PSC 151 Laboratory Activity 10 Electromagnetic Radiation I

The Electromagnetic Spectrum

Regions of EM Spectrum in order of Increasing Energy Decreasing Wavelength

Dependence of EM Radiation on Temperature An object emits all types of EM radiation but at varying intensities. The type of radiation emitted with the greatest intensity depends on the object’s temperature. Wien’s Law

Examples: If an object has a temperature of 50°C what type of radiation is emitted with the greatest intensity? Far Infrared

The surface temperature of the sun is approximately 6000K. What type of radiation is emitted by the sun with the greatest intensity?

How Does Radiation Interact With Matter?

T object T environment

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 amount of radiation emitted by an object depends on its absolute temperature T object. 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: 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 absorbed by an object depends on the absolute temperature of its environment T environment. The absorbing properties of a surface are expressed in terms of its absorbance, a (0 > a >1). The absorbance of a surface is defined as the ratio of the amount of energy it absorbs to the energy it would absorb if it were a perfect absorber (a = 1). “At thermal equilibrium, the emissivity of a body (or surface) equals its absorbance.” Kirchhoff's Law of Thermal Radiation

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 to determine the initial temperature (T i ) 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 f ). 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 f = 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, T f, 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