PHYSICS 103: Lecture 20 Agenda for Today: Blackbody Radiation Light Bulbs Thermal or Blackbody Radiation All matter contains electric charges These particles are in constant motion and are accelerating Accelerating charge produces electromagnetic radiation All materials emit electromagnetic radiation Click here Electromagnetic Waves <a href="/keyword/shorter-wavelength/" >shorter wavelength</a> light has higher energy 1 Blackbody Spectrum The distribution of wavelengths emitted and the intensity (how bright) depend on the temperature of the object P = T4 A 1) is the Stefan-Boltzmann constant. 2) 5.7 x 10-8 W/m2.K4 Wien's Law Wavelength of peak emission 1/temperature or max = 0.29 / T Note: ... is in cm ...T is in Kelvins ~ 480 nm (v) ~ 48 nm (uv) Example: object at 6000 K object at 60,000K Summary of Radiation The sun, stars, you, everything in this room radiates electromagnetic waves (light) The hotter the object --- the higher its temperature --faster the motion of the constituent particles --- the greater the power radiated (ie, more light) The hotter the object --- the higher its temperature --faster the motion of the constituent particles --- the shorter the wavelength at which most of the light is radiated 2 Example: Incandescent Bulbs Electrical connections deliver power to a tungsten filament The electrons collide with the tungsten atoms and transfer some of their energy to them The tungsten filament gets hot (2500 C) Hot things radiate more light at <a href="/keyword/shorter-wavelengths/" ><a href="/keyword/shorter-wavelength/" >shorter wavelength</a> s</a> Tungsten is used because it doesn't melt at these temperatures Glass envelope is their because tungsten burns in air Bulb is filled with nitrogen, argon, krypton (inert) which prolong life Operation Issues Filament temperature - determines color of bulb and efficiency of bulb - higher temperature = more efficient (more <a href="/keyword/light-emitted/" >light emitted</a> in visible; typical 12%) - higher temperature = shorter life Filament heating - filament wires need to be long and thin (so that most of electrons energy is converted to tungsten atoms) - filament in a typical bulb is 0.5 m of 25 micron coiled up into a length of 2 cm Filament lifetime tungsten atoms sublime filament gets thinner with time inert gas added to prolong life but gas adds to convective heat loss (total efficiency is 10%) dark spot at top of bulb is actual tungsten atoms 3-way Bulb Larger filament has greater surface area so it radiates more power 3 Test your understanding If you remove the gas in an <a href="/keyword/incandescent-light/" >incandescent light</a> bulb, will it become more efficient, less efficient, or stay the same When you operate a 50-100-150 W bulb at its lowest setting, it emits yellow-white light. If you use a dimmer switch to operate a regular 150 W bulb on only 50 W electrical power, it emits orangeish light. Why? Main Points from Today's Lecture Thermal Radiation You should understand that all things radiate electromagnetic waves (o...