PHY 242 Laboratory for 9/10/98

Measuring Resistance

The resistance between two points is defined as the ratio of the voltage difference between those points to the current flowing between them. That is,

This definition is often confused with Ohm's Law (V = IR) for ohmic devices, in which R is assumed to be constant for all values of V and I. For many resistive devices this is true, if the temperature is relatively constant. But if the current varies significantly, so will the temperature and hence the resistance. The light bulb is such a case.

1. Plot the voltage vs the current for the light bulb provided. Choose increments of 10 volts for a range of 10V to 120 V by adjusting the variac voltage supply and reading the values from the voltmeter and ammeter. Both the voltage and current readings are in rms AC units and may be treated the same as DC values.
2. Calculate the resistance at each voltage. Plot resistance vs the current and fit a smooth curve through the points.
3. What is the power consumption of the bulb at 20V, 60V, and 120V?

Resistors in Circuits

1. You are provided with three resistors. Using the ohm meter, find the resistance of each by holding the probes to each end. Using the alligator clips, connect the resistors in all possible parallel and series combinations of 2 and 3. Measure and record the resistance for each combination and compare them to the theoretical values.
2. You are provided with two "special" high ohm resistors. Repeat the above analysis and explain your results. (What may have effected your results?)

A Row of Bulbs

The power rating of a light bulb is the power consumption (and output) of the bulb at 120V. The light bulb platform contains four bulbs of unknown power ratings. Using the jumper leads, you may connect the bulbs in any series combination. Do so and record your observations of the relative brightness of the bulbs in each case. From these observations how would you rank the bulbs in terms of the their power ratings? Justify your choices.