Energy, Heat and Temperature
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  • 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!

Kinetic Energy

  • Energy of motion: An external "organized" form [KE = mv2/2].

Potential Energy

  • 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, ...

Internal Energy

  • 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 on?

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?

  • 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>, modified 29 sep 2012