My chemistry teacher defines it as “the ability to do work.” Can someone give me a more concrete definition (in layman’s terms?) I know it manifests itself different ways (heat, motion, light, etc.) so if you could explain how this works it would be great.
I think your teacher is giving you an oversimplfied answer. Not that I’ll do much better, though. It’s been way too long
Energy can be broken up into two parts: kinetic and potential.
Kinetic energy is energy of motion. Potential energy is more of energy at rest.
I really wish I could give formulas for figuring it each out, but the last science class that I had was 6 years ago. Anyway, think of a pendulum. At the top of its swing, the pendulum is composed entirely of potential energy. Slowly, it converts its energy over to kinetic as it travels downward, becoming as close to fully kinetic as it can when the string holding it is perpendicular to the ground. As it rises, it gains potential energy, losing kinetic energy until it is 100% potential energy again at the top of its arch during the split second that it’s motionless.
Gravity, in effect, steals some of the energy. Friction with the air does as well. This is why, while the pendulum is happy to convert potential energy to kinetic and back again forever, it won’t. Energy is lost to other, outside, forces.
I know what kinetic and potential energy are. I’m trying to figure out what energy is though. Someone told me its made of quanta. How did my teacher define quanta? Packets of energy.
Enderw23:
Gravity, in effect, steals some of the energy.
Yes, but it always gives it back. The pendulum loses potential energy by moving with gravity (down) but gains it back when it swings back up.
Energy is an abstract concept (it’s not possible to form a mental picture of energy except in the simplest cases, like the pendulum example above. Energy isn’t a substance, it’s a property, like temperature). Because it’s an abstract, mathematical concept, the most direct explanation of it is mathematical.
Early in studies of mechanics (the motion of bodies and the forces that cause it), scientists noticed that the laws of motion could be converted into a form in which a certain quantity was unchanged during a physical process. This quantity they called energy. In the pendulum example, the total energy (kinetic plus potential) of the pendulum remains constant, except for the small amount the pendulum loses by rubbing against the air and moving it and heating it up, and by rubbing against its pivot and heating it up. This small rate of loss is what makes the pendulum slow down and stop eventually.
Later, when scientists began to study more complicated things like chemistry and electricity, they wrote laws for those things. And, like with mechanics, the laws could be converted to an “energy” form. But now, the definition of energy got more complicated; it had to account for the influence of these new phenomena. This trend has continued to the present day, as new things like nuclear forces were discovered. The interesting thing is that as new phenomena are studied, scientists have always been able to modify the definition of energy so that they still have a quantity that is unchanged during a process. The energy concept still holds up.
Energy is useful in solving problems. It lets you take sort of a checkbook-balancing approach: as long as you account for all the parts of energy and where they go, you can be sure that the total remains unchanged. And you generally don’t have to worry about the fact that the definition of energy is becoming more complicated; usually, only one kind of energy, like electrical energy, is important in a given situation.
The business about quanta is not very important until you get quite advanced in your studies. It goes like this: before about 1900, people thought that energy could be exchanged in any amount. There was thought to be no limit to the smallness of the amount that could be exchanged, like pouring water from a pitcher. But it turns out that energy is not like water, it’s like sand. You can only pour out one grain of sand at a time, no smaller amount. But the “grains of sand” – the quanta – are so small that in most everyday situations energy acts as if it’s continuous, like water.
Note for the pedantically correct: Yes, I know that water really isn’t continuous, and I know that energy and mass are interconvertible, but these ideas can be explained later without the student having to “unlearn” anything.
>> My chemistry teacher defines it as “the ability to do work.”
Your teacher is right. That is what energy is.
>> I know it manifests itself different ways (heat, motion, light, etc.) so if you could explain how this works it would be great.
All those forms of energy can be transformed into work. Energy is measured in terms of work. Heat is transformed into useful work by means of an ingenious device called an internal combustion engine (found under the hood of your car).
