Phys1020 Homework 7
Due Thursday March 6 at 11AM
1. (2 pts) The Sun provides most of the energy that makes life on earth possible. The Sun's surface reaches a temperature of about 5800 C and emits sunlight which travels the 150,000,000 km to Earth at a speed of 3*10^8 m/sec in about 8 minutes. Electromagnetic radiation is emitted when charged particles accelerate. Sunlight is a blend of electromagnetic waves of different wavelengths.
a. (1 pt) Describe the distribution of electromagnetic radiation emitted from the sun (what wavelengths are emitted? is it all visible light? is there more blue light than red light?).
b. (0.5 pt) Why you get a distribution of wavelengths instead of just one wavelength?
c. (0.5 pt) How does the temperature of the sun affect this distribution?
Questions 2-7 (6 pts)
The sun provides the energy that powers life on our planet everyday (even the energy stored in fossil fuels came from the sun). At noon on a typical mid-June day in Boulder, each 1 cm2 of the surface is exposure to 20 mW (milliWatts) of visible radiation. You are considering adding solar panels to your house and want to see whether they will provide enough energy for your household needs.
2. (1 pt) If your solar panels covered an area of 1 meter by 5 meters, how much solar power (in Watts) would be hitting the surface of the panels at noon on a clear day? (check your answer with your fellow students!)
3. (1 pt) Over the period of 1 hour, what is the total amount of energy of the visible light that has hit the solar panels? (assume that the exposure is pretty steady for that hour at 20 mW/ cm2).
4. (1 pt) Red photons and violet photons have slightly different energies. If we take the average energy of the visible light photon hitting the solar panels as that of a green photon (550 nm), how many visible light photons hit the solar panels during that hour?
5. (1 pt) If the solar panels were 15% efficient at converting this energy into electrical energy, how much electrical energy would have been harnessed in 1 hour?
6. (1 pt) For how long could this amount of energy power a 60 W bulb (in hours)?
7. (1 pt) As we stated above, in Boulder at noontime in the summer there is 20 mW of visible light hitting each 1 cm2 of the surface. However, there is only about 0.022 mW (milliWatts) of UV radiation between 290 nm and 320 nm (UV B) hitting each 1 cm2 of the surface. (Only a small portion of the sunlight's energy is at UV wavelengths.) Explain why it is that 0.022 mW of UV exposure can cause damage to our DNA and result in skin cancer, but exposure to 20 mW of visible radiation leaves our DNA unharmed? Why doesn't exposure to so much more visible radiation harm our DNA? (hint: think in terms of energy, photons and wavelength...what’s the difference between being hit with 1,000 individual marshmallows that collectively weigh 10lbs and one small rock that weighs 10 lbs ?)
8. (2 pts) Visible light interacts with the molecules and particles in the air. As a result, the color of the sky and the color of the sun depend on such factors as the location of the sun in the sky (high noon vs sunset), the weather, the level of pollution, etc. (Note: We know that in thinking about an electrical circuit it is helpful to put yourself in place of the electron and imagine what you would do. Here, in thinking about light, it is helpful to imagine you are the sunlight coming streaming in from the sun through the atmosphere and how you would be affected by the molecules and particles in the air. Imagine you are blue light and then red light
a. (1 pt) On a very clear, dry day when there is no dust or water in the air, the sky is a very dark blue. Why? As always, your explanation should include the physics principles and reasoning to support your answer.
b. The sun appears red during sunset and sunrise. Why?
9. (1 pt) The moon has no atmosphere. Explain what you would see if you looked toward the sky while standing on the moon and why?
10. (0.5 pt) In discharge lamps like we used in lab, free electrons are accelerated along the axis of the lamp, exciting atoms as they smash into them. The light emitted from a neon discharge lamp appears red, because
(a) free electrons in the discharge are accelerated with exactly the right amount of energy so it produces red photons.
(b) the inside of the lamp is coated with phosphors that emit red light.
(c) the excited electrons in the neon atoms most often transition between two energy levels whose energy spacing corresponds to that of a red photon.
(d) the neon lamp runs at a cooler temperature than an incandescent light bulb and cooler objects emit redder light.
11. (0.5 pt) T/F: the colors of different discharge lamps are different because the molecules are moving differently. But if they were not moving around they would all look the same color.
12. (1 pt) In making a laser what are three critical conditions in order to make it work:
13. (1 pt) What are three (relatively) unique features of laser light (as compared to a regular lightbulb) ?
14. (1 pt) Pick one of those features (from question 13) that makes the laser unique … what does this allow you to do that a lightbulb wouldn’t be able to do.
Extra credit (2 points) – Pick a question from last week’s homework you got wrong, explain the thinking that led you to the incorrect answer and what change in thinking is required to get the correct answer. Don’t forget to indicate what the question it is you are correcting.