ABSTRACT
This paper presents the report of the experiment carried out on the general concepts of thermal radiation. The experiment is made up of various small experiments which were carried out one after the other. The inverse square law of radiation was investigated using the Stefan-Boltzmann lamp. The relative rates of absorption of four different surfaces to two varying radiation source at different temperature were also investigated. Also, the Stefan-Boltzmann law at high temperatures, the rate of cooling of different surfaces relative to time, and the Stefan-Boltzmann relationship at near room temperature were investigated.
At the end, conclusion was made that that the quantity of the emitted thermal radiation by an object is dependent both on the constituent material and the object’s temperature.
INTRODUCTION
According to the laws of thermodynamics, thermal Radiation is the electromagnetic radiation that is emitted by a body emits due to its temperature (Cengel 1997). In as much as the temperature is above the absolute zero, every matter would emit thermal radiation. (Cengel 1997).
This experiment has been broken into small parts in order to examine the properties of thermal radiation together with the interaction that occur between thermal radiation and the body of the material. The aim of this experiment is to quantitatively show that the emitted radiation by four different surfaces varies from one another.
- In the first experiment, the thermal radiation cube and the radiation sensor were setup as illustrated in the figure given Radiation Sensor was used to examine the relative magnitudes of the radiation emitted from various objects around the room. The Sensor was placed at approximately 5 cm from the black surface of the Radiation Cube and record the reading.
For the second experiment, the apparatus was set up as illustrated in the given diagram. With the lamp OFF, the sensor was slided along the meter stick. The millivolt meter at every 10cm intervals were recorded. The ambient level of the thermal radiation was determined by averaging the recorded value. A graph of the voltage reading (radiation intensity) was plotted against the function of x-2, and the best-fit trend line was gotten.
In the third experiment, relative measurements of the power unit area emitted from the Stefan Boltzmann lamp were made at different temperatures. The recorded measurements were compared with that gotten in the first part.
In the fourth experiment, the apparatus was set as illustrated in the given figure. In this experiment the Stefan-Boltzmann relationship was investigated at a much lower temperatures using the Thermal Radiation Cube. From the collected data investigation was made in order to find out if the intensity of the radiation from the filament is dependent on the 4th power of temperature. A plot of the intensity as a function of T4 was made and fitting a linear trend was traced.
RESULTS
Data from the result (graphs and table) shown below indicates that the different level of thermal radiation is emitted by different surfaces exposed to the same light bulb. In most of these experiments, the sides did not match up in temperature, meaning that they didn’t emit the same level of thermal radiation, even though the ohmmeter was allowed to attain thermal equilibrium before the process is started.
DISCUSSION
- According to the amount of emitted radiation, the order is Black, white, dull, and mirror. Although, the same heat source was used for all the four surfaces of the cube for the same period of time, each side exhibited different temperatures, this shows that they do not emit the same amount of thermal radiation. However, the above order is independent of temperature.
The graph of Radiation vs 1/x2 gives a more linear trend. However, this linear trend does not spread over the whole range. Intensity exists at the nearer distances fall off. The existence of this intensity is as a result of the non-point characteristics of the lamp.
- The Stefan-Boltzmann Lamp cannot be said to be a true point source. If it were, there won’t have been any falloff in the level of the light, especially for measurements taken close to the lamp, as seen in the course of this experiment. (Pasco)
- They do not eventually become equal since a large part of the radiation is absorbed while just a little of the radiation is reflected by the dark object. Conversely, a huge part of the radiation is reflected by the shiny object while just a little portion is retained. This shows why the black colored object retains much more heat that makes it hotter.
- Variations exist in the result of the two experiments this is because the sensor radiation for the lower temperatures experiment must be taken into consideration while the radiation of the sensors has no effect on the high-temperature experiment. The influence of the ambient temperature is also negligible during the high-temperature experiment.
- The lamp filament cannot be referred to as a true black body. If the lamp filament were to be a true black body, it would be totally and completely black at standard room temperature. However, it is seen as a fairly good approximation of a true black body, in as much as the temperature is high enough that the light emitted is much more than that of the incident light.
- Any other thermal source in the room would have an influence on the results, including the room itself and the warm body of the experimenter. These other sources would introduce some error, but these errors can be negligible in as much as the temperature of the lamp is relatively higher than that of the other sources.
- The level of variation between the materials in terms of their surfaces and constituent materials explains why the cooling rates of these materials vary. A good example can be seen in the first experiment in which all the sides of the Leslie’s Cube were at the same temperature what the polished side emitted was less than 10% as much radiation as that of the black side. This shows that the cooling rate of the polished side is faster.
- The graph is linear but cannot be said to be completely linear; this is due to the fact that there is a characteristic falloff in the intensity at some places in the neighboring distances. These occurrences of the falloffs are most likely as a result of the non-point characteristics of the lamp.
- The linearity of this graph has ascertained the veracity, precision, and accuracy of the Stefan-Boltzmann equation, even at low temperatures. (ScienceWorld 2010)
- Conclusion
The amount of thermal radiation that an object emits is dependent both on the constituent material of the object and the temperature of the object. While the absorption of thermal radiation results in a change in the temperature, with the particular change determined by the level of thermal radiation that gets to the object and the properties the materials possess.
- References
- Thermal Radiation. Wolfram Research.
- http://scienceworld.wolfram.com/ physics/ ThermalRadiation.html Accessed 26 October 2010. (Shortlink -http://goo.gl/4vg2)
Cengel Yunus. “Introduction to Thermodynamics and Heat Transfer”. Chapter 12: Radiation and Heat Transfer, McGraw Hill, 1997, pp. 625 – 700.
- Instruction Manual and Experiment Guide for the PASCO scientific Model TD-8553/8554A/8555. ThermalRadiation System.ftp://ftp.pasco.com/Support/Documents/English/TD/TD-8554A/012-04695D.pdfTD/TD-8554A/012-04695D.pdf (Short link -http://goo.gl/XDKh)
