Interactive Lecture Demonstration 4 – Temperature of a Light Bulb Filament

The purpose of this activity is to decide whether a light bulb filament obeys Ohm's Law, and to determine the temperature of the filament.

Recall the definition of the resistance of a material: R = -DV/I where DV is the voltage difference between two ends of the material and I is the conventional current that the voltage difference causes to flow through the material. The - sign is important to include, because DV will be a negative number whereas I and R are positive numbers by definition. (We can see that DV is negative, because the conventional current I will always flow from a higher voltage at one end of the material to a lower voltage at the other end). Also recall that a material is said to obey Ohm's Law if its resistance is a constant, independent of the voltage difference applied across the material.

We will determine the resistance of a light bulb filament for several different values of applied voltage difference. The voltage source will be a Variac, which supplies a voltage difference that can be varied continuously. We will measure the current supplied by the Variac using a digital multimeter functioning in ammeter mode.

1. First, we will measure the resistance of the light bulb directly, using the digital multimeter's resistance mode. In this mode, the multimeter determines the resistance of the bulb by applying a very tiny known voltage difference across the bulb, measuring the current through the bulb, and using the definition of resistance (dividing DV by I to get R). In the data table below, record the value of resistance that the multimeter shows for this case. What was the approximate temperature (in degrees Celsius) of the light bulb filament when this resistance was measured ?

2. Now we will determine the resistance for three more values of the applied voltage. In the data table, record the values measured for voltage difference and current in each case. Then use the definition of resistance to calculate the filament's resistance for each case.

3. Explain why your four values for the resistance are different.

4. What metal is the light bulb filament made of ? (If you don't know from previous reading or because you know it's the metal with the highest melting temperature, you can find the answer on p. 563 of Serway and Faughn).
5. Now use the relevant temperature coefficient of resistivity from the table on p. 563, plus the equation on p. 564 that relates resistances at two temperatures, to calculate the temperature of the light bulb filament for the last three entries in the data table (assuming that you already know the temperature for the first entry in the table, you can use it to determine all the other temperatures).

6. Based on the results of this activity, does the light bulb obey Ohm's Law ? Why or why not ?