PHY 212 Laboratory for 2/10/2004

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

The power consumption of the bulb filament (which is then given up as heat and light) is given by

The Set-Up The circuit shown below uses a variable AC transformer as the voltage source. Think of it simply as a variable voltage battery. It has a standard AC outlet with two "slots", one of which can be thought of as the negative terminal and the other as the positive terminal. (We will study the true nature of AC potentials later.) An ammeter (A) is placed in series with the variac and the light bulb and measures the current in the circuit. The ammeter has very little resistance and therefore has very little effect on the circuit. The voltmeter (V) is connected in parallel with the light bulb and measures the potential drop across it. The voltmeter has very high resistance and therefore, like the ammeter but for the opposite reason, has very little effect on the circuit.

Tungsten Filament

The standard household lightbulb uses a tungsten filament. Screw the standard bulb into the socket board and wire the ammeter, voltmeter and variac together as shown in the diagram. Be sure that the instructor has examined your work before powering in the variac.

1. Choose increments of 10 volts for a range of 20V 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, but may be treated the same as DC values. Record these values in a table. (Leave room for two additional columns in this table.)

2. Calculate the resistance of the filament at each voltage and record the values in the table. Calculate the power consumption of the filament at each voltage setting and record these values in the table.

3. Plot voltage vs the current.

4. Plot resistance vs the power consumption.

Carbon Filament

Repeat the procedure above for the special carbon light bulb provided. It can be easily identified by the clearly visible double loop filament. The carbon filament is similar to the original filament used by Edison. Within a few years it was replaced with the cheaper and more durable tungsten filament.

Report

In your report, discuss the voltage vs current plot. Particularly, what does the shape of the plot indicate about the resistance of the filament. Similarly, discuss the resistance vs power consumption plot.

Describe qualitatively how the carbon and tungsten filaments differ. What can you say qualitatively about the temperature dependence of the resistance for each filament? (Temperature increases with increasing power consumption.)

2. The Kirchoff Circuit Challenge (3-5 pts)

When you enter the lab, you will be given a diagram of three circuits consisting of resistors and a battery. The circuits are assigned point values of 3, 4, or 5, consistent with their difficulty.

1. Choose one of the circuits and determine what the voltage will be across the specified resistor. Although the approximate values of the resistors are specified by the color coded bands, use the ohmeter to obtain more exact values.
2. Carefully construct the circuit. Do not turn on the power supply that will serve as the battery.

• When you are ready, the instructor will collect your calculations, turn on the power supply and check the voltage. If the voltage reading matches your calculation (within a reasonable error), you will receive the points for the circuit you have chosen. There will be no further work required for this section.

• If the values do not match, you must either redo the calculations and submit them with the lab report AND/OR you will be required to reconstruct and re-test the circuit before leaving the lab, depending upon the error you have committed.

May Kirchoff's Rules guide you to success! Good Luck!