PhysicsLAB CP Workbook
Power Production

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Does it take a lot of water to light a light bulb?
That depends on its wattage and how long it glows.
Refer to the following information for the next five questions.

In this practice page, you are to calculate the mass and volume of water that falls over a 10-rn high dam to keep a 100-W light bulb glowing for 1 year.
First, calculate how many joules are required to keep the bulb lit for 1 year.

What mass of water elevated 10 m has this much PE?

But this assumes 100% efficiency. A hydroelectric plant is typically 20% efficient. This means only 1 part in 5 of the PE of the falling water ends up as electricity. So the mass above must be multiplied by 5 to get the actual amount of water that must fall to keep the 100-W bulb lit.

This is an impressive number of kilograms! To visualize this amount of water, convert it to cubic meters.

For comparison, an Olympic-size swimming pool holds about 4000 m3 of water. How many such "poolfuls" of water are required to keep a 100-W bulb lit for one year?

Does it take a lot of water to light a light bulb? To light a city full of light bulbs? Now you have a better idea!

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