If I had a hot cup of tea and I added to it cold milk, what would happen is that tea will lose some of its thermal energy to the milk. But why do we notice that our liquid is no longer as hot as before adding milk? After all, the total thermal energy of the system (the cup) is conserved even if there has been a transfer of heat between the two. Suppose the cup is an isolated system.
[Physics] Thermal energy of a system
energy-conservationtemperaturethermodynamics
Best Answer
Before mixing the average kinetic energy of the molecules which make up the tea is greater than the average kinetic energy of the molecules which make up the milk.
This is restatement of the fact that the tea is hotter than the milk as the temperature of a substance depends on the average kinetic energy of the molecules which make up the substance.
When you mix the tea and the milk together the molecules of the tea and the milk collide and eventually reach an average kinetic energy (temperature) which is less than the average kinetic energy (temperature) of the tea before mixing and is greater than the average kinetic energy (temperature) of the milk before mixing.
So the collisions between the molecules redistribute the kinetic energy amongst the molecules and what is called thermal equilibrium is reached.
If there is no loss of heat or chemical reaction then the kinetic energy of the hot tea plus the kinetic energy of the cold milk is equal to the kinetic energy of the mixture but on average the tea molecules have less kinetic energy after mixing and the milk molecules have more kinetic energy after mixing.