Energy, Heat and
External Links page.
- What is energy? Ultimately,
the ability of an object to produce forces and move things (do
"work" for us); can be stored in different ways, comes
in interchangeable forms.
- On Earth: All of our energy,
basically, comes from the Sun (including that of hurricanes);
How does the Sun produce so much energy?
- Units: One unit of energy is
the joule (J), the energy a 2-kg mass moving at 1 m/s has just
due to its motion.
- Comparisons: A 200-lb person
walking at 1 m/s (normal walk) has 100 J of KE, the same amount
that a 100-W light bulb uses in 1 s. An average human uses about
10 million J/day. Burning 1 liter of oil gives 12 million joules.
In a nuclear reactor, only a small part of the uranium mass is
converted to energy, but 1 kg of uranium gives 56 trillion joules!
- Energy of motion: An external
"organized" form [KE = mv2/2].
- Idea: A form of energy that
can "potentially" become motion.
- Gravitational potential energy: Depends
on how far an object is from bodies that attract it gravitationally;
Makes objects gain speed as they fall.
- Electric potential energy: Depends
on where objects are located with respect to electric charges;
Supplied by the electric company, or batteries and alternators;
Runs all our appliances, lights, ...
- Idea: All forms of energy that
the molecules in matter have.
- Chemical energy: An "invisible"
potential energy inside matter; Depends on how atoms are combined,
and is released in burning, explosions; Runs our candles, cars,
- Mass energy: Energy matter possesses
just because of its mass, E = mc2;
In most cases it is by far larger than any other form of energy,
but only nuclear reactions can release a significant portion;
Runs the Sun.
- Thermal energy: Heat...
Thermal Energy and Heat
- Thermal energy: An "invisible" form
of kinetic energy of the random motion of atoms or molecules (500
m/s is common), including rotations and vibrations; Contrary to the
macroscopic kinetic energy, this internal one is disordered.
- Related concepts: Heat is thermal energy
transferred between two objects. What's the difference between temperature
and heat? What is the lowest possible
T? Absolute zero, 0 K = –273°C = –460°F.
- Example: It is possible to transfer
heat to an object without changing its temperature; What's going
Any Other Energy?
- What about radiation? Are light
and other forms of radiation matter (photons) or energy (radiative
energy)? Is there a clear distinction?
How Does Heat Travel?
- Conduction: Direct contact between
a hotter and a colder material; e.g., metals.
- Convection: Warmer fluid material
moves and carries heat with it; e.g., hot air rising.
- Radiation: Electromagnetic (or
other) waves carry energy with them; e.g., sunlight.
What's the Big Deal with
- Energy is conserved: It is not
created nor destroyed, even when matter and energy transform
into each other!
- The bottom line: When interesting
things happen in nature, energy changes from one form to another;
Thinking about processes in terms of energy makes it much easier
to keep track of them and predict what will happen.
page by luca bombelli <bombelli at olemiss.edu>,
modified 29 sep 2012