# Electric Potential,I ly need help please

1. Jun 15, 2005

### saltrock

Electric Potential,I urgently need help please

I am really confused with the term 'electric potential'.I know its the amount of potential energy that a coulum positive charge would have if placed in an electric field but this isnt enough to make me clear.I am also looking for its formula.My exam will start in about 5 hours.If anyone can help me with this i'd really be greatful.Thanks

2. Jun 15, 2005

### OlderDan

Perhaps the statement of "what you know" is what is confusing you. Electric potential is not the potential energy of any particular charge. In fact, it is not potential energy at all, and has different units than energy. Electric potential is a ratio of energy to charge and has units that reflect that ratio. The most common unit, the Volt, is the ratio Joules/Coulomb. If you placed a one coulomb charge at some point where the electric potential was one volt, then its electric potential energy would be

$$PE = (1 Volt) \cdot (1 Coulomb) = 1 Volt \cdot Coulomb = 1 Joule$$

For some other amount of charge, and some other potential the potential energy would be

$$PE = qV$$

which is equivalent to the definition of electric potential

$$V = \frac{PE}{q}$$

3. Jun 15, 2005

### saltrock

I thought the formula for electric potential was V=kq/r where k=9.0 times 10 to the power 9

4. Jun 15, 2005

### saltrock

This is whats written in my book.' THE ELECTRIC POTENTIAL v AT ANY POINT IN AN ELECTRIC FIELD IS THE POTENTIAL ENERGY THAT EACH COULUMB OF POSITIVE CHARGE WOULD HAVE IF PLACED AT THAT POINT IN THE FIELD.I am an A level student,i think i just need to know a general equation rather than some complicated ones.To be honest i had never seen the formula you stated above.many thanks for help

5. Jun 15, 2005

### JFo

Just to add to what has been said,

An important destinction between electric potential and electric potential energy, is that potential is a property set up by a single system of charge. Whereas electirc PE is a property shared by two systems.

For example if we consider each system to be only a point charge of charge q1 and q2 respectively, then when when looking at the potential energy, we have to take into account both point charges. The potential energy of the system of both point charges is given by

$$PE = \frac{kq1q2}{r}$$

notic that both charges, q1 and q2, show up in this equation. It would be wrong to say that only charge q1 has the potential energy given by the above equation since PE is a shared property, it refers to the system of both point charges. If we take away charge q2, then q1 no longer as that same PE. This is analogous to gravitational PE. The grav PE of a ball 15m above the earth is a shared property of the ball and the earth.

But potential is a property of only one of the point charges. For example, the first charge sets up a potential of

$$V = \frac{kq1}{r}$$

this has nothing to do with q2. the potential set up by q1 is unaffected by q2's presence.

6. Jun 15, 2005

### JFo

Notice that OlderDan's forumula fits this one if you consider the system to be made up of a point charge.

OlderDan's formula is just more general, that is to find the potential set up by a system, just find the PE between that system and another system, and divide by the charge of the second system.

7. Jun 15, 2005

### OlderDan

This is a special case for the potential at a point in space at distance r from a point charge of magnitude q. As is always the case with potential energy, the point at which the potential energy is defined to be zero is arbitrary. In this case the zero of potential energy, and the potential, has been chosen to be zero at infinite distance from the charge.

8. Jun 15, 2005

### OlderDan

If that is the way your book states it, it is dimensionally incorrect. The numerical value of the potential and the numerical value of the potential energy of one coulomb placed at that point would be the same, but their units would not be the same.

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