Physics 12: Homework #4: due May 3
Here are some supplemental questions in addition to the assignment from the book.
- (modified from Q&P 3.2) Some might claim that we would pollute less if we heated our water with
electricity generated from natural gas rather
than direct burning of natural gas at our houses. The logic is that
electricity plants are equipped with pollution control devices that are not
practical on every gas water heater. But things are not that simple. The
gas-to-electricity plant is a heat engine with an efficiency of, say, 40%.
The transmission of electricity to your home is 90% efficient.
Comparatively, natural gas water heaters tend to get 70% of the fuel energy
into the water (the rest lost to the local environment, pipes, etc.).
- Which scheme uses more natural gas to heat a given quantity of water?
- By what factor (ratio)?
- If gas water heaters emit 50% more pollution per unit
of energy than an electricity plant, which scheme for heating water ends up
polluting more, given what you just computed about the energetics?
- If an ordinary household refrigerator is left operating in a closed,
perfectly insulating room with the refrigerator door standing open,
eventually will the temperature in the room go up, down, or stay constant.
Justify your reasoning.
- When the sun is straight overhead in a perfectly clear sky, we receive
about 1000 W/m² of solar flux onto a horizontal patch of ground. Yet
the average flux throughout the year for a place like San Diego is about one
fifth of this figure. Name the three main factors that are responsible for
diminishing the full-sun value.
- If San Diego has an average insolation of about 200 W per
square meter, how much energy (in kWh) would be delivered by a single panel
with an area of 1.0 m² in an average day, if the panel converts
incident light into electricity with an efficiency of 15%?
- Using the result from the previous problem, how many panels would it
take to satisfy the demand of a modest home requiring 7.2 kWh of
electrical energy per day? If the number seems absurd to you, you might
need to revisit the previous problem.