Advanced Ohm's Law

In this lab we'll use Ohm's Law to look at voltages, resistances and currents in different situations.


A) Internal Resistance of the Function Generator
If you take a close look at your function generator, it says "50Ω OUT" by the outputs. What this means is that the power supply acts as if it were a voltage in series with a 50Ω resistor. For circuits that have large resistances, this can usually be ignored, but at times it needs to be accounted for. In this section we will verify the internal resistance.

Plug in your function generator, set it to 50Hz sine waves, and measure the voltage with a DMM. Make sure that the DMM is set to measure AC voltages (in practice we can treat the DMM as if it was a very large resistor, and hence ignore the internal resistance of the DMM). Next connect a 50Ω resistor to the function generator. You will find that the voltage across the new resistor is roughly half the previously measured voltage. Draw a ciruit diagram with the internal resistance in series with your "load" resistor. Use your voltage measurements and the resistance of the load to verify that the internal resistance of the function generator is what it says it is. Replace the 50Ω resistor with other resistors to verify the internal resistance of the function generator.

B) Light Bulb
Make a series circuit with a 100 ohm resistor and a light bulb. Incrementally increase the voltage to the circuit and measure the current through the circuit by observing the voltage across a resistor. Since we are looking for the resistance of the lightbulb, we'll need to know the voltage across the bulb, which should be the voltage of the power supply minus the voltage across the resistor. Plot the current as a function of voltage across the lightbulb, keeping an eye on your intercepts. What should it look like? What does it look like? What does this tell you about the lightbulb?

C) Batteries
What does it mean when a battery is "dead"? Here we'll look at the internal resistance of both new and dead batteries. Take a battery and plot its voltage vs. current for a variety of resistances. How can you use this to determine the internal resistance? Do this for a selection of new and dead batteries, making sure you select pairs (for example, both a new and old C battery).

Write up your observations and conclusions in your lab notebook.