Sunday, February 3, 2008

JEE Revision - Internal energy, work and heat

every substance possesses definite quantity of energy which depends upon factors such a chemical nature of the substance, temperature and pressure. This is known as internal energy or intrinsic energy and is represented by the symbol U.

Internal energy is made up of kinetic energy and potential energy of the constituent particles. The kinetic energy arises due to motion of its particles and includes their translational motion energy, rorational motion energy, and vibrational motion energy etc. We studied various potential energies in physics. The total potential energy of a substance arises from different interactions between the particles due to gravitation, coulomb forces, magentic forces, and nuclear forces.

It is important to note that different substances have different internal energies even at the same temperature and volume. This fact is important when we talk of internal energy of reactants and internal energy of products being different at constant temperature and volume. Internal energy of a substance depends upon the nature of constituting atoms and bonds, apart from temperature and volume.

while internal energy cannot be measured, change in internal energy during a process may identified.

ΔU = Up - Ur

ΔU = change in internal energy of a chemical reaction
Up = internal energy of products
Ur = internal energy of reactants

If internal energy of products is more than the internal energy of reactants, the reaction requires input of energy in the form of heat (endothermic reaction).

If internal energy of products is less e than the internal energy of reactants, the reaction gives out energy in the form of heat (exothermic reaction).

U = q + w

Internal energy of matter is equal to kinetic energy and potential energy.

The change in internal energy is equal to heat transferred and work done between the system and the surroundings.

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