Transcript What is energy???

What is energy???
Usually, in most textbooks and online resources, you will see energy being
described as “the ability to do work”
• Sort of imprecise and is rather confusing if you don’t already have a firm
understanding of what “work” actually is
Because of this, we will take a slightly different approach and define energy in
the following manner:
Energy is a conserved, substance-like quantity with the capability to produce
change in physical systems
James Joule’s research led to the idea
Energy is measured in Joules (J)
• 1 J = 1 N*m
James Joule
behind “conservation of energy”
• Led to development of 1st law of
thermodynamics
• Energy can be changed from one
form to another, but it cannot be
created or destroyed
The total amount of initial energy, plus or minus any energy
transferred into or out of a system, must equal the total final
energy
Perpetual Motion Machines
Perpetual motion devices or “free energy” machines cannot exist
• Though we usually say they are “impossible” what we really mean is that
based on the our understanding of how the universe works and the laws
that govern it, there can be no such device or substance that would have
more final energy that initial energy without any energy transferred into the
system
• Therefore, these cannot exist due to the fact that the idea violates the 1st
law of thermodynamics
Even after all we have learned about
energy and the 1st law of Thermo. has
never failed once….proposals for such
inoperable machines have become so
common that the US Patent and
Trademark Office has made an official
policy of refusing to grant patents for
perpetual motion machines without a
working model
"examples" of perpetual motion
machines in action
A Firm Grasp on Conservation of Energy
After a day
goes by
After some investigation, turns
out one was under the rug
Again, careful investigation
indicates that the window
was open and, upon
looking outside, the other 2
blocks are found
Apparently a friend came
over, with his own blocks,
and had left some at the
house
So the mom disposes the extra blocks from the friend, closes the window, and does not let
the friend in. Everything is going along alright until one time she counts and finds only 7
blocks!!!
• However, there is a box in the room, a toy box in which the blocks are put away in
• Mom goes to open the box, but the boy screams, “No! Don’t open my box!”
• Being extremely curious, and somewhat ingenious, she develops a scheme!
16 oz
3 oz
Even after mom has figured out this new scheme for finding out how many blocks are in
the toy box without looking in it, a problem has occurred. The boy is hiding the blocks in a
bathtub full of dirty water!! The water is so dirty that she can’t see how many blocks are
in there. This requires her to come up with yet another ingenious scheme.
6 in
6.25 in
So what’s the point of the analogy?
Take away the first term in our equations and we are calculating more or less abstract
things
1.) when we are calculating the energy, sometimes some of it leaves the system and
goes away, or sometimes some comes in
2.) In order to verify the conservation of energy, we must be careful that we have not
put any in or taken any out
3.) Energy has a large of number of different storage modes (the toy box & the dirty
water) and there is a formula for each one
• Gravitational, kinetic, potential, heat, elastic, electrical, chemical, thermal, nuclear,
mass energy
4.) If we total up the formulas for each of these contributions, it will not change
Energy Storage
Energy stored in the arrangement of matter in a force field is potential
energy
• Gravitational Potential Energy
• Elastic Potential Energy
• Chemical potential energy
Energy Storage
Energy stored in motion is called kinetic energy
•
•
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Random kinetic energy of molecules (heat, sound) can
rapidly spread out to more molecules and become
untransferrable to other storage modes
Unrecoverable internal energy in a system is called
dissipated energy (Ediss)—not “lost” energy
Friction is usually the culprit for dissipating the kinetic
energy of matter
Once energy has been dissipated, it cannot be
transferred back to useful energy storage mechanisms
• 2nd law of thermodynamics