The First Law of Thermodynamics is the engineering statement of energy conservation. It says energy cannot be created or destroyed, only transferred or transformed between forms such as heat, work, kinetic energy, potential energy, and internal energy. Engineers use it to analyze engines, turbines, compressors, refrigerators, pumps, nozzles, and heat exchangers.
A thermodynamic system diagram helps track every energy interaction crossing a boundary.
Understanding Engineering: First Law of Thermodynamics
A useful first step is choosing the system carefully. A system can be a sealed gas cylinder, the water inside a kettle, or the working fluid moving through a turbine. Everything outside is the surroundings.
The boundary is real or imaginary, but it must be drawn clearly. Energy crosses that boundary in different ways. Heat transfer occurs because of a temperature difference.
Work transfer occurs when a force causes motion, such as a piston moving or a shaft turning. Mass flow carries energy with it when material enters or leaves an open device.
Internal energy is often the hardest part to picture because it is stored in microscopic motion and interactions. Molecules translate, rotate, vibrate, and attract one another. Heating a gas usually raises this internal energy.
Compressing a gas can raise it too, even if little heat enters. This explains why a bicycle pump becomes warm when used quickly. The work done on the trapped air increases its internal energy.
Temperature is related to internal energy, but they are not identical. During melting or boiling, energy can enter while temperature stays constant because the molecular arrangement is changing.
Engineers must use a consistent sign convention. In one common convention, heat entering a closed system is positive and work done by the system is positive. A gas that expands and pushes a piston delivers work outward.
If the gas is compressed, work enters the system. The energy balance then tells whether internal energy, speed, height, or a combination of these must change. Units matter throughout.
Energy is measured in joules or kilojoules. Power is the rate of energy transfer and is measured in watts or kilowatts. Mixing energy with power is a common mistake in calculations.
For devices with flowing fluids, enthalpy becomes especially useful. It combines internal energy with the energy needed to push fluid across a boundary. Steam entering a turbine has high enthalpy.
As it expands, its enthalpy often falls while the turbine shaft produces useful power. A compressor works in the opposite direction. Electrical power drives a shaft, which raises the energy of the flowing gas.
In a nozzle, a drop in fluid enthalpy can become a large increase in speed. In a heat exchanger, one stream loses energy while another gains it, with little or no shaft work.
Good problem solving starts with a simple sketch. Mark each inlet, outlet, heat interaction, and work interaction. State whether the process is steady or changing with time.
Decide whether changes in speed and height are important. They matter for jets, waterfalls, turbines, pumps, and tall buildings, but may be tiny in a tabletop heating experiment. Finally, check whether the answer makes physical sense.
A refrigerator needs work input to move heat from a cold space to a warmer room. An engine cannot produce shaft work without energy arriving from fuel, hot gas, or another source. The balance exposes missing energy paths and unrealistic assumptions.
Key Facts
- First law for a closed system: ΔE = Q - W
- Total energy of a system: E = U + KE + PE
- Change in total energy: ΔE = ΔU + ΔKE + ΔPE
- Kinetic energy: KE = 1/2 mv^2
- Potential energy: PE = mgz
- Steady-flow control volume balance: Qdot - Wdot + Σ mdot(h + v^2/2 + gz)in - Σ mdot(h + v^2/2 + gz)out = 0
Vocabulary
- Thermodynamic system
- A thermodynamic system is the matter or region selected for energy analysis.
- Boundary
- A boundary is the real or imaginary surface that separates a system from its surroundings.
- Internal energy
- Internal energy is the microscopic energy stored in a substance due to molecular motion, vibration, rotation, and intermolecular forces.
- Heat
- Heat is energy transferred across a boundary because of a temperature difference.
- Work
- Work is energy transferred across a boundary by a force acting through a distance or by other organized effects such as shaft rotation or electrical current.
Common Mistakes to Avoid
- Using the wrong sign for work: in the common engineering convention, W is positive when work is done by the system, so compression work on a gas is negative in ΔE = Q - W.
- Treating heat as a property stored in a system: heat is energy in transit across a boundary, while internal energy is stored in the system.
- Ignoring kinetic or potential energy changes without checking: they may be small in tanks but important in nozzles, turbines, pumps, and elevation changes.
- Applying a closed-system equation to an open system: mass crossing a control volume carries enthalpy, kinetic energy, and potential energy, so a flow energy balance is required.
Practice Questions
- 1 A closed piston-cylinder contains a gas. It receives 500 J of heat and does 180 J of work on the surroundings. If changes in kinetic and potential energy are negligible, what is ΔU?
- 2 A steady-flow turbine has one inlet and one outlet. Heat loss is 12 kW, shaft work output is 85 kW, mass flow rate is 2 kg/s, and kinetic and potential energy changes are negligible. What is the decrease in specific enthalpy from inlet to outlet?
- 3 A rigid sealed tank is heated with an electric resistor inside it. No mass crosses the boundary and the tank volume does not change. Explain which energy transfers occur and how the internal energy changes.