Objectives
• Discuss the nature of internal energy.
• Define the concept of heat and three mechanisms of heat transfer: conduction, convection, and radiation.
• Define the concept of work, including electrical work and several forms of mechanical work.
• Introduce the first law of thermodynamics, energy balances, and mechanisms of energy transfer to or from a system.
• Define energy conversion efficiencies.
• Implications of energy conversion on the environment
Introduction
The first law of thermodynamics: energy can be neither created nor destroyed during a process; it can only change forms.
Energy can transfer between a system and surroundings
*the total energy (E) of a system constitutes all the forms of energy inside a system (Internal energy + mechanical energy)
*Thermodynamics deals only with the change of the total energy.
Forms Of Energy
• Energy can exist in numerous forms:
Internal energy, U, (thermal energy, chemical and nuclear energy),
i. Kinetic energy, KE: The energy that a system possesses as a result of its motion relative to some reference frame
ii. Potential energy, PE: The energy that a system possesses as a result of its elevation in a gravitational
iii. Flow energy, Ė: The energy that a system possesses as a result of its
motion due to pressure difference
Physical Insight to Internal Energy
Sensible energy: The portion of the internal energy
of a system associated with the kinetic energies of
the molecules.
Latent energy: The internal energy associated with
the phase of a system.
The various forms of microscopic energies that make up sensible energy
*sensible and latent energy*
Energy Transfer By Work
The energy transfer associated with a force acting through a distance.
A rising piston, a rotating shaft, and an electric wire crossing the system boundaries are all associated with work interactions
Formal sign convention: Heat transfer to a system and work done by a system are positive; heat transfer from a system and work done on a system are negative.
Alternative to sign convention is to use the subscripts in and out to indicate direction.
*energy transfer by work*
Sign convention and units
- Known as the conservation of energy principle
- States that energy can be neither created nor destroyed during a process, it can only change forms.
ENERGY CHANGE OF A SYSTEM
The net change in the total energy of the system during a process is equal to the difference between the total energy entering and the total energy leaving the system during that process.
KE & PE : macroscopic form of energy ( those a system possesses as a whole with respect to some outside reference frame)
U : sum of all the microscopic form of energy ( those related to a molecular structure of a system and the degree of the molecular activity)
Energy Conversion Efficiency
- It indicates how well an energy conversion or transfer process is accomplished.
- It indicates how well an energy conversion or transfer process is accomplished.
