1. Measuring and Calculating Energy
January 22, 2014 –
Objective: I can convert between units of energy and calculate efficiency, explaining results in terms of the Laws of Thermodynamics.
Measuring and Calculating Energy
Do Now:
Section 15-1 in the book
What is net energy and why is it important?

2. Net Energy
the difference between the energy expended to harvest an energy source and the amount of energy gained from that harvest
we were talking about petroleum coming from the Middle East, and we also talked about how energy is used for transportation – well energy has to be used to transport petroleum to the U.S. from the Middle East, so that’s an example of expending energy in order to harvest energy

3. Why people care about net energy:
Usually because they want to make money off of selling energy.
Energy is a valuable commodity. People have become very dependent on external sources of energy. So, there’s profit to be made and energy businesses want to make sure they’re not missing out on that.

4. First Law of Thermodynamics
The total energy of an isolated system is constant; energy can be transformed from one form to another, but cannot be created or destroyed.
2 ≠ 1
2 = 1 + 1

5. Second Law of Thermodynamics
entropy of an isolated system never decreases, because isolated systems spontaneously evolve toward thermodynamic equilibrium—the state of maximum entropy
As time passes, energy tends to go from more concentrated forms to less concentrated forms.
more concentrated energy
less concentrated energy

6. Units of Energy Joule (J) British Thermal Unit (BTU) Calorie
standard international unit of energy
equivalent to energy released when an apple
British Thermal Unit (BTU)
equivalent to heat required to raise 1 pound of water 1° F
Calorie

7. Units of Power Watt (W) rate of energy
(energy produced in a specific amount of time)
1 W = 1 J/s
1 Watt = 1 Joule/second

8. Metric Conversions How many watts are there in a kilowatt?
A powerplant produces 15 megawatts of energy per hour. How many watts are produced per minute?
3. A computer has 10 gigabytes of memory. How many kilobytes is this?

9. The average home in central Massachusetts needs about 75 million BTUs of heat in an average winter. The efficiency of heaters varies, but assume that the heat can be delivered with about a 75% efficiency.
How much energy (in Btu’s) must be consumed to be able to deliver 75 million BTUs of heat?
Efficiency = Energy in the form you want / Total Energy

10. 2. The average incandescent light bulb is about 5% efficient
2. The average incandescent light bulb is about 5% efficient. For every 150 joules of electrical energy that is consumed, how much light energy is produced?

11. 3. A typically coal-fired power plant will produced approximately 60,000 joules of electrical energy for every 200,000 joules of coal consumed. What is the efficiency of a coal-fired power plant?