Motion (kinetic energy) is also trasformed into useful work in turbines, sailboats etc. A hammer is a good example of an energy-storing device-machine. As you swing it, it stores kinetic energy which is used to drive the nail. If you miss the nail and hit your finger you will most definitely feel the energy.
So, something has energy in the measure it can do work. That is the definition of energy. More work, more energy.
Energy per unit of time is power
Feynman’s take on the subject, from the Lectures (I-4):
Just a slight nitpick on quanta of energy: Energy can, indeed, come in any amount, but in many situations, there are only certain discreet amounts you can have. An electron moving through empty space, for instance, can have any energy whatsoever, but an electron bound to a proton in a hydrogen atom can only have certain specific energies.
Actually, we learned day three of chemistry that its the Law of Conservation of Matter (or mass) AND Energy. This is because matter can be changed into enegery (E = MC2) and it is possible (I don’t know if its been done) to change energy to matter.
So when I stretch a piece of rubber in my hands, I transfer kinetic energy (my arms pulling) into potential energy (the rubberband becomes tense?) When I let go of the band and it snaps back, thats kinetic energy, correct?
Correct me if I’m wrong, and feel free to add to the list:
heat = the speed at which molecules “move.”
electiricty = motion of electrons (through a wire, etc.)
magnetism = pull of a north and south pole on each other
The E=mc^2 thing is correct, but it only comes into play in very high-energy situations, like nuclear explosions or nuclear reactors, and the interiors of stars. The reason it’s mentioned in your chemistry book is probably because (1) it’s interesting and (2) the situations where it has been used by man are spectacular and important (nuclear bombs and reactors). It’s basically irrelevant to purely chemical processes. It’s a completely separate issue from the kinetic vs. potential energy definitions. The potential energy isn’t “stored” by being converted to additional mass, it is stored in the gravitational or electric field when work is done against it. There’s no mass change in ordinary circumstances.
Your description of kinetic and potential energy in the rubber band is correct.
In your definitions:
Temperature is proportional to the average kinetic energy of a single molecule in a quantity of matter. Since kinetic energy is 1/2 m v^2, the temperature thus depends on both the speed (v) and the mass (m) of the molecules. Heat is the amount of that energy that can be transferred out of a quantity of matter. Thus, if you have 1 kg of matter at 100 degrees, then you combine it with another kg of matter at 100 degrees, you have twice as much heat but still the same temperature.
Electrical energy changes when a charge (like an electron) moves in an electromagnetic field. It’s much the same as the potential energy you obtain when you lift a mass up through a gravitational field.
Magnetism is really just another manifestation of electricity. That is why people usually talk of “electromagnetism” rather than just electricity and magnetism. If you’re interested in how the two are related, you can read any account of Michael Faraday’s experiments
I would add only one comment to sailor’s remarks: While energy is indeed the “ability to do work”, it’s important to recognize that the work need not actually be done. For instance, the amount of gravitational potential energy in a rock on top of a cliff is easily determined and very real, even if the rock never falls off the cliff. It’s the “ability” that is important, not whether that ability is ever used.
This website explains electricity, and you can see that it is not a flow of electrons in a wire: http://www.amasci.com/amateur/elecdir.html
Well, MY high school chemistry teacher told me that if you were to figure out exactly what energy is (more specifically than “the ability to do work”), you’d win a Nobel Prize.
Potential energy is mass. It’s just that the change in mass is the change in energy divided by c[sup]2[/sup]. Since c[sup]2[/sup] is so large, it takes a very large change in energy to have a measurable change in mass. I would say: There’s no measurable mass change in ordinary circumstances.
I thought “quanta” only applies to the energy of electromagnetic radiation, and not to potential energy, kinetic energy, gravity, etc.
Yes/no?
I like the teachers way of putting it. Its simple & to the point, however, what he said is just what the dictionary said, so I bet he read it. Also try howthingswork.com search for ‘energy’
en•er•gy "e-ner-je\ noun pl en•er•gies [LL energia, fr. Gk energeia activity, fr. energos active, fr. en in + ergon work — more at work] (1599)
1 a : dynamic quality <narrative energy>
b : the capacity of acting or being active <intellectual energy>
2 : vigorous exertion of power : effort <investing time and energy>
3 : the capacity for doing work
4 : usable power (as heat or electricity); also : the resources for producing such power syn see power
©1996 Zane Publishing, Inc. and Merriam-Webster, Incorporated. All rights reserved.
