# Electric Potential and Potential Difference

#### LEARNING OBJECTIVES

By the end of this section, you will be able to:

- Define electric potential, voltage, and potential difference
- Define the electron-volt
- Calculate electric potential and potential difference from potential energy and electric field
- Describe systems in which the electron-volt is a useful unit
- Apply conservation of energy to electric systems

Recall that earlier we defined electric field to be a quantity independent of the test charge in a given system, which would nonetheless allow us to calculate the force that would result on an arbitrary test charge. (The default assumption in the absence of other information is that the test charge is positive.) We briefly defined a field for gravity, but gravity is always attractive, whereas the electric force can be either attractive or repulsive. Therefore, although potential energy is perfectly adequate in a gravitational system, it is convenient to define a quantity that allows us to calculate the work on a charge independent of the magnitude of the charge. Calculating the work directly may be difficult, since

and the direction and magnitude of

can be complex for multiple charges, for odd-shaped objects, and along arbitrary paths. But we do know that because

, the work, and hence

, is proportional to the test charge

. To have a physical quantity that is independent of test charge, we define **electric potential**

(or simply potential, since electric is understood) to be the potential energy per unit charge:

#### ELECTRIC POTENTIAL

The electric potential energy per unit charge is

(3.2.1)

Since U is proportional to q, the dependence on q cancels. Thus,

does not depend on

. The change in potential energy

is crucial, so we are concerned with the difference in potential or potential difference

between two points, where

#### ELECTRIC POTENTIAL DIFFERENCE

The **electric potential difference** between points

and

,

is defined to be the change in potential energy of a charge

moved from

to

, divided by the charge. Units of potential difference are joules per coulomb, given the name **volt (V)** after Alessandro **Volta**.

The familiar term **voltage** is the common name for electric potential difference. Keep in mind that whenever a voltage is quoted, it is understood to be the potential difference between two points. For example, every battery has two terminals, and its voltage is the potential difference between them. More fundamentally, the point you choose to be zero volts is arbitrary. This is analogous to the fact that gravitational potential energy has an arbitrary zero, such as sea level or perhaps a lecture hall floor. It is worthwhile to emphasize the distinction between potential difference and electrical potential energy.

#### POTENTIAL DIFFERENCE AND ELECTRICAL POTENTIAL ENERGY

The relationship between potential difference (or voltage) and electrical potential energy is given by

(3.2.2)

Voltage is not the same as energy. Voltage is the energy per unit charge. Thus, a motorcycle battery and a car battery can both have the same voltage (more precisely, the same potential difference between battery terminals), yet one stores much more energy than the other because

. The car battery can move more charge than the motorcycle battery, although both are

batteries.

#### EXAMPLE 3.2.1

#### Calculating Energy

You have a

motorcycle battery that can move

of charge, and a

car battery that can move

of charge. How much energy does each deliver? (Assume that the numerical value of each charge is accurate to three significant figures.)

#### Strategy

To say we have a

battery means that its terminals have a

potential difference. When such a battery moves charge, it puts the charge through a potential difference of

, and the charge is given a change in potential energy equal to

. To find the energy output, we multiply the charge moved by the potential difference.

#### Solution

For the motorcycle battery,

and

. The total energy delivered by the motorcycle battery is

Similarly, for the car battery,

and

#### Significance

Voltage and energy are related, but they are not the same thing. The voltages of the batteries are identical, but the energy supplied by each is quite different. A car battery has a much larger engine to start than a motorcycle. Note also that as a battery is discharged, some of its energy is used internally and its terminal voltage drops, such as when headlights dim because of a depleted car battery. The energy supplied by the battery is still calculated as in this example, but not all of the energy is available for external use.

#### CHECK YOUR UNDERSTANDING 3.4

How much energy does a

AAA battery have that can move

?

