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Name: Dave
Status: student
Age: 15
Location: N/A
Country: N/A
Date: 7/13/2004


Question:
What is the difference between heat and internal energy?


Replies:
First, internal energy is not an absolute number, unless you consider the mass equivalent given by E=m*C^2. The CHANGE in the internal energy of a substance or system is the sum of the heat added to or released by the substance or system and the work done by or done upon the substance or system, give by D(E)= q - w where d(E) is the change in energy, 'q' is the heat, and 'w' is the work. This is also called the first law of thermodynamics.

Vince Calder


Dave,

Internal energy is energy stored within the vibrations and structure of the molecules of an object or material. For an object's temperature to rise, internal energy must increase. Heat is energy passing from one object or material to another BECAUSE OF a difference in temperatures. Heat can be in any of three forms: conduction, convection, radiation.

Conduction is quickly vibrating molecules of a hot surface making contact with slowly vibrating molecules of a cold surface. When the fast molecules bang into the slow molecules, the faster molecules slow down and the slower molecules speed up. The hot surface has cooled down and the cold surface has heated up. When temperatures are equal, conduction is balanced: no more heat flows.

Convection is energy carried from one place to another by the flow of a liquid or gas. An excellent example of this is wind from the south. Air molecules are heated in the south, either by the sun or by hot ground and hot seas. They then move north, carrying the energy with them. The air makes contact with your body, making you hot. Carrying the energy from the south to your body is convection. A hot object will add energy to the wind. A cold object will absorb energy from the wind.

Radiation is energy carried by electromagnetic waves: radio waves, microwaves, infrared light, visible light, ultraviolet light, x-rays, gamma rays, etc. Atoms with extra energy can release this energy as light waves. The waves travel through space and get absorbed by other atoms. Hot atoms emit more energy because they have more extra energy. As a result, more radiation energy passes from hot to cold atoms than from cold to hot. The hotter atoms cool off and the cooler atoms warm up.

Ken Mellendorf
Math, Science, Engineering
Illinois Central College



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