Internal Energy

Understanding Internal Energy(I.E). We know that material may exist in three states: solids, liquids, and gases. Remember that the particles in a solid do not move from place to place but they do vibrate. Particles in a liquid can move around each other and particles in a gas move extremely rapidly. So, it means that all of these particles have got kinetic energy. The force that keeps the particles together is intermolecular forces. If there are molecules (more than one group of atoms) then they must contain chemical bonds together. This bond gives potential energy to the particles.

The internal energy is the energy stored in a system by the particles. It is the total kinetic energy and potential energy of all the particles and that includes atoms and molecules that make up a system

I.E is really important during changes of state. If we test solid and increase the I.E. At some point, the solid can turn into a liquid and we call this as melting. If we continue to heat to increase this energy and at some point, the liquid will turn to a gas. We call this process as boiling.

Now, if we cool the gas down again, then we would use the internal energy. At some point, the gas turned back to liquid and we call that condensation. If we cool further the liquid down further, then we would use the energy even more. Eventually, the liquid turns to a solid and we call that freezing now.

What is Internal Energy?

It is the energy of a gas, simply the sum of all the different types of energies of all the molecules found in our system. It Includes the vibrational energies, the translational kinetic energies, the rotational kinetic energies and all the types of energies.

let’s say that we have the closed container and inside this closed container we have n number of molecules where each molecule is a monatomic molecule (means it’s composed of a single atom).

Because we’re taking the example of monatomic ideal gas molecules that implies that each molecule only has translational kinetic energies. So, the I.E of our system is equal to the sum of all the translational kinetic energies of all the molecules found in our system.

I.E Translational  = K1+K2+ K3+……..+Kn

Change in Internal Energy

It is represented by an uppercase letter U. This is the energy associated with atomic motion and it is a quantity that is directly proportional to the temperature. Higher temperatures mean more internal energy, lower temperatures mean less I.E. In this way, we must understand that a substance does not contain heat, it contains I.E, like the kinetic energy of all the particles, and it is the transfer of this energy from areas of high temperature to areas of low temperature that we can call sometimes as heat.

• ΔU = +ve, if the I.E of the system increases.
• ΔU = -ve if the I.E of the system decreases.
• I.E depends on temperature, pressure, volume and quantity of matter.

Internal Energy Equation

When a system is supplied with the heat. This heat energy gets used up in increasing the I.E of the system and also in doing some work. It has applications in real life. Consider a gas confined by a piston in a cylinder and the heat is provided to gas particles by an external source. This results in the pushback of piston and does some work. Hence, the heat we are supplying is getting used up in increasing the I.E of the gas and a part in moving the piston.

• ∆Q = heat gain by gas particles from the surroundings.
• ∆W = Work done(pushback of piston) by the system.
• ∆U = Change in I.E.

∆Q = ∆W +∆U

Must Read: Ist Law of Thermodynamics.

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