OKState BAE 2023  Electromagnetic Properties (29 pages)
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Electromagnetic Properties
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 Oklahoma State University
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 Bae 2023  Physical Properties of Biol Mat
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Electromagnetic properties Part I Electrical and magnetic properties Electromagnetic fields are propagated through and reflected by materials Characterized as Current flow at low frequencies Magnetism in metals Optical absorbance reflectance in light etc Frequency is a major factor in the primary characteristics Low frequency electrical properties High frequency optical properties Fundamentals of high frequency electromagnetic waves Light Light Energy radiant energy Readily converted to heat Light shining on a surface heats the surface Heat energy Light Electro magnetic phenomena Has the characteristics of electromagnetic waves eg radio waves Also behaves like particles e g photons The electromagnetic spectrum Relationship between frequency and wavelength Plus c Plus Minus Minus Wavelength speed of light divided by frequency miles between bumps miles per hour bumps per hour Wavelength m Frequency Hz c 3x108 m s in a vacuum Relationship between frequency and wavelength Plus Antenna Plus Minus Minus KOSU KOSU 3 x 108 97 1 x 106 3m 6 40 x 10 7 m 640 nm Bohr s Hydrogen 5 x 10 11 m red Plants light harvesting structure model Jungas et al 1999 Light emission absorption governed by quantum effects Planck 1900 E nh E n h is light energy flux is an integer quantum is Planck s constant is frequency Einstein 1905 E p h One photon Frequency bands and photon energy Changes in energy states of matter are quantitized Bohr 1913 h E k E j Where Ek Ej are energy states electron shell states etc and frequency is proportional to a change of state and hence color of light Bohr explained the emission spectrum of hydrogen Hydrogen Emission Spectra partial representation Wavelength Measurement of reflected intensity Typical Multi Spectral Sensor Construction One Spectral Channel CPU Photo Diode detector Amplifier Analog to Digital Converter Optical Filter Collimator Radiometer Computer Target Illumination Measurement of reflected intensity Fiber Optic Spectrometer One Spectral Channel at a time Optical Grating Analog to Digital Converter CPU Element selection Computer Photo Diode Array Optical Glass Fiber Visual reception of color Receptors in our eyes are tuned to particular photon energies hn Discrimination of color depends on a mix of different receptors Visual sensitivity is typically from wavelengths of 350nm violet to 760nm red Wavelength 400 nm 500 nm 700 nm Quantification of color Spectral measurements can be used to quantify reflected light in energy and spectral content but not very useful description of what we see Tri stimulus models represent color as perceived by humans Tri stimulus models RGB most digital work CYM print HSI HSB or HSV artists CIE L a b YUV and YIQ television broadcasts CIE XYZ model Y Attempts to describe perceived color with a three coordinate system model X Z luminance CIE Lab model An improvement of the CIE XYZ color model Three dimensional model where color differences correspond to distances measured colorimetrically Hue and saturation a b a axis extends from green a to red a b axis from blue b to yellow b Luminance L increases from the bottom to the top of the three dimensional model Colors are represented by numerical values Hue can be changed without changing the image or its luminance Can be converted to or from RGB or other tri stimulus models Photo Chemistry Light may be absorbed and participate drive a chemical reaction Example Photosynthesis in plants 6CO2 6 H 2 O h C6 H12 O6 6O2 The wavelength must be correct to be absorbed by some participant s in the reaction Some structure must be present to allow the reaction to occur Chlorophyll Plant physical and chemical structure Silicon Responsivity Primary and secondary absorbers in plants Primary Chlorophyll a Chlorophyll b Secondary Carotenoids Phycobilins Anthocyanins Chlorophyll absorbance Chla black Chlb red BChla magenta BChlb orange BChlc cyan BChld bue BChle green Source Frigaard et al 1996 FEMS Microbiol Ecol 20 69 77 Radiation Energy Balance Incoming radiation interacts with an object and may follow three exit paths Reflection Absorption Transmission I I 1 0 and are the 0 fractions taking each path Known as fractional absorption coefficient fractional transmittance and reflectance respectively out I 0 I I 0 0 Internal Absorbance Ai Lambert s Law The amount of light absorbed is directly proportional to the logarithm of the length of the light path or the thickness of the absorbing medium Thus I 1 Ai log 0 I kl out l length of light path k extinction coefficient of medium Normally in absorbance measurements the measurement is structured so that reflectance is zero I 0 Ai log I out 1 log kl T Reflectance Ratio of incoming to reflected irradiance Incoming can be measured using a white reflectance target Reflectance is not a function of incoming irradiance level or spectral content but of target characteristics Solar Irradiance UV NIR Soil and crop reflectance 0 6 17 Corn 73 Cotton 9 Sunflower Fractional Reflectance 0 5 27 Soybeans 0 4 25 Potatoes 0 3 43 Soils 0 2 P S Thenkabail R B Smith E De Pauw Yale Center for Earth Observation 0 1 0 300 400 500 600 700 800 Wavelength nm 900 1000 1100 Soil Reflectances Oklahoma Electrical properties Current and Voltage Current Flow of electrons The quantity of electrons per unit time flowing through a conducting medium Units Amperes A abbreviated amps or fundamentally coulombs per second coulomb 6 03x1023 electrons Voltage Electromotive force EMF A potential or tension between two points of a conducting medium that can drive the flow of electrons through the medium expressed as work per number of electrons Analogous to pressure in a fluid that can drive flow of fluid through a pipe Units of Volts V or fundamentally joules per coulomb the energy potential per unit of electrons Resistors and Ohms Law Property of a resistor Flow of current is proportional to voltage or vice versa The proportionality constant is known as resistance v Ri For the following circuit Vsupply Ri Resistance has units of Ohms fundamentally volts per amp The current could be computed in the circuit above given Vsupply and R i 5V 10 000 0 0005 V 0 5 mV Resistivity The fundamental property of materials defining resistance is resistivity RA L m Where L length of conductive path A Crossectional area of conductive path R Resistance
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