Few questions about heat?

[scilearner]

Hello everyone,

 

Ok the definition of heat says it is the total kinetic energy due to random motion of molecules. My question is it really the total kinetic energy. For example I know in inelastic collision heat is released. So is thermal energy really heat released in inelastic collison of molecules. Also if there is a bunch of molecules in a container and they all had elastic collisions, would there be heat in the container. Thanks !!

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Hello everyone,

 

Ok the definition of heat says it is the total kinetic energy due to random motion of molecules. My question is it really the total kinetic energy. For example I know in inelastic collision heat is released. So is thermal energy really heat released in inelastic collison of molecules. Also if there is a bunch of molecules in a container and they all had elastic collisions, would there be heat in the container. Thanks !!

 

Edit: Oh I think I worked it out. Tell me if it is right. The total kinetic energy of individual particles in an object is thermal energy (This is what gives temperature). In inelastic collision heat is given off (which is like like loss of thermal energy because heat is energy transfer). Heat is given off when molecules collide with surrounding air and give some energy I think, this is for an example. In elastic collisions, the shape comes back to orginal shape, so no energy is lost. Also this definition from another website helped me.

 

Question: In detail, what is the difference between kinetic energy

and thermal energy?

marsh

 

Answer:

Kinetic energy is a general term describing the energy

associated with the motion of objects (large or small objects).

You can calculate the kinetic energy of an object of mass m

with a velocity (speed) v from the formula K.E. = 1/2 mv^2.

Thermal energy refers to the kinetic energy of the microscopic

particles (atoms and molecules) that make up all samples of matter

- i.e. all objects. When you add heat to an object, you increase

the temperature of the object (usually) and that heat increases

the kinetic energy of the molecules that comprise that object.

In fact, temperature is a measure of the average kinetic energy

of the microscopic particles that make up an object.

Hope this helps....

Dr. Brown

Edited May 23, 2010 by scilearner
Consecutive posts merged.

[Mr Skeptic]

Heat thermal energy is the average kinetic energy of the molecules, if you are not counting that kinetic energy as part of the kinetic energy of another object (eg a bullet). That is, the molecules are moving/vibrating in random directions. Molecules, in general, have elastic collisions only, unless they are reacting.

 

When two large objects collide inelastically, their component molecules still collide elastically (more or less), but the now move in random directions unlike if the collision were elastic (they'd move in the same direction and thus be counted as kinetic energy of the object rather than as heat thermal energy).

Edited May 23, 2010 by Mr Skeptic
Thanks swansont, I should know better than to participate in the confusing of these words.

[scilearner]

Heat is the average kinetic energy of the molecules, if you are not counting that kinetic energy as part of the kinetic energy of another object (eg a bullet). That is, the molecules are moving/vibrating in random directions. Molecules, in general, have elastic collisions only, unless they are reacting.

 

When two large objects collide inelastically, their component molecules still collide elastically (more or less), but the now move in random directions unlike if the collision were elastic (they'd move in the same direction and thus be counted as kinetic energy of the object rather than as heat).

 

Thanks Skeptic. That last point I don't think I would have figured out myself

 

EDIT: Wait that also answers one of my other questions I think. The difference between kinetic energy and heat. Is kinetic energy simply energy due to movement in one direction and heat just randon motion of molecules? If an object is moving at constant speed and I want to work out its total energy, do I add heat+kinetic energy or is heat simply its total energy. I think it is heat+kinetic energy. This is just to confirm.

Edited May 23, 2010 by scilearner

[swansont]

Hello everyone,

 

Ok the definition of heat says it is the total kinetic energy due to random motion of molecules. My question is it really the total kinetic energy. For example I know in inelastic collision heat is released. So is thermal energy really heat released in inelastic collison of molecules. Also if there is a bunch of molecules in a container and they all had elastic collisions, would there be heat in the container. Thanks !!

 

Heat is energy being transferred, it is not contained in a sample.

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Heat is the average kinetic energy of the molecules

 

That's temperature.

 

temperature ≠ heat

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The difference between kinetic energy and heat. Is kinetic energy simply energy due to movement in one direction and heat just randon motion of molecules? If an object is moving at constant speed and I want to work out its total energy, do I add heat+kinetic energy or is heat simply its total energy. I think it is heat+kinetic energy. This is just to confirm.

 

Kinetic energy is in the category of internal energy; you can have translational (which is the source of temperature) and also rotational and vibrational, and these additional modes are why the heat capacity of some materials are larger than that of an ideal gas.

 

Generally the KE of the center-of-mass of a system does not contribute to the temperature. You measure it in the CoM of the system, i.e. at rest with the system.

 

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heat.html#c1

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/inteng.html#c2

[scilearner]

Thanks swansoft Anyway I want to clarify again. If an object is moving and someone asks me what is its internal energy. Is it thermal energy+other energies you decribed or thermal energy+other energies+kinetic energy of centre of mass. Kinetic energy of centre of mass is this always less than the thermal energy. Thanks

[swansont]

Thanks swansoft Anyway I want to clarify again. If an object is moving and someone asks me what is its internal energy. Is it thermal energy+other energies you decribed or thermal energy+other energies+kinetic energy of centre of mass. Kinetic energy of centre of mass is this always less than the thermal energy. Thanks

 

The KE of the CoM is not restricted to being less than the thermal motion. In my lab we continually move atoms around at several m/s, but the thermal rms speed is about 1 cm/s

[scilearner]

The KE of the CoM is not restricted to being less than the thermal motion. In my lab we continually move atoms around at several m/s, but the thermal rms speed is about 1 cm/s

 

Ok I see. Thanks