Journal 10/16/14

1. Journal 10/16/14 Infrared goggles can sense heat, or infrared light. If you point them at a person, you can see their heat. This means that people emit infrared light. Do you think all objects make some kind of light? Explain. Objective Tonight’s Homework To learn how all the kinds of light work together in ideal conditions Read section 3.4 (pp 66-71) Do “problem” 7 on pg 75

2. Notes on Blackbody Radiation Our Journal question has revealed that all objects put out light of some kind. If we look at different objects, we see that they put out different kinds of light. If we look at the sun, we find that it’s putting out ALL the kinds of light. If we were to graph the sun’s light, we’d get something like this:

3. Notes on Blackbody Radiation We can see here that the sun is putting out all kinds of light. It peaks in visible light (meaning it makes more of that than any other kind of light) but it still makes all kinds. We call this a “blackbody radiation curve”. It shows all thelight being putout by an object.

4. Notes on Blackbody Radiation We can create blackbody radiation curves for any object. Even people have a blackbody radiation curve, although ours peaks in infrared light and doesn’t make it to visible light.

5. Notes on Blackbody Radiation So what use is this? We know that each object of a different temperature makes a different curve. Knowing this, if we look at something new and graph its curve, we can work backwards to get the temperature of the object. This is one way we know the temperatures of the stars. We do this with the following equation: 2.9 mm Where T λpeak = is the highest wavelength in mm T = temp in kelvins λpeak =

6. Notes on Blackbody Radiation Example: A star has a peak wavelength of 300 nm. What is the temperature of this star?

7. Notes on Blackbody Radiation Example: A star has a peak wavelength of 300 nm. What is the temperature of this star? λpeak = 300 nm 300 nm 1 mm 1 1 x 106 nm λpeak = 0.0003 mm T = (2.9 mm) / (0.0003 mm) T = 9,666 K

8. Notes on Blackbody Radiation As we can see, this is a powerful tool. We can get the temperature of something just by looking at what kind of light it makes the most of! We can use another equation to get the total energy an object puts out. Etotal = AσT4 Etotal = Total energy made every second A = surface area of the object T = temperature of the object in kelvins σ= 5.67x10-8 W/m2●K4

9. Notes on Blackbody Radiation Example: The sun has a temperature of 5800 K and a radius of 7x108 m. How much energy does it output each second?

10. Notes on Blackbody Radiation Example: The sun has a temperature of 5800 K and a radius of 7x108 m. How much energy does it output each second? Etotal = AσT4 Etotal = [4 π (7x108)2][5.67x10-8][58004] Etotal = [6.16x1018][5.67x10-8][1.13x1015] Etotal= 3.95x1026W In comparison, a nuclear bomb emits 4x1016 W. The sun puts out a billion times more energy than a nuclear bomb every second!

11. Estimating Energy Let’s take these equations one step further. Some day, the sun will blow up to be 100 times bigger in radius than it is now. Assuming the total energy output of the sun stays the same, what will the surface temperature be? Let’s say you had futuristic technology and wanted to crush the sun down to 1/100 its current radius. Assuming the energy output is the same, what would the surface temperature be now? The Earth is 150,000,000,000 m from the sun. Using this number and the total energy output from the sun, find the total amount of energy from the sun absorbed by the Earth every second.

12. Exit Question #17 X-Rays start at a size of 10-8 m. What temperature star would you need to have to get a peak of energy in x-ray? a) 2,900 K b) 29,000 K c) 290,000 K d) 2.9 million K e) 29 million K f) 290 million K