SPU 25 Energy and Climate: Vision for the Future, Spring 2018
SPU 25 Energy and Climate: Vision for the Future, Spring 2018
Homework Set #3 Electricity and Energy. DUE Monday 3/12/18 (1:00 pm in lecture)
Name: _____________________________________ Section: __________ TF: _____________________ Score: ______/_______
Electricity consumption is a large fraction of our overall energy use. Electricity is generated using both fossil
fuels and renewable energy sources. In this assignment, you’ll review the basics of electricity, some energy
sources, and examine electricity flow and efficiency. [40 points total]
Conceptual and quantitative problems. You will only receive credit if you show all your work and write
I. Electricity Basics
1. Coulomb’s Law of Charged Particles can be likened to: [2 pt]
a. Einstein’s theory of relativity
b. Newton’s law of gravitation
c. Edison’s law of direct current
d. Tesla’s theory of free energy
2. True or False. An ideal insulator is a medium that cannot provide a source of free electrons and is
consequently unable to conduct electricity. [1 pt]
3. _______________ is the capacity is to do work. ____________ is the rate at which work is done. [2 pt]
4. A circuit includes a 9 V battery and a copper wire of a given length and diameter. You measure the
current in the wire to be 1.5 Amperes.
a. What is the resistance of the wire? Your answer should be in Ohms. [1.5 pts]
b. With this battery and wire, how much power can be provided? Report your answer in Watts.
c. What changes could you make to the wire to increase the power? [1.5 pts]
d. If you half the resistance of the wire, what is the power output? [1.5 pts]
e. If you double the voltage of the battery, what is the power output? [1.5 pts]
5. You have been using a 75W traditional incandescent light bulb in your desk lamp. However, in a SPU
25 section, you have learned about LED light bulbs and decided to replace your inefficient light bulb
with a 12W LED that provides the same amount of light.
a. Assuming that you are using your lamp for 5 hours per day and the electricity rate is 15 cents
per kilowatt-hour, what is your annual energy cost savings? [2.5 pts]
b. The incandescent bulb cost about $0.75, and the LED bulb cost about $6.00. Based on the
energy savings, does the LED bulb save you money in the long run? For more information,
visit: http://energy.gov/energysaver/lighting-choices-save-you-money. [2 pt]
II. Electricity Generation and Transmission
6. Although our small electronics like cell phones and flashlights use direct current, the electricity
coming into our homes is alternating current. What is the advantage of transporting electricity on the
large scale using AC instead of DC? [1 pt]
a. Voltage can be stepped up or down using a transformer with A/C.
b. More materials can be used as conductors with A/C.
c. Less electrical energy is lost to heat with A/C.
d. Ohm’s Law restricts D/C, but not A/C.
7. At the large scale, electricity cannot be effectively stored with current technology. In other words, the
electricity generated and transmitted over the power lines needs to match the electricity that is being
used. ISO New England is an organization that carefully predicts how much energy will be used in the
New England power grid during every hour of every day. They then communicate with energy
generating companies (nuclear, solar, gas-burning power plants, etc.) to supply the most economic
electricity to the grid at a certain time. Visit the ISO New England website: http://www.iso-ne.com.
a. What is the current actual demand (MW) and fuel mix (percentages)? [1 pt]
b. Describe the daily pattern of system demand (i.e. what time of day is energy use high or low).
c. At the top of the page, click on the “About Us” tab. Under “Our Three Critical Roles,” click on
“Operating the Power System.” Read the section “Forecasting New England’s Electricity Use.”
What three factors largely determine the hourly demand for electricity? [2 pt]
d. At the top of the page, click on the “About Us” tab again. Under “Key Grid and Market Stats,”
click on “Electricity Use.” Scroll down and read the section “Long-Term Changes in Annual and
Peak Demand.” What is the biggest determinant of electricity demand in any given year? What
are three additional factors that have played a role in the decreased energy demand since
2005? [2 pts]
e. Read the report from ISO New England on air emissions from electricity generation:
https://www.iso-ne.com/about/key-stats/air-emissions. What are the top 4 factors that have
contributed to a reduction in emissions from electricity generators? [2 pts]
III. Energy Flow
8. The Lawrence Livermore National Laboratory publishes flow charts for energy for multiple countries
and states. Visit https://flowcharts.llnl.gov/commodities/energy.
a. In 2016, the United States consumed __________ Quads of energy per year. Of this, _________ Quads
is lost as rejected or waste energy. _________ Quads of the rejected energy comes from electricity
generation. [1 pt]
b. If the going rate for electricity is $0.15/kWh, what is the “cost” of the wasted energy from
electricity? Report your answers in trillions of dollars. [2 pts]
c. What is the fate of most of this “rejected energy?” Energy cannot be created or destroyed, so
where does it go? [1 pt]
9. The Lawrence Livermore National Laboratory published flow charts for both energy and carbon
Solar 0.130 0 0.427 0
Nuclear 8.40 0 8.33 0
Hydro 2.50 0 2.47 0
Wind 0.920 0 1.73 0
Geothermal 0.210 0.4 0.202 0.400
Natural Gas 25.0 1286 27.5 1440
Coal 21.0 1985 17.9 1710
Biomass 3.50 0 4.78 0
Petroleum 36.0 2362 34.8 2260
Total 98.0 5632 98.3 5410
a. Note that between 2010 and 2014, CO2 emissions decreased even though energy use
increased. Look closely at which energy sources and emissions increased and which decreased.
Explain the decrease in CO2 emissions based on these data and what you have learned in class.
IV. Energy Efficiency
10. According to energycalculator.com, an average 14-15 inch laptop runs at about 60 W.
a. Assuming that every undergraduate in Harvard College (about 6,700) uses their laptop for six
hours per day, how much energy is consumed per day? Report your answer in kWh. [1 pt]
b. How much of each fuel below is needed to supply this amount of energy each day? Assume
100% efficiency. Show your work and report your answers in kilograms in the table below. [2
c. How much CO2 results from the needed amount of fuel each day? Show your work and report
your answer in kilograms in the table below. [2 pts]
Fuel Energy Content
(g CO2 eq/kWh)*
Daily Harvard College laptop use
Fuel (kg) CO2 Footprint (kg)
Coal 8.3 900
Oil 13.0 650
Natural Gas 15.4 420
Uranium (breeder) 2.2*107 5
11. What did you find most surprising or interesting from this homework assignment? Why? [3 points]
12. BONUS: Can you identify immigrants who had a major influence on US technology? If you have a
candidate or candidates, give a brief summary paragraph on why you think they belong in this
technology Hall of Fame. [4 points] We already mentioned:
a. Alexander Graham Bell (1847-1922)
b. Nikola Tesla (1856-1943)
c. Elon Musk (1971- )