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Experiment 1 Properties of Thermal Radiation Objectives The aim of this laboratory is to introduce the basic properties of thermal radiation. Through a series of simple experiments we will examine the emissive power of energy radiated by a body as a function of its temperature, its propagation through space and the temperature dependence of spectral distribution of thermal radiation. *We will also investigate the relative emissivity of different surfaces* and the relative transmission properties of glass as a function of temperature. Theoretical Background Thermal Radiation Thermal radiation is the energy emitted by a body as result of its finite temperature. In contrast to heat transfer through convection and conduction, radiation heat transfer does not require a medium and can occur in a vacuum. This is because thermal radiation energy is a type of electromagnetic (E-M) radiation and like other types of E-M radiation it can travel can travel through vacuum at the speed of light. Since it is the only mode of heat transfer that can take place through vacuum, radiative heat transfer is the mode of heat exchange between the Sun and Earth; hence the term solar radiation. Radiation Transmitted, τGRadiation Emitted, εG Radiation Reflected, ρGRadiation Absorbed, αGIncident Radiation, G Figure 1 - Radiative properties of a surface Figure 1 shows that when radiant energy with an intensity G (W/m2), is incident on a surface, portions of it can be reflected, absorbed and/or transmitted. The relative fractions that are reflected, absorbed and transmitted are determined by the radiative properties ρ, α and τ, referred to as the reflectivity, absorptivity and transmissivity, respectively, of that surface. From conservation of energy we also know that: ρ + α + τ = 1 In addition to the above, the surface also emits energy via radiation where the amount of energy emitted by the surface is given by the Stefan-Boltzmann Law, given as: 4ATEεσ=In this equation, σ is called the Stefan-Boltzmann constant and is equal to 5.67 x10-8 W/m2K; A is the surface are, ε is the emissivity of the surface, a surface property similar to ρ, α and τ and T is the absolute temperature of the body in degrees Kelvin. The emissivity of a body can vary between 0 and 1. A surface with ε = 1 is a perfect radiator and is referred to as a black body radiator; for all real surfaces, ε < 1. The Stefan-Boltzmann law will be verified in this experiment. Thermal Radiation Spectrum Electromagnetic radiation, like all other forms of radiation, travels at the speed of light, which is related to its wavelength, λ, and frequency ν by the well known equation: C = υλ where c = 3 x108m/s is the speed of light. Thermal radiation spans only a portion of the entire electromagnetic spectrum, which ranges from X-rays to Microwaves. The thermal spectrum spans a range of 0.1μm - 100μm, which, as shown in Fig. 2 includes the entire visible spectrum. Whether thermal radiation is visible, and at what color, is a function of the portion of the radiation that falls within the visible spectrum. Not only is the total amount of thermal radiation emitted by a surface − described by the Stefan-Botzmann’s law − a direct function of temperature, how this energy is distributed over the thermal spectrum as a function of wavelength, is also a related to the surface temperature. Figure 2 – Thermal radiation portion of the Electromagnetic Spectrum The Planck Distribution gives the spectral distribution of thermal radiation of a Black Body as a function of temperature. This distribution function can be found in any standard undergraduate heat transfer text. Using Planck’s Distribution, the spectral distributions from black bodies at various temperatures are shown in Figure 3. The figure shows that energy radiated varies continuously with wavelength at any given temperature. It also illustrates that at lower temperatures most of the energy is outside the visible spectrum. However, as the temperature rises, more and more energy is shifted to shorter wavelengths and into the visible spectrum region. The dependence of the spectral distribution on temperature also explains why the color of a body changes as it is heated: from black to dark red to bright red to yellow and finally to white hot. The overall intensity of the visible light from an object also increases with temperature since a larger percentage of the total energy radiated is in the visible spectrum.Visible Spectrum Figure 3 – Spectral distribution of a Black body emissive power using Planck’s Distribution Solar radiation has a spectrum very similar to that of a black body at 5800K. As a result, a large portion of solar radiation is visible. The reflectivity, absorptivity and transmissivity of most materials are also a function of wavelength of the incident radiation. Hence they may transmit radiation from sources above (or below) certain temperatures, i.e. within a certain range of wavelengths, while blocking light from sources at temperatures outside that range. This wavelength dependence of material transmission properties is responsible for the greenhouse effect and will also be explored in this experiment. Experimental Apparatus The following main components will be used to conduct different parts of this experiment. Stefan-Boltzmann Lamp: This lamp will serve as a high temperature source of thermal radiation. By adjusting the voltage and current supplied to this lamp, filament temperatures as high as 3000°C can be obtained. The voltage and current supplied to this lamp can be monitored using a voltmeter and an ammeter, respectively. These values can be used to determine the resistances of the filament at various temperatures, which can then be used, calculate the temperature of the filament.Variable Power Supply: This power supply will be used to operate the Stefan-Boltzmann lamp. Be very careful while using this supply, the current output is very high. (IMPORTANT: Never supply more than 13V and 3A to the lamp) Radiation Sensor: The Pasco TD-8553 radiation sensor produces a voltage output, which is proportional to the intensity of radiation incident upon the sensor. Note that the output voltage ranges from a few microvolts (μV)to ~ few hundred millivolts (mV). Additional Comments The ‘Intensity of radiation is an important term used in radiative heat transfer and can be used for emitted or incident


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FSU EML 4304L - Radiation

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