Wednesday, May 30, 2007

Direct Proportionality and Gasoline Prices

Ahhh, another example of why Mathematics is Important, Reason #231! Recall the Ideal Gas Law, pV=nRT, which means that the volume V of a gas is directly proportional to the temperature T of that gas held at a fixed pressure. Why is this important? Gasoline!

In an article in the Los Angeles Times on May 9, 2007, "Not just inflated prices but inflated fuel":

The U.S. government defines a gallon of gas this way: At 60 degrees, a gallon is 231 cubic inches. But when fuel is warmer than 60 degrees, the liquid expands. When it's colder, the fuel contracts.

U.S. oil companies and distributors account for temperature when they sell to each other. Wholesale facilities are equipped with devices that adjust volumes to bring the gallon tally in line with the 60-degree standard.


That equity, however, stops short of retail fuel pumps. Service stations dispense gas and diesel as if every drop is flowing at 60 degrees — and they charge customers as if they are getting government-standard gallons.


Gasoline expands or contracts 1% for every 15-degree change in the fuel's temperature. Diesel volumes change 0.6% per 15-degree change. The difference seems small, but it adds up fast in California, where fuel temperatures can be much higher than 60 degrees and prices are steep. It's a bigger hit for truckers, whose rigs gulp 20,000 gallons or more of diesel a year.
So, as you think about rising gas prices, recall the principle of direct proportionality from your high school algebra class.


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