention is rooted in the definition.

Whenever you are trying to find a charge's potential difference between two points, consider it's pathway between the two points. A positive charge 5cm from a negative charge would naturally move to a point 3cm from the negative charge, so no work must be done to move that charge. That means it went from a point of higher potential to a point of lower potential. At its new point it has less chance to achieve a large kinetic energy, because its distance of travel is less, so it has less distance to accelerate and build up kinetic energy. Its change in potential is kqq/r2 - kqq/r1, which is defined as a negative number, because its associated with a drop in potential.

Potential should be thought of a potential to move, not as a force or potential energy (despite the similarities). Although the force is stronger when the charges are closer, the charge farther from the central charge has a greater chance to move a greater distance than the closer charge.

To reconcile the sign difference, consider the potentials relative to one another, and ignore absolute signs. The sign really isn't important if you understand the concept of higher and lower when it comes to potentials.