Sample Lab Report

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“The Stefan - Boltzmann Equation - Is the Exponent Really T4? ” Objective: To examine the energy transfer that occurs in a light bulb as a function of filament temperature and determine if the amount of energy emitted per second is proportional to the fourth power of temperature. Introduction: Electromagnetic radiation absorbed and emitted by any substance is dependent on the temperature of the substance. Josef Stefan showed experimentally in 1879 that for a perfect emitter (a 'black body') the rate at which energy is emitted is related to the object's temperature by the following equation: P = A T 4 Here, the P term is the amount of Joules leaving the object per second, the A term is the surface area and the term (epsilon) the emissivity (color) of the surface. The coefficient (sigma) is the constant that links all of these parameters to our system of Joules, degrees Kelvin, meters and seconds and is equal to the Stefan-Boltzmann constant: = 5.6704 × 10-8 Joule / meter 2 sec Kelvin 4 We examined this equation by using an ordinary tungsten filament lamp and by taking measurements attempt to show that the exponent is indeed a fourth order type. The output power of the bulb is easy to obtain. It is simply the product of the voltage applied and the current flowing through the bulb filament. That is: P = V I Where V is the applied lamp voltage and I the lamp current. The temperature of the filament is a problem to measure. We cannot get inside the bulb to place a thermometer onto the filament. However by measuring the electrical resistance of the filament inside the light bulb for every value of voltage we examined the change in resistance of tungsten as a function of temperature and comparing this to Chart 1 we were able to deduce a temperature for every power level that we apply. The resistance of the filament is given by:

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