The Laws of Thermodynamics deals with entropy, temperature, pressure energy etc of a system. There are four Laws of Thermodynamics which include Three Laws of Thermodynamics and a fourth law which is zeroth law.
Zeroth Law of Thermodynamics says that if there are three thermodynamic systems such that two systems are in equilibrium with the third system then those two systems will also be in equilibrium with each other. For example if T1 system is in equilibrium with T3 system and also T2 system is in equilibrium with T3 system then the systems T1 and T2 will be in equilibrium with each other.
First and Second Laws of Thermodynamics :First law of thermodynamcs: This states that energy cannot be created, neither can it be destroyed. Energy can change its form from one form to another as said by law of conservation of energy. it also says that internal energy of a system increases when heat is given to it to raise its temperature. Or we can say that when temperature of a system rises, its internal energy increases.
Internal energy is the sum of kinetic energy associated with the movement of the particles inside the system and the potential energy associated due to bond between the particles. Also the law says that the forms of energy transfer are heat and work done. For example heat energy is transferred from a body at higher temperature to a body at lower temperature; when we lift a object to a certain height by doing work on it, we transfer energy to the object which is potential energy in this case.Internal energy, heat and work are related to each other by this law.
It says that the change in internal energy (U) of a system is equal to the heat given(Q) to the system minus work done(W) by the system:Second law says that when two isolated systems say A, B (not in equilibrium with other) are brought in contact with each other such that they can exchange matter or energy, they will reach to equilibrium after sometime by exchanging. The sum of the entropy of the two isolated systems (A, B) will be less than or equal to the entropy of the final system (combining fist two systems).So, during the process of exchanging and reaching to equilibrium the entropy increases or remains same but never decreases.Third law says that entropy of a system reaches to a constant as temperature moves towards zero. This means that processes or systems ceases as the temperature becomes absolute zero.