Cracking the SAT Physics Subject Test

be the final temperature of both the Teflon and the water, then

Qlost by Teflon = mTefloncTeflon(Tf − 430°C)

and

Qgained by water = mwatercwater(Tf − 20°C)

By conservation of energy (ignoring the heat transfer to the air or the beaker), the
heat lost by the Teflon is equal to the heat absorbed by the water. Since Qlost by Teflon

is heat going out, it’s negative, and since Qgained by water is heat coming in, it’s

positive. To account for the signs, we have to write

Qlost by Teflon = −Qgained by water

so

Conservation of Energy

Energy is never simply

“lost.” Thermodynamics

accounts for the missing

energy, which is often

transformed into heat.