1 Lecture 17 : Incandescent lightbulbs How they work Why they are inefficient Reminders: HW 7 online, due Monday 24th at midnight Extra evening class Wed 26th at 7pm Check results on CULearn regularly Reading quiz in a second Preclass notes for lecture 18 available by Wed Lightbulbs How many scientists does it take to change a lightbulb? Undergraduates: None “Bright light - hurts... must go back to bed”. Postgraduates: Funding for a new lightbulb ran out six months ago - will have to borrow from their parents. Apple engineers: Seven. One to change the bulb, six to design t-shirts and new gadgets Russian military scientists: It‟s top secret. Nuclear engineers: One to install the bulb and six to figure out what to do with the old bulb for the next ten thousand years At end of semester I will: • Scale the 2 hardest midterms up, so that all 3 have the same class average • Delete your lowest scaled midterm score. • Add up all points possible during class to get final score and % • Based on that %, you are guaranteed the following grade: > 90 % = A > 80 % = B > 70 % = C > 60 % = D > 50 % = E < 50 % = F I reserve the right to scale everyone’s scores upwards, if after looking back at the last few years that seems appropriate. End of semester grade policy Physics: • Introduction to electromagnetic radiation (light and other stuff) • EM radiation emitted by all objects - Spectrum – range of colors - Power – Stefan Boltzman Law Lightbulbs Use vast quantities of power- ~25% of all power for lighting. Incandescent (regular) lightbulbs waste 88% of that!! • How do they work? • What determines color? • Why do they “burn out”? • Why are fluorescent lights are more efficient? • Why physics makes it hard to improve efficiency? Electric Current How does a lightbulb work? How does an incandescent light bulb work: • Electric current flows through filament • Filament gets hot • Hot filament emits EM radiation • Electrical energy  EM radiation energy prism incoming light beam What can we see coming out of a lightbulb? We can see white(ish) light: - Visible light is one type of electromagnetic radiation - Form of energy - Consists of many different colors – what‟s a color? White light is made up of many different colors. There are other „colors‟ emitted that we can‟t see Electromagnetic radiation EM radiation travels in waves, like sound, waves on a string etc Recall: sound is periodic modulation of air pressure Sound wave Pressure Distance or time2 Electromagnetic radiation • EM radiation also travels in waves, like sound • But light can travel through a vacuum so what exactly is „waving‟? • EM radiation is a periodic modulation of “electric field” Strong field • Color of light depends on its wavelength. Electric field distance Electric field distance l l What is Electric field? • Light is periodic modulation of “electric field” Strong field Electric field distance l • Electric field exists everywhere in space • Describes the force on a charged particle at each point in space • Vector – has a magnitude and direction • Units: Newtons/Coulomb • Contains energy • Created by charges (and created in other ways) • Analogy: Like gravitational field describes the force on a particle with mass • Balloon demo See this next week: • All EM radiation is a periodic modulation of an electric field • Visible light is just one part of the electromagnetic spectrum with specific range of l (and f) • Visible light is the strongest part of the EM spectrum emitted by the sun • It is the part of the EM that our eyes can detect The electromagnetic spectrum Electromagnetic radiation Light travels in waves, like sound Speed of sound, in air = 330 m/s Frequency of concert A = 440 Hz Wavelength of concert A = 0.75 m Speed of light, in vacuum = 3 X108 m/s Frequency of red light = 4.6 X 1014 Hz Wavelength of red light = a) 0.75 m b) 7.5 X 10-5 m c) 6.5 X 10-7 m d) 1.38 X 1023 m The spectrum of white light? incoming white light beam light made up of many colors • Defined as the spectrum of EM radiation emitted by the sun • All visible l present with roughly equal intensity • LED demo White light detector angle (wavelength or color) detected power IR UV Visible A spectrometer measures the spectrum (range of wavelengths or frequencies) in light detected power IR UV Visible A B C For this spectrum, rank power of light at each color: a. C greater than B greater than A b. A greater than B greater than C c. C greater than A greater than B d. B greater than A greater than C e. Cannot tell from this data. Understanding a spectrum: An object is giving off light with this spectrum. What color in the emitted light has the most power? a. IR, b. Yellow/green c. UV, d. red, e. blue, Wavelength3 • Everything that has a non-zero temperature emits EM radiation •The spectrum of EM radiation coming from a black object is called the “blackbody spectrum.” • Go to the blackbody spectrum simulation • BB spectrum determined by temperature only. •The temperature of the object affects both - The total power of EM radiation emitted by the object - The range of wavelengths emitted (the spectrum) Blackbody spectrum Look at light bulb with variac to control how much electrical power goes into it. If I put half as much electrical power into it, what will happen? a. color will change, get whiter, brightness decrease b. color will stay the same, brightness decrease c. color will get redder, brightness decrease d. color will get redder, brightness the same e. color will get whiter, brightness the same. Blackbody spectrum and temperature Stefan-Boltzman law gives total electromagnetic power (energy/second) out of a hot object at temperature T Power = e X  X T4 X a How does temperature affect emitted power (brightness)? Area of surface Temperature of object (in Kelvin!) Stefan-Boltzmann constant,  = 5.67 x 10-8 J/(s m2 K4) e = “emissivity”; how well the light gets out Two burners on the stove are at the same temperature, but the left-hand burner has twice the area. How much more infrared radiation is it putting out? a) The same amount b) Twice as much c) Half as much d) Four times as much e) Sixteen times as much. Power = e X  X T4 X a • Property of surface, • Scale