Question

What is the net increase or decrease in internal energy of the system, when 25J is transferred by heat into the system and 45J work is done by the system?
1. -70 J
2. 70 J
3. -20 J
4. 20 J

Answer 1

Correct Answer - Option 3 : -20 J

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 = +25 J

Work done by the system W = +45 J

By the first law of thermodynamics Q = ΔU + W

25 = ΔU + 45

ΔU = -20 J

So the correct answer is option 3.