Watts, BTUs, joules – oh my. It is easy to get confused with all the different acronyms, and units, being bandied about. Well, In a spirit of pure public service, I am going to try to explain what the different energy terms mean. Hey, I said try…
Although this is not meant to be a first year physics text (there would be much more “between the lines”), before we can understand the units, we must somewhat understand the concepts of work, and force. And how the two are inter-related. We also have to understand what energy is.
While energy can be an abstract concept, in our particular context energy means the potential (or ability) to do work. Work is what shifts energy from one system to another. Energy can exist in two general forms potential (stored) or kinetic (in motion). Gasoline contains potential energy. It is just “sitting there” taking up space. Once that energy is shifted to the mass of your car, by doing work – acceleration - the mass of your car then contains energy in the form of kinetic energy. It will contain exactly the amount of energy that was transferred to it.
“It is important to realize that in physics today, we have no knowledge what energy is. We do not have a picture that energy comes in little blobs of a definite amount. — Richard Feynman”
- So, Energy is the ability to do work.
Now, work seems a simple concept, and it is, sort of. Work, in physics, is of course mechanical work, The moving of something over a distance. If you push your car up the driveway, it is obvious you have done work. Obviously! You have done that work by transferring your bodies’ potential energy into the mass of the car, accelerating the car up the driveway.
But, Interestingly, if you push and push, and the car does not move, according to the laws of physics, you have not done any work. Your energy was expended as waste heat and was not transferred to the car. Physics demands that something be moved for work to happen. Hence no work. Unfair, I know, but that’s the way it is.
You have, however, transferred energy from one system (you) to another (the air) via heat. Well, in thermodynamics, there is another, broader, definition of work. Work can also be defined as the transfer of energy from one system to another. Both definitions are important to the understanding of the terms below.
So work is either the movement of a mass (stuff) over a distance in physics, or, the transfer of energy from one system to another in thermodynamics. The distinction is that in Physics we are dealing with the transfer of energy by mechanical means. In Thermodynamics, energy is transferred, not mechanically, but via heat.
At the risk of being overly simplistic, these are two of the four primary ways that energy is transferred. The other two are electromagnetic waves and gravity. Note there are many defined FORMS of energy – as I stated, energy as a concept is elusive – but it is transferred between systems mainly in these four ways.
But, back to work:
- Work is the movement of a mass over distance, or the transfer of energy from one system to another.
Force is the influence (or applied energy) that causes work to be performed. When a force acts to move an object, we say that work was done on the object by the force. That force can be electricity, and the object a motor shaft. The force can be chemical energy, and the object a piston or turbine. The force can be gravity, and the object a skydiver falling out of an airplane. Force when applied to a mass (thing) to cause a displacement over a period of time is causing work to be done. In physics, force is mass times acceleration. (F=ma).
- So, force is an influence that causes an object to speed up or slow down (i.e. accelerate or decelerate), or that causes work to be performed.
And, finally, there’s power. Power is a measurement of how fast that object is speeding up or slowing down, or how fast energy is being transferred between systems. In other words, how much work is being done? As we all know, the more power we have, the faster the work is done. Conversely, the faster a certain amount of work is done, the more power is required, and the more power has been used. Power equals work divided by time (P=W/T). Power is an aggregate of all the former terms. It is the measurement of the final result, so to speak, of using energy to apply a force to do work.
- So, if work is defined as moving (accelerating or decelerating) something, then force is what causes it to happen. Energy is the push behind the force, and power measures how much work has been done.
But, you came here to learn about the units we measure this stuff with, right? Well, if you are still here, we are about done!
“For those who want some proof that physicists are human, the proof is in the idiocy of all the different units which they use for measuring energy. — Richard Feynman.”
We measure the quantity of energy (or force) with the units Kilowatt hour, BTU, or Joule. These are all measurements of “units of energy”. Conversely, the same units can be used to measure how much energy a process has used. We used so many BTU’s of fuel, so many Kilowatt-hours of electricity, or so many Joules of energy. If energy were water, the equivalent unit would be the gallon.
There are 1055 Joules in a BTU, and there are 3414 BTU’s in a Kilowatt-Hour.
We measure the amount of work that is done with units of power. The Horsepower, the Watt, or the BTU per hour (BTU/Hr). In our water example, these would be equivalent to Gallons Per Hour.
A horsepower is a unit of power sufficient to raise 550 pounds One foot in 1 second. Equivalent to roughly 745.7 Watts, or 2,546.6 BTU/Hr.
There you go! So much background. Well, the concepts I briefly explained (and it was brief) before defining those terms, are extremely important to all fields of energy. They are the foundation principles. The above explanation only scratched the surface.
I will finish this up with a couple of examples.
First with water. The garden needs to be watered with a gallon of water every hour (big garden). This is the amount of energy (water) we need. SO we will be doing work at the rate of one gallon per hour. At the end of ten hours we will have used ten gallons (units) of our energy.
Now lets make the garden a 100-watt light bulb. We need 100 watts of energy every hour to light our room. So we will be doing work at the rate of 100 watts of electrical energy every hour. At the end of ten hours we will have consumed 1 Kilowatt hour (1 unit) of electrical energy.
Our Furnace needs 50,000 BTUs of energy every hour to keep our toes toasty. So we will be doing work at the rate of 50,000 BTUs per hour. At the end of ten hours, we will have consumed 500,000 BTU’s (units) of energy.
Our car needs two gallons of gas (248,000 BTUs) every hour to go a steady sixty MPH. So, we are doing work at the rate of 248,000 BTUs per hour. At the end of ten hours, we will have consumed 2,480,000 BTUs (units) of energy. Of course we ran out of gas 6 hours ago…
I am going to elaborate on that last example, lest you think this is all academic. Simple math tells us that car is getting 30 MPG on the highway. So, how much horsepower did we need from the engine? Well, if you have been attentive, you would realize you divide that 248,000 BTUs by the amount of BTUs in one horsepower and get 97 Horsepower. Right? Well, sort of, but wrong. In the average gasoline internal combustion engined car, 75% of those BTUs were wasted, they did not make any horsepower. The went out the tailpipe, and kept your radiator warm. SO, the correct way to do it is to take 25% of those BTUs, 62,000 (which is what was actually used to make Power), and divide by 2,546.6. Which gives us 24 Horsepower. And, that indeed, as a rule of thumb, is what the average 30 mpg car needs to go down the road at 60 MPH.
And, now you know.