Potential Difference Confusion

[brentd49]

There seems to be two different uses of potential difference that either contradict each other, or at the very least make it very confusing. Let me write the three different forms:

Definition: Vf-Vi= -E*d
Differential: dV=-E*dr (Vi=0)
Point Charge: V=E*r (Vf=0)

What is with the different reference potentials (Vi,Vf=0)? Can someone please make sense out of this.

Edit: I avoided writing integrals above because I don't how to use latex, and the book I'm studying is Halliday, Resnik.

 

[Astronuc]

Here is a nice discussion of potential energy. It is really the change in potential energy that matters.

http://hyperphysics.phy-astr.gsu.edu/hbase/pegrav.html#pe

Gravitational potential energy - http://hyperphysics.phy-astr.gsu.edu/hbase/gpot.html#mgh

Electric potential energy - http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elepe.html#c1

Reference potential - potentials are relative!

The nature of potential is that the zero point is arbitrary; it can be set like the origin of a coordinate system. That is not to say that it is insignificant; once the zero of potential is set, then every value of potential is measured with respect to that zero.

ref Hyperphysics

Read also the third plate in the link on electric potential - Potential Reference at Infinity.

 

[BerryBoy]

There seems to be two different uses of potential difference that either contradict each other, or at the very least make it very confusing. Let me write the three different forms:

Definition: Vf-Vi= -E*d
Differential: dV=-E*dr (Vi=0)
Point Charge: V=E*r (Vf=0)

What is with the different reference potentials (Vi,Vf=0)? Can someone please make sense out of this.

Ok, so you V_f & V_i are values of potential. So if you substract them, you get the difference in potential between them.

Your second is just substituting 0 into your first equation.

Your third equation is not potential difference. It is a value of electric potential at a distance r from a point charge (It is the energy required to bring a unit charge of identical polarity from infinity to the distance r from the point source; that's the way I think about it).

Does this help you?

Regards,
Sam