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Energy & Power Units

Energy & Power Units. Energy has units Joule (J) Rate of energy expended per unit time is called power, and has units Watt (W) Example: a 100 W = 100 J/s light bulb emits 100 J of energy every second Nutritional Value: energy your body gets out of food, measured in Calories = 1000 cal = 4200 J.

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Energy & Power Units

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  1. Energy & Power Units • Energy has units Joule (J) • Rate of energy expended per unit time is called power, and has units Watt (W) • Example: a 100 W = 100 J/s light bulb emits 100 J of energy every second • Nutritional Value: energy your body gets out of food, measured in Calories = 1000 cal = 4200 J

  2. Stefan’s Law • A point on the Blackbody curve tells us how much energy is radiated per frequency interval • Question: How much energy is radiated in total, i.e. how much energy does the body lose per unit time interval? • Stefan(-Boltzmann)’s law: total energy radiated by a body at temperature T per second: P = A σ T4 • σ = 5.67 x 10-8W/(m2 K4)

  3. Doppler Shift From Wikipedia

  4. Doppler Shift Can use the Doppler shift to determine radial velocity of distant objects relative to us Transverse velocity can be measured from the motion of stars with respect to back-ground over a period of years

  5. Doppler Shift of Hydrogen spectrum • The discrepancy between the wavelength of a line measured in the lab versus measured on an object is proportional to the velocity of the object • Apparent/ true wavelength = 1+ velocity/c • Example: • Observed(or apparent): 698 nm • Actual(or true or lab) wavelength: 656.3nm • velocity = (698nm/656.3nm -1) c = 19100 km/s

  6. Atomic Energy Levels • For Hydrogen, the energies of the atomic levels are given by a simple formula that just depends on the (excitation) number n of the orbit: En = – Ry / n2 where Ry = 13.6 eV = 2.179 x 10-18J  E1, E2=¼ E1, E3=1/9 E1,… • Electrons in higher levels will cascade down, producing many different spectral lines • Formula can be converted to frequency, wavelength

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