Answer 1) For calculating ΔU;
We can use the expression for the first law of thermodynamics which is,
ΔU = Q + W
Where ΔU is the Change in internal energy, Q is heat added to the system and W is the work done on the system.
As, Q = + 51 KJ and W = -15 KJ.
So, ΔU = 51 -15 = 36 KJ.
Hence, ΔU = 36 KJ
Answer 2) Using the same first law of thermodynamic equation for calculating ΔU (Internal Energy o the system) ;
ΔU = Q + W;
Given, Q = + 100 KJ and W = - 65 KJ
On substituting the values in the above equation we get,
ΔU = 100 - 65 = 35 KJ
Therefore, ΔU = 35 KJ
Answer 3) Using the same first law of thermodynamic equation for calculating ΔU (Internal Energy o the system) ;
ΔU = Q + W;
Given here, Q = - 65 KJ and W = - 20 KJ
On substituting the above values in the equation we get,
ΔU = -65 -20 = - 85 KJ
Therefore, ΔU = - 85 KJ for this system.
Answer 4) According to the answers in 1), 2) and 3) it can be seen that there has been change in the internal energies of the system for all, but some are positive changes and some are negative.
As per the definition of the internal energy, when the system does work on its surroundings, energy is found to be lost, and E is observed to be negative; and when the surroundings do work on the system, the internal energy of the system becomes larger, converting E into positive.
So, it is clear that in case 3) the system did work on the surroundings as E value is found to be negative, rest have positive E values.
