Physics Lab: Internal Resistance
Goal: To measure the internal resistance of one or more batteries.
Materials
- Two multimeters
- Several light bulbs
- two D batteries, one used if possible, and one fresh
- leads
- battery housing with end conductors
Procedure
Make sure that you understand the proper series connection to use one multimeter as an ammeter in the circuit. Note that an ammeter has very low internal resistance; therefore if connected across a power supply, it will cause a short circuit, so connecting the ammeter parallel may cause a fuse burnout. Use a multimeter with an mA scale. The second multimeter should be connected across the battery.
- Use the example color-coding shown below to set up the circuit correctly. If you have different color test leads, adapt it to the order of color-coding provided in the example. In the circuit shown below, r is the internal resistance of the battery, R1 is the light bulb, and R2 is the total resistance of all the test leads used.
- Connect the circuit as shown and measure the voltage Vab (this is closed circuit voltage of the battery) using the fresh battery.
- Measure the current I.
- Disconnect one of the leads to the battery case, and measure the emf(e) across its terminals (this is open circuit voltage).
- Record your measurements for Vab, e, and I with the proper units.
- Connect another light bulb parallel to the first, and repeat the measurements.
- Repeat both sets of measurements (different number of light bulbs) using the used battery.
Data Handling
- For each circuit (one, two, or more light bulbs), find r using
- Compare your results. These values should represent the internal resistance of the battery. Check the web page of the battery manufacturers to see if they give any information about what the internal resistance should be for a fresh battery.
Report
Follow standard reporting procedures and include any conclusions you draw on differences for r in your measurements.
Post your report to the Moodle.
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