Evolution of Earth
Scientific Explanations...
Work, Energy, & Environment
Global Warming vs. Ozone Depletion

WORK, ENERGY, AND ENVIRONMENT

QUESTION ONE:
Name the three fundamental forces that are at the basis of all the energy sources we use. For each, give one example of a practical energy source.

QUESTION TWO:
In each of the following cases, trace the chain of energy transformations from the sun to the energy in its final form:

A. A pot of water is boiled on an electric stove.
B. An automobile accelerates from rest on a level road, climbs a hill at constant speed, and comes to stop at a traffic light.
C. A windmill pumps water out of a flooded field.

QUESTION THREE:
A Calorie is a measure of energy, usually used when energy is transferred in the form of heat. Assume the body is 20% efficient (that is, it converts 20% of the food energy one eats to usable work). How many kilograms of hamburgers would you have to eat to supply the energy for:

A. a half-hour of digging?
B. a three hour hike at 3 miles per hour?

 

Approximate Energy Content of Various Foods
(in Calories per Kilogram)
Butter 7000
Chocolate (sweetened) 5000
Hamburger beef 4000
Bread 2600
Whole milk 700
Raw apples 500
Lettuce 150

 

 
Approximate Rates of Energy Use During Various Activities
(in Calories per Hour)
Sleeping 70
Lying down awake 80
Sitting Still 100
Standing 120
Typewriting rapidly 140
Walking 220
Digging a ditch 400
Running fast 600
Running in a race 1200
Source: U.S. Department of Agriculture

 

QUESTION FOUR:
The burning of carbon (coal) in air (in O2) is given by the equation:

C + O2 CO2

Actually, this reaction involves the making and breaking of bonds. C is bonded to other C with a "C-C bond" in coal, O is bonded to other O with a "O=O bond," and in CO2, the bonds are C=O. In terms of bonds, the reaction looks like:

(C)-- C + O = O O = C = O + ( C )
break
break
make
ignore

The bond energies for the carbon burning are:

C - C
82.6 kcal/mole
O = O
179.4 kcal/mole
C = O
178 kcal/mole

 

A. Write the equation that represents burning of carbon. Write the mole equivalents near each reagent.
B. Calculate how many kilocalories of heat energy are released when 1 mole (12 g) of carbon are burnt? How many moles of O2 does it take?

[This reaction is called oxidation (adding oxygen). This type of calculation can be used for example to figure out how much energy we get from the "burning" of sugar in our bodies: C6H12O6 + 6O2 --> 6CO2 + 6H2O ]
C. How much pure carbon has to be burnt every second in a 1000 Megawatt (electricity) power plant if the plant is 30% efficient in converting heat to electricity?

[30% efficiency means that for every 30 Watts needed as output, we have to burn fuel (input) worth 100W]
D. How much water has to fall a distance of 100 meters to release the same amount of energy as 1 mole of carbon? (1 kilocalorie = 4184 Joules) What difference would there be in this problem if we lived on the moon instead of the earth?
E.

Calculate the mass of each product and the total energy generated (in Joules, Calories, and BTU) from the complete combustion of 1 pound of methane gas. The heat of combustion of methane is 58.8 Joules/kilogram. Combustion means burning, or the addition of oxygen.


  ©Copyright 2003 Carnegie Mellon University
This material is based upon work supported by the National Science Foundation under Grant Number 9653194. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.