1. Feb 4, 2013

### mc2_phy

Came across these questions

What is meant by?
Energy use
Energy storage
Energy expenditure

2. Feb 4, 2013

### Staff: Mentor

Let's look at what the definition of energy is.

A very basic definition is: The ability for one system to perform work on another. Or just, "The ability to perform work."

Well what is work? A basic definition of work is: In physics, a force is said to do work when it acts on a body so that there is a displacement of the point of application, however small, in the direction of the force. Thus a force does work when it results in movement.

The key here is that energy is a term that allows us to quantify work BEFORE it's performed. Note that both work and energy have units of measurement in joules. This is because they are *almost* the same thing. Instead of saying a bullet will perform X amount of work through the application of a force when it impacts a target, I can simply say "The bullet has X amount of kinetic energy". Plus we can break things up into different "types" of energy, such as kinetic, electric, thermal, etc to make them easier to work with.

So what is Energy Use? It's simply the result of performing work. To perform work you require energy, as energy is the ability to perform work!

How about Energy Storage? Consider a battery. I charge the battery and throw it on a shelf. A little later I take that battery and I use it to perform work in an electric circuit. The fact that I can take that battery, charge it, and then use it later on is pretty much what energy storage is. It's quite simply the storage of energy.

Energy expenditure is the same thing as energy use.

3. Feb 4, 2013

### AbsoluteZer0

If you're interested in the mathematics of Energy, here is a mathematical take of it:

To have energy is the ability to perform work.
There are several types of energy, the most commonly known being Potential and Kinetic.
Kinetic energy is the energy of an object as a result of its motion.

$E_k = \frac{1}{2} mv^2$
where m is mass and v is velocity.

Work is defined as the product of Force and Displacement or the change in energy.

$W = Fd$
or
$W = \Delta E$

The storage of energy is known as having potential.
From Wikipedia: "Potential energy is the energy of an object or a system due to the position of the body or the arrangement of the particles of the system"

One example of potential energy (Ep) is Gravitational Potential Energy, which is possessed by all objects within a gravitational field. In the case of the Earth, it is possessed by all objects above the earth's surface. It is given by the following formula:

$E_p = mgh$ where m is the mass, g is the gravitational field strength (acceleration due to gravity,) and h is the altitude.

**This formula is only effective up to a certain height, I believe it is 1km.
____

One thing to note is that energy is ALWAYS conserved. You cannot destroy or create energy, it can only be transformed from one form to another. In order to illustrate this, take the following example: Say you are standing on top of a cliff and you are holding a rock. The energy that is possessed by this rock is Potential Energy, more specifically Gravitational Potential Energy as it is above the surface of the earth. When you release it, this potential will be converted to kinetic until it hits the ground.

$E_p = E_k$ and therefore:

$mgh = \frac{1}{2} mv^2$

4. Feb 4, 2013

### ZapperZ

Staff Emeritus
Please note that to use this forum effectively, you should have done some of the investigation yourself. Did you try to look up these things by doing a search first?

If you did and you didn't understand something, that's when it is best to ask your question here and specifically point out what part you didn't understand. If not, you're just asking to be spoon-fed.

https://www.physicsforums.com/blog.php?b=3588

Zz.

5. Feb 8, 2013

### tomdam

I have several questions. I'm not asking to have my questions answered in totality here, although I would be appreciative if that happened. Instead, if someone could point me to a site, book, etc. that could answer this. My professors and my google searches haven't been able to get me through my curiosity on this topic.

I am curious about the units that comprise joules: kg * m2/s2.

I understand that a joule is essentially a Newton * Meter. I understand that a Newton is a measurement of acceleration and that a joule, therefore, is simply the measurement of acceleration over a distance (hence, Joules equals Newton*M).... that is, if I'm not misunderstanding things.

However, this description of energy is confusing to me. To me, this doesn't seem to describe what energy is. Instead, it seems to just describe what energy does. Energy is defined as the ability to perform work, but that just sounds like we're describing the effect of energy. The idea I have of energy is elusive and abstract; so it seems there is an injustice to having such a simple, "materialistic" definition of it.

Some examples scenarios of what confuses me: When we say that heat is energy, are we just describing the distance molecules will accelerate in response to heat stimuli?

When electrons relax and emit light, where is the energy there? Are photons the mass that is being accelerated? Or is light just the byproduct of negative energy? And if light is just a byproduct of negative energy, shouldn't there be something more to the definition of energy if something as phenomenal as light can be produced from it?

Maybe there is too much baggage to the word energy? Maybe I am conflating the popular idea that energy is a pseudo-natural presence with the physics definition of energy?

6. Feb 8, 2013

### ZapperZ

Staff Emeritus
7. Feb 8, 2013

### Staff: Mentor

A newton is the unit for force. It is the amount of force required to accelerate one kilogram at one m/s2. Note that a newton is NOT acceleration. It is a force.

But energy doesn't "do" anything. It is simply an abstract quantity that represents certain things about a system of interacting objects.

I don't think heat is energy, rather heat is energy transfer between objects through thermal interactions. I think thermal energy is what would be most appropriate here.
http://en.wikipedia.org/wiki/Thermal_energy

The acceleration of mass is not the only form of work. And there are several different ways to define a Joule, such as the work required to produce one watt of power for one second. When you move charges to produce an EM wave you must perform work on those charges, and thus it requires energy to do so.

I don't even know what "pseudo-natural presence" means. Energy is clearly defined in physics, even if it is not easy to understand.
This isn't to say that my post is ALL there is to say about energy of course...

8. Feb 8, 2013

### tonyjk

mc2_phy : i asked for myself the same questions about 2 years ago:P... and i found it by myself and am happy now Drakkhith is "confirming me" my knowledge:P thanks Drakkhith haha.. and i posted a thread about heat and thermal energy and i think i just found the answer in Drakkhith post so thanks again:P

9. Feb 8, 2013

### tonyjk

I have also a question : the energy carried by an ElectroMagnetic wave has the ability to perform work like what?

10. Feb 8, 2013

### Staff: Mentor

Really?
"What it is, what it shall be, what it was." -Robin Williams as Adrian Cronauer as Walter Cronkite

What it does is what it is. [me]

11. Feb 8, 2013

### Staff: Mentor

Like moving an electric car.

12. Feb 8, 2013

### Staff: Mentor

Energy is very easy to understand because it is clearly defined. The problem is that people are not satisfied with the definition because for some reason they feel like it should be more complicated than it is.

13. Feb 9, 2013

### Staff: Mentor

How about moving the charges in an antenna so you can listen to the radio or watch TV? Or ejecting electrons from metals? Or breaking molecular bonds and damaging our DNA?

14. Feb 23, 2013

### tomdam

Thank you all for your replies. You have definitely helped me to better understand energy. I also better understand why I get confused about it and how I can get beyond my confusion.