Students will learn the difference between Emf and Voltage (i.e. ideal voltage and the output voltage) and how to calculate the internal resistance of a battery.
Key Equations
The terminal voltage (or 'output voltage') is equal to the emf (it's 'ideal voltage') minus the voltage drop across the internal resistance.
Example 1
You have a battery with an EMF of 5 V and an unknown internal resistance. You hook the battery up to a circuit with one resistor and measure the current through the resistor to be 1.5 A. What is the internal resistance of the batter and how much power is the battery's resistance dissipating.
Solution
To start this problem we'll first find the terminal voltage of the battery using the information we know about the resistor. We know the voltage drop across the resistor must be equal to the terminal voltage because there the total change in voltage must be over the whole circuit.
We can plug this value into the equation given above to find the internal resistance.
Now we can find find the power dissipated by the resistor
Watch this Explanation
Time for Practice

In the circuit shown here, the battery produces an
emf
of
and has an internal resistance of
.
 Find the total resistance of the external circuit.
 Find the current drawn from the battery.
 Determine the terminal voltage of the battery
 Show the proper connection of an ammeter and a voltmeter that could measure voltage across and current through the resistor. What measurements would these instruments read?

Students are now measuring the terminal voltage of a battery hooked up to an external circuit. They change the external circuit four times and develop the
Table
(
below
); using this data, complete the following:
 Graph this data, with the voltage on the vertical axis.
 Use the graph to determine the emf of the battery.
 Use the graph to determine the internal resistance of the battery.
 What voltage would the battery read if it were not hooked up to an external circuit?
Terminal Voltage Current 
You have a battery with an emf of
and an internal resistance of
. Some
are drawn from the external circuit.
 What is the terminal voltage
 The external circuit consists of device , and ; device , and , and two different resistors. Show how this circuit is connected.
 Determine the values of the two resistors.
Answers to Selected Problems
 a. b. c.
 b. c. d.
 a. b. c. and