University Physics, Lab 3: Internal Resistance of A Battery

In this experiment you will measure the internal resistance of an ordinary battery (Size D, 1.5 Volt).

Apparatus:

Two multimeters, a kit containing several light bulbs, two D batteries, leads, battery housing with end conductors.

Precaution:

Make sure that you understand the proper connection of Ammeter to the circuit.  If you are not sure, ask the instructor.  Connecting the ammeter parallel may cause a fuse burnout.  Note that an ammeter has very low internal resistance; therefore if connected across a power supply, it will cause a short circuit.  Proper choice of test lead connection to the meter and proper choice of knob position is required to prepare the multimeter to be used as an ammeter.  Reserve the large multimeter as an ammeter.  Its leads need to be connected properly and its scale should be in mA scale.

Procedure:

I-One light Bulb:

Use the example color-coding shown below (next page) to set up the circuit correctly.  If 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 resistnace 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) and the current I.  Then disconnect one of the leads to the battery case, and measure the emf(e) across its terminals (this is open circuit voltage). Record the answers below with proper units.

Vab: _________

e: ____________

I: _____________

II. Two light bulbs in parallel:

Connect another light bulb parallel to the first, and repeat the measurements.

Vab: _________

e: ____________

I: _____________

Calculations:
For each data set find r from,

and compare the two set result.

r (using one bulb data)___________r (using two bulb data) ________

These values should represent the internal resistance of the battery.  Check the web page of some of the manufacturers to see if they give any information about what the internal resistance should be for a fresh battery.

You may ask the instructor to provide you—if she/he has one-- with a fresh battery to repeat the experiment.