Energy, Power Questions?

Question

Hi few questions on how these are done, for my college Engr 10 class ( intro to Engr). Please tell me equations and show how done.

1. A person running at the pace of 5mph expends 300 kilocalories of energy per hour.

a)Find the power of the runner is exerting while running.

b)Find the total energy exerted by the runner in a 10k run

I got 343.02 kc is that right?


2. It took you 2 hours to drive to Berkeley and back with a total distance of 100 miles traveled

a) Your car gets 34.0 miles per gallon. How much energy have used for this trip?

b) What is the average power consumption for this trip?

c) We know that energy does not vanish ( energy conservation). Where does the energy go ( consumed) after the trip?

Answer 1

1. is all about conversions. If you got everything into metric and converted cal to J you can't lose.
E = E/t * distance / (distance/t) * conversions
= 300000 cal/hr * 10000 m / 5 (mi/hr) * J/cal * mi/m
Answer is in J.
2.a) needs knowledge of gasoline volume-specific energy (e.g., J/liter). Then do the rest of the conversions. E = distance / (distance/volume) * (energy/volume) * conversions, that is 100 (mi) / 34 (mi/gal) * J/liter for gasoline * liter/gal.
Answer is in J.
2.b) P(ave) = E/t, in W.
2.c) Since you make a round trip there's no potential energy change. Kinetic energy is lost in air and machine friction, and deceleration by brakes and engine ("engine braking"). If you have a hybrid car that recovers deceleration energy you reduce some of that part of the energy loss; however limited energy storage capacity may prevent recovery of all the decel energy on a large descent, and a large ascent may use all the stored energy and require gasoline assistance.
Note: You can easily get the needed conversions (except for the gasoline number) by googling. E.g., for J/cal google "cal to j".

Answer 2

1. Yes, there is the production aspect of the making the solar. A factory has to make the solar panel in the first place which takes electricity, deliver it to your door which takes gas, etc. 2. This is a totally open ended question. Solar panels can be used for heating, electricity, etc. It depends on the type and number of panels. For example a 75 Watt 12 v Panel silicon solar electrical panel which is roughly 3 x 1 feet might cost you a $450 USD. Next you have to consider the application (ie. what are going to use it for and how are you tying into your existing applications). You need something to store or use the electricity as well. 3. Your last question doesn't make sense. Are you asking how much you would save on your month billing? Again it depends on how much average sunshine you are getting. The solar panel ratings are typically peek power ratings. If you are only getting enough to power 1/2 the cell you are only getting 1/2 the benefit. You would take the average power and times it by the cost of electricity in your community. Hope that helps

Answer 3

Calculator available:

http://www.a2dvoices.com/realitycheck/Energy/

Select the "Energy" tab