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This is one of those words which everybody knows, but is hard to define well. Energy is:
the ability (or capacity) of a system to do work or supply (or produce) heat.
Of course, there are words in there which require definitions also, but I'll get to that in a moment. First some other energy-related stuff.
(1) Kinetic energy is the energy associated with motion; the faster an object moves, the more kinetic energy it has. There is an equation which governs this:
K.E. = (1/2) mv2
m means mass and v is velocity. This equation means that the general units on kinetic energy are:
(mass) (distance)2 (time)¯2
Since any mass, time or distance unit could be used, it has been agreed to standardize on specific units for these three quantities and they are the kilogram, second and meter. Inserting them in the above equation gives:
(kg) (m)2 (s)¯2
This unit has been given a name: Joule. This is in honor of James Prescott Joule, who in the mid-1800s did pioneering work on energy. The Joule is the standard metric (or SI) unit for all energy and it will be used on this web site.
By the way, an older energy unit is still around. It is called "calorie" and it gets a bit of a mention on this web site, but not much.
(2) Potential energy is energy that is stored by virtue of position. There are several different types of storage, of which these four are examples.
(a) Gravitational - this is the most familiar. A rock poised to roll down a hill has potential energy. A ball thrown into the air gains more and more potential energy as it rises. The higher in the gravity field you go, the more potential energy you gain. Generally speaking, chemistry does not concern itself with the potential energy from gravity.
(b) Electrical - in certain materials, you can remove electrons from one area and send them to another. The area losing the electrons becomes more and more positive and the area gaining them becomes negative. The greater and greater the charge difference, the more energy is stored within the system. An example of this is a storm cloud about to "hurl" a lightning strike Earthwards.
(c) Chemical - this is slightly more complex. Certain chemicals have bonds which require little energy to break. This energy must be put into the bond to break it. However, during the course of the chemical reaction, new bonds form which give off MORE energy than that which was put in. Commonly, these reactive compounds are said to "store" energy, but the truth is that the energy released came from a process of first putting in and then getting back more than you put in.
The positional aspect comes from first breaking bonds between atoms (which takes energy) and then rearranging the atoms in new positions to form new bonds (which gives off energy).
If you get back more than you put it, this is called exothermic. The net potential energy converted in the reaction shows up as heat, that is the area around the reaction goes up in temperature. If you get back less than you put in, this is called endothermic. The increase in potential energy of the newly made compounds is reflected in a heat flow from the surroundings into the chemicals, resulting in a temperature drop in the surroundings.
(d) Nuclear - the famous equation E = mc2 governs this source of potential energy. We can consider the mass itself to be potential energy, since it can be converted from a form not being used (while it is the mass), to kinetic energy. This type of potential energy is released (in measurable amounts) during radioactive decay, fission and fusion.
This last potential energy type leads to an interesting development of modern science.
Of these types of potential energy, electrical and chemical are of the most interest in chemistry. A small warning - in the sources the ChemTeam has looked at, gravitational is by far the most common example and is many times the only example. The problem, of course, is that gravity plays little to zero role in chemistry.
The usual definition of work is as follows:
a force acting over a distance
A more wordy definition is:
the transfer of energy from one mechanical system to another. It is always completely convertible to the lifting of a weight.
In chemistry, the primary type of work discussed is called "PV work."In fact, the concept of work is usually introduced in chemistry in order to discuss PV work's role in the definition of Enthalpy. The result? A good understanding of work tends to "disappear" - especially in introductory classes - since it gets lumped into enthalpy. And enthalpy is very, very important!!
I guess what I'm saying is that work is a bit of a side issue on the way to a more useful concept of enthalpy. So it tends to get a short discussion in most chemistry textbooks. It's not that work is unimportant because it's not. It's just not the main focus.
There is a lot of misunderstanding about what heat is, so let's try and make it real clear: heat is not a thing, heat is a process. Here's the definition:
heat is the transfer of energy between two objects due to temperature differences.
Notice that the name of the transfer process is heat. What gets transfered is energy. Heat is NOT a substance although it is very convenient to think of it that way. In fact, it used to be thought that heat was a substance.
There is some circularity to the definitions used:
(a) energy does work or produces heat, but
(b) heat is a transfer of energy.
I think this traces back to the fact that energy is something like obscenity: you know it when you see it, but it's very difficult to define. Ultimately, energy is expressed in the motion of substances. If it is moving, it has energy. If it has the capacity to move, there is some potential energy stored away.
Generally speaking, the temperature discussed is absolute temperature, measured in Kelvins. Here's the definition:
temperature is a property which is directly proportional to the kinetic energy of the substance under examination.
By the way, it's OK to use temperature differences measured in degrees Celsius. That's because the "size" of one degree Celsius equals one Kelvin. Also, the term "degrees Kelvin" is NOT used.
Here's another definition I found:
temperature is the property which determines the direction heat will flow when two objects are brought into contact.
It turns out that temperature is a rather sophisticated concept, but this short discussion will suffice until you get into the more sophisticated classes that chemistry majors take in college. We'll measure temperature with various devices and use it in calculations and pretty much leave it at that.
Two last points, just by the by:
(1) When two bodies are in thermal equilibrium with a third body, then they must be in thermal equilibrium with each other. This is called the Zeroth Law of Thermodynamics and is the basis for temperature measurements, since the thermometer must come to thermal equilibrium with the object being measured.
(2) An important issue in temperature measurement is the ability to accurately and reroducibly measure temperature. To that end, there are on-going efforts at the international level to set temperature standards and ensure that the scientific world gets good data. A recent issue of discussion concerns how to accurately measure temperatures below 0.1 Kelvin. After all, your ordinary laboratory thermometer just will not do at those very low temperatures.
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