Download Review Energy Versus Power, Energy Bills, Oil Usage - Slides | PHYS 3070 and more Study notes Physics in PDF only on Docsity!
- Homework #2 due today at 5 pm (in the wooden box)
- Extra Credit #1 due today
- Homework #3 due next Wednesday (now posted)
- Last problem you can (not required) sign-out Power-Meters for 2 days (thus you can measure a few of your usages)
- Exam #1 (in class) – September 24, 2008 [Two Weeks!] -- exam details to be given in one week
Topics for today:
Review energy versus power, energy bills, oil usage…
Inquire/Inform Assignment #1:
Everyone must write a 1-2 page (double spaced, 12 point font) letter to one of the Presidential candidates. You have the open of sending the letter, and you need to hand-in an addressed copy of the letter for credit.
You should read the candidate’s position on an issue of energy and/or the environment. You should then do a little investigation into the issue. Then write them a letter informing them of some scientific information on the issue relevant to the position. Include all references. You need not agree or disagree with their position. Grades are based only on the scientific information in the letter. If you are only expressing an opinion, no credit will be given. Feel free to check with me if you have questions.
First one is due Friday, October 03, 2008 (in class).
Electromagnetic Radiation Energy
Light is an electromagnetic wave (like radio waves, X-rays, and also visible light!). These waves carry energy. Sunlight is electromagnetic radiation emitted by the sun that travels through empty space to us on earth. It is the source of most of the energy available on earth (even via the creation of fossil fuels a long time ago).
Think about getting a sun burn!
Now we know most of the different forms of energy and many of the different units that this energy can be expressed in.
Lets work out a few more examples to get some feeling (“intuition”) for energy and also Power.
Review the relationship between Energy and Power.
Power
Power is the rate of doing work. It is the energy given to or taken away from an object per unit time.
time
Energy
Power =
Power is a rate. It is not the same as energy!
Analog: You might get paid a salary as a rate (e.g. $10 / hour). If you work 40 hours in a week, you have earned $400.
The $400 is like energy and the $10/hour is like power.
Energy =Power× time
Clicker Question
1 2 3 4 5 6 7 8 9 10
80 70 60 50 40 30 20 10
Total (cumulative) Energy consumed
Energy (Joules)
Time in minutes At what time is the instantaneous Power consumption the maximum? A) time = 2 minutes B) time = 5 minutes C) time = 9 minutes D) It is the same at all times shown.
Power = Change in Energy / Change in Time = slope on graph.
Electric Bill
I buy electricity from the “Power Company”
Here in Colorado
My bill charged me for approximately 1000 kiloWatt-hours.
Note that kiloWatt-hours is (Power [kWatt]) x (Time [hours]) and is thus a unit of energy.
So Xcel does not charge me for Power, they charge a certain rate for my total electrical energy usage.
For example, they do not care if I used 1000 kWatts for one hour or 100 kWatts for 10 hours, etc. They only care about the total energy.
Note that 1 kWatt-hour can be converted into Joules.
sec
( 1 )^1000 hour ond
E kW hour J × ⎟ ⎠
×⎛
× ⎛
1 min
60 sec 1
60 min hour
E = 3. 6 MegaJoules
Xcel charges approximately $0.09 (9 cents) per kWatt-hour.
Thus, since I used ~ 1000 kWatt-hours, my electric bill was for approximately $90 for one month.
Does that energy usage make sense?
If I had ten 100 Watt lightbulbs on all the time for one month, how much energy is that?
E = 10 ×[( 100 Watts )× ( 1 month )]
However, we want kWatt-hours, not Watt-months. So we need to use the fact that there are 720 hours per month.
×
month
720 hours
E = 720 kiloWatts-hours Close to my usage.
Do I really leave 10 lightbulbs on all the time? No. However, I use the electric oven (~1000 Watts) and I also use the dryer a lot (~3000-5000 Watts). The hot tub definitely does not help /
Note that this does not include heating my house and hot water, since that is from natural gas (another 1000 kWatt- hours) and another ~ $100 per month in the winter!
My Great Idea (or Not)
I have a Concept2 Rowing Machine at home. If I row for 20 minutes, it says I have burned “250 Food Calories” (like the one jelly donut).
That is about 10^6 Joules in 1200 seconds or a Power of 800 Watts. However, I am only about 10% efficiency at producing useful work (the chain pulling a wheel), and thus I only could get out about 80 Watts of electricity generation (for example).
So instead of paying $100 for my 1000 kWatt-hours of electricity last month, I could have just rowed for 12,000 hours ~ 500 days!
85% of US energy supply comes from fossil fuels!
This includes:
- Petroleum = gas (methane) and liquid (oil)
“natural gas”, propane, butane, gasoline, kerosene, etc.
- Coal
- Tar Sand and Oil Shale are sometimes considered separately from petroleum but are still fossil fuels.
