PHY 242 Laboratory for 9/9/99

RESISTANCE

(Is Not Useless)

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,

R = V/I

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. In this section, you will examine a standard tungsten filament light bulb and a special bulb with a carbon filament.

Tungsten

  1. Plot the voltage vs the current for the standard light bulb provided. Choose increments of 10 volts for a range of 10V to 120V 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. (We will study AC circuits later.)
  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?

Carbon

  1. Plot the voltage vs the current for the special carbon light bulb provided. (It can be easily identified by the clearly visible double loop filament.) As before, 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.
  2. Calculate the resistance at each voltage. Plot resistance vs the current and fit a smooth curve through the points.

  3. Describe qualitatively how the carbon and tungsten filaments differ. Offer an explanation of the behavior of each material. (Consult your text for this information.)

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.