Physics Exercise Radiation

Part One Emissivity, e

The reflection, emission and absorption of radiation depend on the characteristics of the object's surface. In general polished, shiny surfaces are good reflectors, poor emitters and poor absorbers; while rough, dark surfaces are poor reflectors, good emitters and good absorbers. The emitting properties of a surface are expressed in terms of its emissivity, e (0 > e >1). The emissivity of a surface is defined as the ratio of the amount of energy it radiates to the energy it would radiate if it were a perfect emitter (e = 1). Also different surfaces reflect and transmit varying amounts of EM radiation. The amount of radiation emitted by an object depends on its absolute temperature T. The Stefan-Boltzmann law of radiation gives the rate at which radiant energy is emitted by an object that has a Kelvin temperature T, surface area A, and emissivity e:

In the first part of this exercise we will investigate the emitting properties of various surfaces. Experimental Apparatus

Step 1. Turn on the multimeter connected to the Radiation Detector (DC, V, 200mV), Turn on the multimeter connected to the Radiation Cube’s Thermistor ( ,200k). Procedure Step 2. Read the resistance of the thermistor and use the Resistance / Temperature Table above to determine the initial temperature (T o ) of the radiation cube. (e.g k  ) Step 3. Choose a temperature (T 1 ) around 15C° above the initial temperature, (39°C), and determine the corresponding thermistor resistance (R 1 ). Step 4. Set the radiation cube's power control at "5" and begin observing the thermistor ohmmeter. Remember that as the cube's temperature increases the thermistor resistance will decrease. R 1 = 53.3k  T o = 24°C

Step 5. As the thermistor resistance begins to approach R 1 take the top off the radiation cube (BE CAREFUL TO ONLY TOUCH THE BLACK PLASTIC KNOB!). This will slow the increase in temperature (slow the drop in the thermistor resistance) until the thermistor resistance becomes constant (cube temperature is constant). You may also need to turn the power down to stabilize the temperature. Record the thermistor resistance k  in Data Table 1. Determine the corresponding temperature of the cube and record it in Data Table 1. This temperature does NOT have to be exactly 15C° above the initial temperature. Step 6. Open the Radiation Detector aperture by sliding the ring forward. Place the detector's posts in contact with the "black" surface.

Step 7. Record the voltage, mV, output of the detector in Data Table 1A. Step 8. Move the detector back and place the glass plate about 5 cm from the black surface. Place the posts of the detector against the glass plate and record the voltage in Data Table 1B. Do not let the glass plate touch the cube. Step 9. Repeat Step 8 with the plastic, record the voltage in Data Table 1C. Do not let the plastic plate touch the cube. Step 10. Repeat Steps 6-9 with the other three sides of the cube (white, polished aluminum, and dull aluminum). Answer Questions.

Questions Part I: Emissivity of Different Surfaces Remember that the radiation intensity is directly proportional to the voltage output of the detector. 1. List the four surfaces in order of decreasing emissivity: a)_________, b)__________, c)___________, d)__________ 2. The reflecting property of a surface is measured in terms of its Albedo. Albedo is defined as the ratio of the amount of radiation reflected from a surface to the amount light incident on the surface. How are the emissivity and albedo of a surface related? ______________________________________________________ List the four surfaces in order of decreasing albedo: a)_________, b)__________, c)___________, d)_________

3. The absorbing property of a surface is measured in terms of its Absorbance. Absorbance is defined as the ratio of the amount of radiation absorbed to the amount of light incident on the surface. How are the emissivity and absorbance of a surface related?______ _______________________________________________________ List the four surfaces in order of decreasing absorbance: a)_________, b)__________, c)___________, d)__________ 4. How are the albedo and absorbance of a surface related? ______ _______________________________________________________ 5. What happens to the radiation incident on a surface which is not reflected or absorbed? ____________________________

Resistance / Temperature Table

Part Two Dependence of Radiation intensity on Distance from Source

As radiation travels from a source it spreads spherically. As it spreads the intensity (brightness) decreases. How does the intensity depend on the distance? Source

Experimental Apparatus

Step 1. Set the Radiation Detector next to the meter stick. Align the detector opening with the height of the light bulb and 20 cm away. Procedure Step 2. Turn on the light to maximum brightness. Slide the ring on the radiation detector forward to uncover the aperture. Turn multimeter selector counterclockwise to 200m (V). Step 3. Record the detector's voltage output in the data table. Step 4. Move the detector to 30 cm from the bulb and record the detector's output. Step 5. Repeat Step 4 for distances to 100 cm in 10 cm increments.

65.3

Plot a graph of Radiation, R versus Distance, d. Let distance be the independent variable. If this graph is a straight line go to Conclusion section. If this graph is not a straight line compare to the various graph shapes and plot a new function of the independent variable, d.

Plot a new graph of Radiation, R versus new function of Distance. Continue until you get a straight line graph. Then go to the Conclusion section.

Conclusion Circle the proportion that gives the relationship between radiation intensity, R and distance from the source, d. Suppose the radiation intensity was proportional to the distance squared, If the distance from the source was doubled (multiplied by 2) the radiation intensity would be multiplied by 2 2 = 4. if the distance was multiplied by 3 (tripled) the intensity would be multiplied by 3 2 = 9. If the distance we cut in half (multiplied by 1/2) the intensity would be multiplied by (1/2) 2 = 1/4.

Suppose that at 50cm from the light bulb the light intensity was 100 mV, according to your graphical analysis, at a distance of 100cm the intensity would be ______mV. Suppose that at 50cm from the light bulb the light intensity was 100 mV, according to your graphical analysis, at a distance of 25cm the intensity would be ______mV.

Conclusion Circle the proportion that gives the relationship between radiation intensity, R and distance from the source, d. ?