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Welcome to our Physics lesson on The First Law of Thermodynamics, this is the fifth lesson of our suite of physics lessons covering the topic of The First Law of Thermodynamics, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
We stated earlier that the two ways to transfer energy to or from a system are doing work and exchanging heat. This statement forms the base of the First Law of Thermodynamics. It says:
The increase in internal energy of a thermodynamic system is equal to the heat added to the system plus the work done on the system.
In symbols, we have:
This equation represents the law of energy conservation in its simplest form. In many cases, we are interested on the work done by the system. Hence, we can write:
or
The last formula is interpreted as:
"The heat supplied to a thermodynamic system, partly goes for the increase in the internal energy of the system and partly for work done by the system on the surroundings."
This interpretation is much easier to understand than the original formulation of the First Law of Thermodynamics. For example, if we give 1000 J of energy to a system in the form of heat and the internal energy of system increases by 400 J, the remaining amount of 600 J is used by the system to do work on the surroundings.
All terms in the expression can be positive or negative. Consider the gas sample above as the thermodynamic system to be studied:
ΔU is positive if the internal energy increases. (Temperature increases).
ΔU is negative if the internal energy decreases. (Temperature decreases).
Q is positive, if heat is added to the system.
Q is negative, if heat flows out of the system.
Wby gas is positive, if work is done by the gas to lift the piston.
Wby gas is negative, if work is done on the gas, decreasing thus the volume.
The internal energy of a gas sample increases by 800 J as it gives out 400 J of heat to the surroundings. Find the work done by the gas.
We have the following clues:
ΔU = + 800 J as the internal energy increases.
Q = -400 J as the heat is given off by the system to the surroundings
Wby gas = ?
From the last version of the First Law of Thermodynamics, we have:
Substituting the above values, we obtain:
Since the result is negative it means that 1200 J of work is done by the surroundings to the gas. From this amount, 800 J have contributed in the increase of the gas internal energy while the remaining amount of 400 J are delivered in the form of heat by the gas to the surroundings.
You have reached the end of Physics lesson 13.5.5 The First Law of Thermodynamics. There are 6 lessons in this physics tutorial covering The First Law of Thermodynamics, you can access all the lessons from this tutorial below.
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