Energy is merely the result of phlogiston evolution.
Sort of. Imagine an electron on a spring - it’s vibrating at a certain energy. When it is moving, it has kinetic energy, and when it is at rest, it has potential energy, just like a pendulum. (Quantum mechanics: please play along.) In order for it to slow down, it must radiate a photon. It can emit a photon of arbitrary frequency up to the point where the photon takes all of the vibrational energy of the electron, but it can emit very low frequency photons as well, to acheive nearly any energy level.
So, since it can decrease its energy by any arbitrary amount, it is not in that sense quantized. Since it must emit a single photon to lose energy, it is in that sense quantized.
This type of ‘quantization’ gives rise to blackbody radiation. This is different than the quantization of an electron in an atom, which you may be familiar with due to the emission lines in atomic spectra. In that case the electron can only give off photons with one of a few discrete energies to descend to a lower level of energy.
You’re correct, but from the classical viewpoint, there’s no provision or explanation for a mass increase, so it’s conventional to say that the energy is stored in the field.
I knew as soon as I hit the button, I forgot to say something.
Although DrMatrix is correct, I would caution against thinking that “potential energy is mass”. In Newtonian mechanics, gravitational potential energy requires the presence of a gravitational field. The potential energy in the mass is only due to its interaction with gravity. Mere mortals would never calculate the path of a mass in a gravity field using the relativistic mass increase; we leave that to the astrophysicists.
If we account for relativistic effects, then yes, there is energy in the rest mass, even without gravity present. But that energy is not ordinarily obtainable and is not related to gravity. An analogy might be to consider the potential energy of a stick of dynamite lifted through a gravitational field. It has lots of chemical potential energy that can be released by lighting it, but in the realm of mechanics, that energy is unobtainable.
Oh, and I didn’t want to leave the impression that relativity is not part of classical (non-quantum) mechanics. I should have said “Newtonian” instead of “classical” in my previous post. You can never be too precise on these here boards.
Quoth douglips:
Bad example, Doug. If it’s on a spring, then it’s a bound state. In QM, all bound states have quantized energy: it can be value A or value B, but not any value in between. For the specific bound state you’re talking about, the harmonic oscillator, these energy states are evenly spaced, and occur at 1/2 hbaromega[sub]0[/sub], 3/2 hbaromega[sub]0[/sub], 5/2 hbar*omega[sub]0[/sub], etc., where hbar is a fundamental constant of the Universe, and omega[sub]0[/sub] is a constant that depends on the strength of the spring and the mass of the thing attached to it. The energy of the electron is, of course, divided into kinetic and potential, but the total energy must always have exactly one of those values.
If you have a free electron, without any springs or fields messing with it, then it can have any energy.
[disclaimer]
add the phrase "According to my understanding, " to each statement below. I’m no physicist, but I read a lot.
Current theory, unless I’m mistaken, has the universe as a giant grid (so to speak), with resolution of 10[sup]-33[/sup]m (the planck distance). Since nothing can move less than the Planck distance in any direction, position is not a continuous function.
Additionally, time is also discontinous, with maximum resolution of the Planck time (I don’t remember the number for this one).
Since gravitational potential energy and kinetic energy are both functions of discrete values, the energies themselves have to be discrete (quantized) as well.
In other words, PE[sub]gravity[/sub] = MGY, where M=mass, G=force of gravity on the object, and Y=height above an arbitrary delimiter. Since M and Y are each limited to one of a number of quanta, PE is quantized as well. Similarly, KE is [sup]d(position)[/sup]/[sub]d(time)[/sub]. Since the calculation is dependent on two quantized values and the result cannot have more precision than the most precise operand, KE also has to be quantized.
I hope that made sense.