Internal Resistance lab

This virtual lab uses the PHET Circuit Construction Kit to study how to determine the emf and internal resistance for a battery by graphing voltage and current.

Internal Resistance Lab (pdf)
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Internal Resistance Lab (editable)
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Tutorial

PHET Circuit Construction Kit

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Part 1: Setting up your circuit

Using the PHET Circuit Construction kit, use a battery, resistor, ammeter, and voltmeter to build the circuit design shown in the schematic diagram on the right
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On the right of the simulation window there is a dropdown menu for “Battery Resistance”. Choose an internal resistance and circle your selection in the table below

​Display the circuit values by making sure that the “values” checkbox is marked

Click on the battery to choose a voltage between 1 V – 120 V and record your selection below. The value that you are setting is your battery’s emf

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Part 2: Collecting Data

​With this set up, you are measuring the total current flowing through the circuit as well as the terminal voltage of the battery. To collect multiple measurements, adjust the external resistance and record the resulting current and voltage in the table below. (see the video tutorial above for an example

External Resistance (Ω)	Current (A)	Terminal Voltage (V)
2.5
5
10
20
40
80

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Part 3: The Mathematical Model

​Copy the table from part 2 into Excel and create a scatterplot from only the “Current” and “Terminal Voltage” columns. Add a trendline with an equation to your graph. Make sure that the current is graphed on the x-axis.

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Part 4: Analysis of the Model

​Compare your mathematical model with the battery values that you selected in part 1:

1. What electrical property does the slope represent?
2. What electrical property does the y-intercept represent?

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​Part 5: Extending our Understanding

​If we were to do this lab in real life, we wouldn’t have a simulation displaying the emf and internal resistance of our battery. Analyze the data shown in the screenshots below to determine the mystery battery’s properties that are covered up by the emoji. 

Battery emf

Battery Resistance

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Part 6: Checking your Work

​To check if your values for the mystery battery are correct, create a test circuit with your battery values to recreate one of the example configurations shown in part 5. All values and measurements should match.

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Answers

This is the plot and properties for the mystery battery shown in the screenshots above.

Battery emf	42 V
Battery Resistance	3 ​Ω

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