Note that the energies calculated in the previous example are absolute values. The change in potential energy for the battery is negative, since it loses energy. These batteries, like many electrical systems, actually move negative charge—electrons in particular. The batteries repel electrons from their negative terminals (

) through whatever circuitry is involved and attract them to their positive terminals (

), as shown in Figure 3.2.1. The change in potential is

and the charge

is negative, so that

is negative, meaning the potential energy of the battery has decreased when

has moved from

to

(Figure 3.2.1)

**Figure 3.2.1**A battery moves negative charge from its negative terminal through a headlight to its positive terminal. Appropriate combinations of chemicals in the battery separate charges so that the negative terminal has an excess of negative charge, which is repelled by it and attracted to the excess positive charge on the other terminal. In terms of potential, the positive terminal is at a higher voltage than the negative terminal. Inside the battery, both positive and negative charges move.#### EXAMPLE 3.2.2

#### How Many Electrons Move through a Headlight Each Second?

When a

car battery powers a single

headlight, how many electrons pass through it each second?

#### Strategy

To find the number of electrons, we must first find the charge that moves in

. The charge moved is related to voltage and energy through the equations

. A

lamp uses

joules per second. Since the battery loses energy, we have

and, since the electrons are going from the negative terminal to the positive, we see that

.

#### Solution

To find the charge

moved, we solve the equation

:

Entering the values for

and

we get

The number of electrons

is the total charge divided by the charge per electron. That is,

#### CHECK YOUR UNDERSTANDING 3.5

How many electrons would go through a

lamp?

#### The Electron-Volt

The energy per electron is very small in macroscopic situations like that in the previous example—a tiny fraction of a joule. But on a submicroscopic scale, such energy per particle (electron, proton, or ion) can be of great importance. For example, even a tiny fraction of a joule can be great enough for these particles to destroy organic molecules and harm living tissue. The particle may do its damage by direct collision, or it may create harmful X-rays, which can also inflict damage. It is useful to have an energy unit related to submicroscopic effects.

Figure 3.2.2 shows a situation related to the definition of such an energy unit. An electron is accelerated between two charged metal plates, as it might be in an old-model television tube or oscilloscope. The electron gains kinetic energy that is later converted into another form—light in the television tube, for example. (Note that in terms of energy, “downhill” for the electron is “uphill” for a positive charge.) Since energy is related to voltage by

, we can think of the joule as a coulomb-volt.

(Figure 3.2.2)

**Figure 3.2.2**A typical electron gun accelerates electrons using a potential difference between two separated metal plates. By conservation of energy, the kinetic energy has to equal the change in potential energy, so . The energy of the electron in electron-volts is numerically the same as the voltage between the plates. For example, a potential difference produces electrons. The conceptual construct, namely two parallel plates with a hole in one, is shown in (a), while a real electron gun is shown in (b).#### ELECTRON-VOLT

On the submicroscopic scale, it is more convenient to define an energy unit called the **electron-volt (eV)**, which is the energy given to a fundamental charge accelerated through a potential difference of

. In equation form,

An electron accelerated through a potential difference of

is given an energy of

. It follows that an electron accelerated through

gains

. A potential difference of

(

) gives an electron an energy of

(

), and so on. Similarly, an ion with a double positive charge accelerated through

gains

of energy. These simple relationships between accelerating voltage and particle charges make the electron-volt a simple and convenient energy unit in such circumstances.

The electron-volt is commonly employed in submicroscopic processes—chemical valence energies and molecular and nuclear binding energies are among the quantities often expressed in electron-volts. For example, about

of energy is required to break up certain organic molecules. If a proton is accelerated from rest through a potential difference of

, it acquires an energy of

(

) and can break up as many as

of these molecules (

per molecule

molecules). Nuclear decay energies are on the order of

(

) per event and can thus produce significant biological damage.

#### Conservation of Energy

The total energy of a system is conserved if there is no net addition (or subtraction) due to work or heat transfer. For conservative forces, such as the electrostatic force, conservation of energy states that mechanical energy is a constant.

Mechanical energy is the sum of the kinetic energy and potential energy of a system; that is,

. A loss of

for a charged particle becomes an increase in its

. Conservation of energy is stated in equation form as

or

where

and

stand for initial and final conditions. As we have found many times before, considering energy can give us insights and facilitate problem solving.

