The First Law
The 1st law of thermodynamics states that internal energy change of a system equals net heat transfer minus net work done by the system.
Learning Objectives
Explain how the net heat transferred and net work done in a system relate to the first law of thermodynamics
Key Takeaways
Key Points
- The first law of thermodynamics is a version of the law of conservation of energy, specialized for thermodynamical systems.
- In equation form, the first law of thermodynamics is [latex]\Delta \text{U} = \text{Q} - \text{W}[/latex].
- Heat engines are a good example of the application of the 1st law; heat transfer into them takes place so that they can do work.
Key Terms
- internal energy: The sum of all energy present in the system, including kinetic and potential energy; equivalently, the energy needed to create a system, excluding the energy necessary to displace its surroundings.
- heat: energy transferred from one body to another by thermal interactions
- law of conservation of energy: The law stating that the total amount of energy in any isolated system remains constant, and cannot be created or destroyed, although it may change forms.
The first law of thermodynamics is a version of the law of conservation of energy specialized for thermodynamic systems. It is usually formulated by stating that the change in the internal energy of a closed system is equal to the amount of heat supplied to the system, minus the amount of work done by the system on its surroundings. The law of conservation of energy can be stated like this: The energy of an isolated system is constant.
If we are interested in how heat transfer is converted into work, then the conservation of energy principle is important. The first law of thermodynamics applies the conservation of energy principle to systems where heat transfer and doing work are the methods of transferring energy into and out of the system. In equation form, the first law of thermodynamics is
Internal Energy: The first law of thermodynamics is the conservation-of-energy principle stated for a system where heat and work are the methods of transferring energy for a system in thermal equilibrium. Q represents the net heat transfer—it is the sum of all heat transfers into and out of the system. Q is positive for net heat transfer into the system. W is the total work done on and by the system. W is positive when more work is done by the system than on it. The change in the internal energy of the system, ΔU, is related to heat and work by the first law of thermodynamics, ΔU=Q−W.
[latex]\Delta \text{U} = \text{Q} -\text{W}[/latex].
Here ΔU is the change in internal energy U of the system, Q is the net heat transferred into the system, and W is the net work done by the system. We use the following sign conventions: if Q is positive, then there is a net heat transfer into the system; if W is positive, then there is net work done by the system. So positive Q adds energy to the system and positive W takes energy from the system. Thus ΔU=Q−W. Note also that if more heat transfer into the system occurs than work done, the difference is stored as internal energy. Heat engines are a good example of this—heat transfer into them takes place so that they can do work.
