On 1/7/2005 9:55 AM EST, Brian Henning wrote:
Okay, okay, before I get cold hosed myself: Heat is the kinetic energy of a particle, whether it be an atom or molecule or whatever.
>> I have a HOT flash for you. HEAT is Radiation, by definition. > > And I have a COLD HOSE for you. > > Wrong. HEAT, by definition, is the kinetic energy of a molecule. > RADIATION is a method by which heat is transferred without contact > or immersion in a fluid (example: the sun heats the earth via > radiation). Other methods of transferral of heat are CONVECTION > (fluid movement) and CONDUCTION (surfaces in contact). In fact, > the heat added to your lap by your laptop is NOT radiation, by > definition; it is conduction.
You guys are mostly correct. Heat transfers from a hotter object to a colder object, whether through conduction (solid or liquid objects), convection (heat conducting into a gas and the hotter gas leaving the vicinity due to bouyancy), or radiation. Radiation is what you feel if you stand in front of a heat lamp or fire; the acual photons of infrared frequencies are heating your skin.
Radiation comes in multiple forms, however. Photons are the carrier of most kinds of radiation (that is commonly considered "radiation"). When electron(s) in an atom transition from an excited state to a lower or ground state, they emit photon(s) (conservation of energy requires this). In the case of a laser and some other excitations, most of the energy is released as phonons (vibrational energy) which can be thought of as thermal photons in the crystal lattice. This is one of the reasons that most lasers (gas and liquid) require cooling.
Energy quanta released from electron transitions are called photons, and from nuclear transitions (i.e., same "spdf" levels in electronic transitions but a lot more added for the nucleus) they are called gamma particles. They are both photons, but gamma radiation is considered more damaging because of its higher energy. When a helium atom is stripped of electrons (ionization), it is called an alpha particle. Beta particles are nothing more than electrons ripped from their atom (but there are positrons, same characteristics of electrons but positive charge). Neutrons come from atoms stripped of electrons and protons, which can cause more damage due to the unstable nature of neutrons (can transition, with a neutrino, to protons and beta particles and back again).
Originally I said "heat" because many people think of radiation as a nuclear property, but it is merely a propagation mechanism (whether it be via photons, elecrons, neutrons, gamma, or others). A nuclear bomb, ironically, has heat as the most damaging property, but that is a mere "waste" product to satisfy conservation of energy.
To elaborate the statments above, heat added to your lap via laptop comes from both conduction and radiation, and if it's under your lap or have a CPU fan blowing out the bottom, also convection. Most of the heat would be absorbed via conduction, though, because of its higher transfer efficiency.
Regards, -Rob
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