#### EXAMPLE 3.2.3

#### Electrical Potential Energy Converted into Kinetic Energy

Calculate the final speed of a free electron accelerated from rest through a potential difference of

. (Assume that this numerical value is accurate to three significant figures.)

#### Strategy

We have a system with only conservative forces. Assuming the electron is accelerated in a vacuum, and neglecting the gravitational force (we will check on this assumption later), all of the electrical potential energy is converted into kinetic energy. We can identify the initial and final forms of energy to be

,

,

,

.

#### Solution

Conservation of energy states that

Entering the forms identified above, we obtain

We solve this for

:

Entering values for

,

, and

gives

#### Significance

Note that both the charge and the initial voltage are negative, as in Figure 3.2.2. From the discussion of electric charge and electric field, we know that electrostatic forces on small particles are generally very large compared with the gravitational force. The large final speed confirms that the gravitational force is indeed negligible here. The large speed also indicates how easy it is to accelerate electrons with small voltages because of their very small mass. Voltages much higher than the

in this problem are typically used in electron guns. These higher voltages produce electron speeds so great that effects from special relativity must be taken into account and hence are beyond the scope of this textbook. That is why we consider a low voltage (accurately) in this example.

#### CHECK YOUR UNDERSTANDING 3.6

How would this example change with a positron? A positron is identical to an electron except the charge is positive.

#### Voltage and Electric Field

So far, we have explored the relationship between voltage and energy. Now we want to explore the relationship between voltage and electric field. We will start with the general case for a non-uniform

field. Recall that our general formula for the potential energy of a test charge

at point

relative to reference point

is

When we substitute in the definition of electric field (

), this becomes

Applying our definition of potential (

) to this potential energy, we find that, in general,

(3.2.3)

From our previous discussion of the potential energy of a charge in an electric field, the result is independent of the path chosen, and hence we can pick the integral path that is most convenient.

Consider the special case of a positive point charge

at the origin. To calculate the potential caused by

at a distance

from the origin relative to a reference of

at infinity (recall that we did the same for potential energy), let

and

, with

and use

. When we evaluate the integral

for this system, we have

which simplifies to

This result,

is the standard form of the potential of a point charge. This will be explored further in the next section.

To examine another interesting special case, suppose a uniform electric field

is produced by placing a potential difference (or voltage)

across two parallel metal plates, labeled

(Figure 3.2.3)

**Figure 3.2.3**The relationship between and for parallel conducting plates is . (Note that in magnitude. For a charge that is moved from plate at higher potential to plate at lower potential, a minus sign needs to be included as follows: .)From a physicist’s point of view, either

or

can be used to describe any interaction between charges. However,

is a scalar quantity and has no direction, whereas

is a vector quantity, having both magnitude and direction. (Note that the magnitude of the electric field, a scalar quantity, is represented by

.) The relationship between

and

is revealed by calculating the work done by the electric force in moving a charge from point

to point

. But, as noted earlier, arbitrary charge distributions require calculus. We therefore look at a uniform electric field as an interesting special case.

The work done by the electric field in Figure 3.2.3 to move a positive charge

from

, the positive plate, higher potential, to

, the negative plate, lower potential, is

The potential difference between points

and

is

Entering this into the expression for work yields

Work is

; here

, since the path is parallel to the field. Thus,

. Since

, we see that

.

Substituting this expression for work into the previous equation gives

The charge cancels, so we obtain for the voltage between points

and

where

is the distance from

to

, or the distance between the plates in Figure 3.2.3. Note that this equation implies that the units for electric field are volts per meter. We already know the units for electric field are newtons per coulomb; thus, the following relation among units is valid:

Furthermore, we may extend this to the integral form. Substituting Equation 3.2.2 into our definition for the potential difference between points

and

, we obtain

which simplifies to

As a demonstration, from this we may calculate the potential difference between two points (

and

) equidistant from a point charge