Monday, August 11, 2008


AIM: To verify Stefan’s law of radiation.
APPARATUS: DC Regulated power supply, Voltmeter, Ammeter, Electric Bulb, Rheostat.
PRINCIPLE: According to Stefan’s law, power radiated from a blackbody is proportional to the fourth power of its absolute temperature.
I.e. P µ T4
Or log P µ 4 log T --------- (1)
Resistance of the tungsten filament of the electric bulb, R µ T
Or log R µ log T --------- (2)
(1) / (2) gives log P/log R = 4
PROCEDURE: Connections are made as shown in figure (1). The voltage through the bulb is varied and the corresponding current is noted. Readings are taken only after the bulb starts glowing. The power radiated and resistance are calculated in each case. A graph is plotted taking log P along Y-axis and log R along X-axis. The slope of the straight line graph is calculated.

Voltage V(Volts) Current I(Amp) R=V/I (Ohms) P=VI (Watts) logP logR

Slope from graph=AB/BC= =
RESULT: Slope of the graph = » 4 which verifies Stefan’s law.

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