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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.
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