**Electric Potential Difference
**

An **electric field **is a region in space where a force is exerted on a **positive test charge **.

**Electric lines of force **represent the direction that a positive test charge would move in an electric field.

- By convention, they
at positively charged objects and*originate*at negatively charged objects.*terminate*

Work

- These two points differ in their
.__electric potential__

This is analogous to the definition of the gravitational potential energy through the work done by the force of gravity in moving a mass through a certain distance.

The units of potential difference, or simply **potential**, are Joules / Coulomb, which are called

- Physically, potential difference has to do with how much work the electric field does in moving a charge from one place to another.
- Batteries, for example, are rated by the potential difference across their terminals.
- In a nine volt battery the
between the positive and negative terminals is precisely nine*potential difference***volts**. - On the other hand the
across the power outlet in the wall of your home is 110*potential difference***volts**.

The

**V = W ÷ Q**

The **volt** (V) is the unit used to measure electric potential difference.

An electric potential difference must exist for current to flow in an electric circuit.

As charge moves from one point to another in an electric circuit, energy is released.

- This results in a decrease in electric potential.
- The decrease in electric potential implies that there is an electric potential difference between the two points.
- The term electric potential difference is preferable to "voltage."

An object connected directly to the ground can be described as being **earthed, or “grounded”.**

The potential at any point in an electric field can be either positive or negative with respect to the earth, depending on the nature of the charge (pos or neg).

__Capacitance__

**Capacitance** is a measure of the amount of **electric** **charge** stored (or separated) for a given **electric potential**. The capacitance is usually defined as the total electric charge placed on the object divided by the potential of the object:

**C = Q ÷ V**

where

C is the capacitance in **farads**

Q is the charge in **coulombs**

V is the potential in **volts**

Note that this formula results in "charge per volt", or Coulombs per volt. 1 Coulomb/volt equals 1 * farad*..

**Capacitance** exists between any two conductors insulated from one another.

The formula defining capacitance above is valid if it is understood that the conductors have equal but opposite charge Q, and the voltage V is the **potential difference** between the two conductors.

The **SI** unit of capacitance is the ** farad** (F).

A capacitance of one farad results in a potential of one volt for one coulomb of charge.

The capacitance of the majority of capacitors used in electronic circuits is several orders of magnitude smaller than the farad.

The most common units of capacitance in use today are the microfarad (µF), the nanofarad (nF) and the picofarad (pF).

It should be noted that the above equation (C = Q/V) is only applicable for values of Q which are much larger than the electron charge e = 1.602x10^{-19} C.

For example, if a capacitance of 1 pF is charged to a voltage of 100 nV, the equation would predict a charge Q = 10^{-19} C, which is smaller than the charge on a single electron.

Homework: Ch 21, problems 32-36, 39,40,41

Video: Electric Potential and Capacitance