The heat given to a thermodynamic system is 200 J and work done by the system is 100 J. Find the change in internal energy of the system.

CONCEPT:

• The first law of thermodynamics gives the relationship between heat, work, and internal energy.
• The first law of thermodynamics applies the conservation of energy principle to systems.
• The first law of thermodynamics states that the change in internal energy of a closed system equals the net heat transfer into the system minus the net work done by the system.

In equation form, the first law of thermodynamics is given by:

Q = ΔU + W

where Q is the sum of all heat transfers into and out of the system, W is the net work done by the system and ΔU is the change in internal energy.

CALCULATION:

Given that:

Heat supplied to the system Q = +200 J

Work done by the system W = +100 J

By the first law of thermodynamics Q = ΔU + W

200 = ΔU + 100

ΔU = 100 J

So the correct answer is option 3.

Important Points 

Sign convention:

• ΔQ = Positive (Heat supplies to the system)
• ΔQ = Negative (Heat rejected from the system)
• ΔW = Positive (Work done by the system)
• ΔW = Negative (Work done on the system).