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Stefan-Boltzmann Law

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The primary purpose of this lab is to see how closely the tungsten filament of a small flashlight bulb approximates a blackbody by measuring the power radiated by the bulb and its temperature, plotting the data on log-log paper and computing the exponent of the P vs T relationship from that plot. The Stefan-Boltzmann law states that for a blackbody the exponent should be 4. The student sees that for low temperatures the exponents is as low as 2.5 and for higher temperatures (approximately 1800K) the exponent lies in the range of 3.6-3.9. Secondary features of the lab include: (1) introducing the student to the use of thermopiles and optical pyrometers, (2) to show that the intensity of the radiation form the bulb is a function of 1/r^2, and (3) that the assumption that the power into the bulb is equal to the power out is a reasonable assumption for this experimental setup.

A second version of the lab takes a more detailed look at the actual power-temperature relationship as described by John W. Dewdney in an article in the American Journal of Physics (1960). In that article Dewdney explains that the power-temperature relationship is actually of the form P = aT^d + bT + C. The students take the same data as in the original form of the lab and use a computer modeling program to determine the values of the coefficients a, b and c and of the exponent d.

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Student Handouts


Number of set-ups available:

Per lab station:

1 HP 6215A DC power supply
2 multimeters
5 banana leads
1 small light bulb
1 socket for the light bulb

Per class (1 setup):

1 optical pyrometer/power supply, meter and lense set
1 HP 6215A DC power supply
5 banana leads
2 multimeters
1 socket for the light bulb/light bulb

Per class (1 setup):

1 HP 6215A DC power supply
4 multimeters
7 banana leads
1 Variac 1 thermopile/ring stand and clamp 1 2 meter meterstick 1 light socket/ring stand 3 ring stands/standard bulb 1 black cloth with hole

Stefan  apparatus


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