Heat is not temperature. Temperature is a measurement. And, it is the way that we describe the warmness or coldness of a substance. The measure of temperature is a scale, generally Celsius (C) or Fahrenheit (F), that has been defined using various reference points, the freezing of water (32 degrees F or 0 degrees C) and the boiling point of water (212 degrees F or 100 degrees C). The ability to measure temperature results from the principal now dubbed the “Zeroth Law of Thermodynamics:” two systems are in thermal equilibrium (i.e. the systems are such that there is no heat transfer occurring between them) with a third system (a thermometer) then they are in equilibrium with each other. Sounds like one of those logic problems, if A and B are the same temperature, and B and C are at the same temperature, then A and C are at the same temperature. This may seem obvious, but when you are writing down the rules to explain physical phenomena, these may or may not be very clear. Hence, the Zeroth Law was written down after the First, Second and Third Laws were established. But, without it, we would be unable to investigate heat transfer.
So, what is heat? Heat is a form of energy. Formally, it is defined as the energy transferred from a higher temperature object to a lower temperature one. Heat flows from hot to cold. And, we typically think of heat in terms of heat capacity, the amount of energy required to raise something by one degree of temperature when temperature is measured in Celsius. The typical units of heat are calories and Joules, the same units that we see for measuring energy (even though you may be more familiar with BTUs or kilowatt-hours as “typical” units of energy). A calorie is defined as the amount of heat (think energy) required to raise one gram (or one milliliter) of water one degree C. But, this calorie is not the same as that calorie on the back of your candy bar. Those calories are “food calories” and 1 food calorie is equal to 1000 calories as defined by the amount of heat. So, think about that for a minute, something that has 10 food calories contains the same amount of energy that it would take to raise one liter of water from 4 degrees C (about 40 degrees F) to 14 degrees C (about 57.2 degrees F).
When you start to think about heat as energy, you can start to understand why our electric bills get higher in the summer. We have to move a lot of heat to reduce the temperature in our homes and cars. In fact, humans are always moving a lot of heat. In some cases, we want the heat to do mechanical work for us. Think about the internal combustion engine. We ignite gasoline or diesel to create a push on a piston, which in turn pushes on a shaft that causes the wheels to turn. We convert the chemical energy stored in the chemical bonds of the gasoline and diesel to mechanical energy through the expansion of gasses. Heat is generated in the process. Or, you can look at a steam engine or turbine, we heat water to create steam, then use the steam to either provide the push on the piston or to spin the turbine. This push or spin is converted into mechanical work.
Unfortunately, we can’t harness all of the energy. We are always losing some of the energy to the environment. (This is a result of the Second Law of Thermodynamics and why we can’t have a perpetual motion machine.) The internal combustion engine is only about 20% efficient, when looking at the usable energy, i.e. that energy that is converted to doing the work that we desire. The thermal efficiency of a typical home central air conditioner or its energy efficiency ratio (the cooling energy divided by the power consumption times 100%) is about 14%. It is this energy conversion to the desired outcome that is what challenges the engineer. How can we get all that energy that we are losing to friction, and heat and put it to work?
This is an excerpt from a column published in the Ponca City News Midweek in July 2016.