Question

What is the change in the internal energy if 10 J of heat is given to the system at constant pressure and 20 J of work is done by the system?
1. 10 J
2. 30 J
3. -10 J
4. -30 J

Answer 1

First law of thermodynamic tells us about the change in the internal energy of the system. The first law of thermodynamics is a version of the law of conservation energy, adapted for the thermodynamic processes. The law of conservation of energy states that total energy of an isolated system is constant.

Complete Step by step answer:
The first law of thermodynamic states that,
$\Delta U = q + W$
Where $\Delta U$denotes the change in the internal energy of a closed system. And q is the quantity of energy supplied to the system as heat and W denotes the work done by system on the surroundings.
As heat is given to the system therefore,
$q = 10J$
As the work is done by the system so the work done will be negative, therefore
$W = - 20J$
Now as per the first law of thermodynamics, that is
$\Delta U = q + W$
Therefore, change in the internal energy of system, $\Delta U$= (10-20) J
Therefore we get $\Delta U$= -10 J

So the correct option is 3rd.

Note: It is to be noted that if heat is given to the system then it will be positive and if the heat is released from the system then it is negative. If work is done by the system then energy will be negative, and it will be positive if work is done on the system.