Heat, Temperature, and Power for AP Physics B

Practice problems for these concepts can be found at:

Thermodynamics Practice Problems for AP Physics B

Heat, represented by the variable Q, is a type of energy that can be transferred from one body to another. As you might expect, heat is measured in joules, because it is a form of energy. Notice that all energy quantities (KE, PE, Work) have units of joules. Heat is no different.

Be careful with phraseology here. Energy must be transferred in order to be called heat. So, heat can be gained or lost, but not possessed. It is incorrect to say, "a gas has 3000 J of heat."

Internal energy, represented by the variable U, is also measured in joules, again because it is a form of energy. It is correct to say, "a gas has 3000 J of internal energy."

Temperature is measured in kelvins or degrees Celsius. There is no such thing as "a degree kelvin"; it's just "a kelvin." To convert from kelvins to degrees Celsius or vice versa, use the following formula:

Temp_Celsius = Temp_Kelvin – 273.

So, if room temperature is about 23°C, then room temperature is also about 296 K.

It is important to note that two bodies that have the same temperature do not necessarily contain the same amount of internal energy. For example, let's say you have a huge hunk of metal at 20°C and a tiny speck of the same type of metal, also at 20°C. The huge hunk contains a lot more internal energy—it has a lot more molecules moving around—than the tiny speck. By the same logic, you could have two items with very different temperatures that contain exactly the same amount of internal energy.

Power is measured in joules/second, or watts:

1 W = 1 J/s.

Power is not an idea limited to thermodynamics. In fact, we often talk about power in the context of mechanics. We might ask, for example, "How much power is needed to raise a block of mass 2 kg a distance of 1 meter in four seconds?"¹ But discussing power now allows us to bridge mechanics and thermodynamics … with this problem.

The force of friction is converting the block's kinetic energy into heat—this is why friction slows objects down and why your hands get hot when you rub them together. We know the rate at which the block's KE is being converted to heat, so if we calculate how much KE it had to begin with, we can find how long it takes to come to rest.

Now that we know the length of time the block was sliding, we can determine the distance it traveled using kinematics. You know how to do this well already, so we won't take you through it step-by-step. Just remember to fill in your table of variables and go from there. The answer is 625 m, or more than a quarter mile… this must be most of the way across the warehouse!

Practice problems for these concepts can be found at:

Thermodynamics Practice Problems for AP Physics B