Lesson Explainer: Electromotive Force and Internal Resistance Physics

In this explainer, we will learn how to relate the electromotive force (emf) of a battery to its terminal voltage and its internal resistance.

Batteries are usually thought of as supplying a potential difference to other components of a circuit in order to produce a current in those components. This is correct. It is also true, however, that a battery produces a potential difference across itself in order to produce a current through itself.

Consider a battery that produces a potential difference across its terminals. When a conducting wire connects the positive and negative terminals of the battery, a circuit is closed. A direct current is produced in the circuit. The current in the wire is given by where is the resistance of the circuit.

The direction of the current is from the positive terminal to the negative terminal. In a series circuit, the current at all points in the circuit is equal. This means that there must be equal currents out from the positive terminal and into the negative terminal. This is shown in the following figure.

From this we see that there must also be a current in the battery equal to the current at its terminals. This is shown in the following figure.

We have seen that

For two resistances in series and , their combined resistance is given by

We see then that for the circuit consisting of a wire and a battery must be the sum of the resistance of the wire and of the battery. We can call the resistance of the wire and the resistance of the battery .

The equation can be rearranged to have as its subject, which gives us

The current in the circuit can therefore be expressed as

is called the external resistance (and is also called the load) while is called the internal resistance.

A potential difference can be expressed as where is the work done by the potential difference on a charge across the potential difference.

The difference in potential across the ends of a wire is the decrease in potential across the wire. This is shown in the following figure.

The potential difference produced by a battery across a wire equals the work done per coulomb of charge on charges that move through the wire from one battery terminal to the other. The potential decreases along the length of the wire.

As well as when moving through the wire, work must be done to move charges through the battery. When this happens, the potential energy of the charges increases rather than decreases. The potential must then increase along the length of the battery. This is shown in the following figure.

For many purposes, a circuit containing a battery is modeled as having purely external resistance. The potential difference across such an external circuit can be measured using a voltmeter connected in parallel to the resistance of the circuit as shown in the following figure.

It is important to note that the wires connecting the battery, resistor, and voltmeter are modeled as having negligible resistance in this diagram.

It might be expected that a voltmeter could also measure the potential difference across a battery by connecting the voltmeter to the battery, as shown in the following figure.

This circuit would not, however, measure the potential difference across the battery. Both voltmeters in the circuit would measure the same value, which is the potential difference across the external circuit.

If we want to measure the potential difference across the battery terminals for charges moving inside the battery, a voltmeter would have to measure the work done on charges moving through the battery rather than through the external circuit.

We see then that a voltmeter in a circuit cannot measure the potential difference across the battery. This appears to show that there is no way to know the value of the internal resistance of a battery or the potential difference across it. In fact, it is possible to determine these values by using multiple measurements.

Considering a battery as a component of a circuit that has an external resistance , we see that there must be a decrease of potential, , across the battery. This is given by where is the current in the circuit.

Consider now the equation where is the potential difference across the external resistance, which can be measured using a voltmeter.

This can be written as

To use a voltmeter to measure the full potential difference that the battery can produce, must be zero.

must be zero if the value of is zero. This would give us the equation

This value of corresponds to all the potential difference of the battery doing work on the external circuit.

Unfortunately, if the value of is zero, then the equation must have the values

This seems to tell us that the only way that a voltmeter can measure the full potential difference of a battery is if that potential difference is zero. This seems inevitable, as a battery with a nonzero potential difference would produce a nonzero current and so a nonzero value of .

This conclusion is incorrect, however. The reason why the conclusion is incorrect is deduced later in this explainer. However, understanding why the conclusion is incorrect first requires us again to consider the equation

We have seen that the reading shown by a voltmeter equals . As we know that the potential difference across the external circuit plus the potential difference across the battery sum to give a total potential difference, we can make the following equation:

There are specific names for the quantities in this equation. is called the lost volts, is called the terminal voltage, and is called the electromotive force or emf.

All these quantities have the unit of volts. emf is denoted by the symbol . emf is, despite its name, not a force but a potential difference.

Let us now look at an example in which the emf of a battery is determined.

Let us now look at an example in which the internal resistance of a battery is determined.

A circuit like the one in the following figure can be used to determine the emf and internal resistance of a battery.

The voltmeter in the circuit measures the terminal voltage of the battery. The variable resistor in the circuit allows the resistance of the circuit to be changed. Changing the resistance of the circuit changes the current in the circuit. The value of terminal voltage for different values of current can therefore be measured.

The measured values can be plotted on a graph.

As the value of decreases, the value of increases. The value of for cannot be found from a voltmeter reading but can be estimated using voltmeter readings where . This is shown in the following figure.

This graph is the graph of a straight line that has a -axis intercept of . The graph has a negative slope.

The graph of a straight line can be written as where is the slope of the graph and is the -axis intercept.

The graph used to estimate has values of on its -axis and values of on its -axis. This shows us that the equation of the line of this graph is

We can rearrange the equation as

Comparing to the equation for the line of the graph used to estimate , we see that

The graph can therefore be used to determine as well as . Both the emf and internal resistance of a battery can be determined.

Let us now look at an example in which the emf and internal resistance of a battery are determined from measurements.

Example 3: Determining the emf and Internal Resistance of a Battery Using Multiple Measurements

The graph shows the change in the current in a circuit with the terminal voltage of the battery that produces the current.

1. What is the electromotive force of the battery?
2. What is the internal resistance of the battery?

Answer

Part 1

The emf of a battery is given by the equation where is the terminal voltage of the battery, is the internal resistance of the battery, and is the current in the circuit.

The value of the emf of the battery is equal to the -axis intercept of the line of best fit for the points plotted on the graph. This is shown in the following figure.

The emf of the battery is 6 V.

Part 2

The internal resistance of the battery is determined using the equation

This equation can be rearranged as and expressed as

This equation can be compared to the equation for the line of best fit where is the terminal voltage, is the current, is the emf, and is the slope of the line.

We see then that and so

The slope of the line of best fit is given by

We can take two very clear values of and from the graph: and , and and .

This gives us a value of as follows:

This gives us a value of as follows:

This gives a value of as follows:

We know that and so, equals 0.125 Ω.

Let us now summarize what has been learned in this explainer.