PRE - Report Thermal Radiation PDF

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PRE - Report Thermal Radiation...

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PHYSICS LABORATORY PRE-REPORT III THERMAL RADIATION

OBJECTIVES: 

Experimentally introduce the concept of thermal radiation.

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Check the Stefan-Boltzmann Law for high temperatures.

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Check the inverse square law for thermal radiation.

MATERIAL AND EQUIPMENT TO USE: 

PASCO TD-8553 radiation sensor.

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Thermal radiation cube TD-8554A.

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TD-8555 Stefan-Boltzman Lamp.

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Voltmeter, Ammeter, Ohmmeter, Voltage Source (12 VDC; 3A).

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Glass window, aluminum coated insulating sheets, small triplex sheet, aluminum sheet, tape measure and some other materials that can test as blockers of thermal radiation.

PROCESS: NOTE: When using the radiation sensor, always protect it from hot objects except for the few seconds it really takes to make the measurements. This prevents heating of the thermopile which will change the reference temperature and alter the reading. 

Stefan-Boltzmann law at high temperatures: IMPORTANT: The voltage at the lamp cannot exceed thirteen volts. Take each reading quickly. In between readings, put on a protector so that the sensor temperature is relatively constant (Styrofoam block).

 The temperature in the Tref laboratory is measured in kelvin (K = C +273). Assume a resistance of (0.3 ± 0.1) Ω for a temperature close to 295 K.  The equipment is assembled as indicated in the Physics Laboratory Guide III. The voltmeter must be directly connected to the lamp. The sensor should be at the same height as the filament, with the front face of the sensor approximately 6 cm from the filament. The angle of entry to the thermopile must not include other nearby objects other than the lamp.  The voltage source turns on. For each voltage in Table 1.2, the heat of the current I read in the ammeter is recorded, and the value of the radiationℜ read on the voltmeter. 

Inverse Square Law IMPORTANT: Each sensor reading is done quickly. Between readings, the two radiation-blocking sheets are placed between the lamp and the sensor, with the silver surface facing the lamp, so that the temperature of the sensor remains relatively constant.  The equipment is assembled as shown in the Physics Laboratory Guide III. 

The tape measure is attached to the table.

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The Stefan-Boltzmann lamp is placed on one end of the tape measure as shown. The zero of the tape measure should line up with the center of the lamp filament.

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The height of the radiation sensor is adjusted so that it is in the same plane of the filament of the Stefan-Boltzmann lamp.

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The lamp and sensor are oriented such that, by sliding the sensor along the tape measure, the axis of the lamp aligns as close as possible to the axis of the sensor.

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The sensor is connected to the voltmeter and the lamp to the voltage source as shown in the Physics Laboratory Guide III.

 With the lamp off, the sensor slides along the tape measure. Millivoltmeter readings are recorded at 10 cm intervals. These values are averaged to determine the environmental level of thermal radiation. This environmental average value will need to be subtracted from your measurements with the lamp on, in order to determine the contribution of the lamp alone.  The voltage source control is actuated to turn on the lamp. A potential difference of approximately 10 V is placed.  The distance between the sensor and the lamp is adjusted for each of the positions in Table 1.4. The reading is recorded on the millivoltmeter for each position. 

Introduction to thermal radiation  Connect the ohmmeter and voltmeter (use the mV scale) as shown in the Physics Laboratory Guide III.  Turn the radiation cube switch to “ON” and turn the knob that controls the power of the bulb to the “HIGH” position. Eyesight is maintained on the ohmmeter reading. When she has dropped to around 40 KΩ, she sets the knob to 5.0.  When the cube finds thermal equilibrium (this part is consulted with the teacher), the ohmmeter reading will fluctuate around a relative fixed value. The radiation sensor is used to measure the radiation emitted by each of the four surfaces of the cube. The sensor is placed such that the terminals are in contact with the surface of the cube (this ensures that the distance of the measurements is the same for all surfaces). Their measurements are noted in Table 1.1. The information provided at the end of the Guide is used to determine the corresponding temperature.  Set the power knob, first to 6.5, then to 8.0, and then to "HIGH". For each of the above values, wait for the cube to reach thermal

equilibrium, then repeat the measurements from the previous step and record your results in Table 1.1.  The sensor is placed approximately 5 cm from the black surface of the thermal radiation cube and the reading is noted. A glass window is placed between the surface and the sensor. Is the glass window an effective blocker of thermal radiation?  The radiation cube lid is removed and the measurements from the previous step are repeated, placing the sensor directly on top of the bulb. It is repeated with other materials.  The radiation cube is turned off and disconnected. QUESTIONS: 

What does thermal radiation consist of? Thermal radiation is radiation emitted by an object at any temperature, the characteristics of which depend on the temperature and the properties of the object. In thermal radiation, a wavelength distribution can be observed. At low temperatures, the wavelengths are mainly in the infrared region and therefore are not observed by the eye. As the temperature of the object increases, it emits a red glow, that is, thermal radiation runs to the visible part of the spectrum since as the temperature increases, the emitted radiation is made up of a continuous distribution of wavelengths of the infrared, visible, and ultraviolet parts of the spectrum.

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What is called blackbody radiation? A black body is an ideal system that absorbs all the radiation that falls on it and the radiation it emits is called black body radiation.

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What does the Stefan-Boltzmann law establish? It is one of the interrelated properties of cavity radiation (blackbody radiation).

The total radiated power per unit of the cavity opening, summed for all wavelengths, is called the radiant intensity I (T) and is related to the temperature: I ( T ) =σ T

4

Where σ = 5,670 x 10-8 w (m2 x k4) is a universal constant called Stefan Boltzmann constant. Ordinary hot objects always radiate less efficiently than cavity radiators do. To generalize the equation: I ( T ) =ε σ T 4 Where ε is a dimensionless quantity, it is called the emissivity of the material's surface, in a cavity radiator ε = 1. The forms of heat transmission are: Conduction, Convection and Radiation....