Energy Concepts in Physics

Kinetic Energy (KE)

• A scalar quantity
• The energy associated with the motion of an object.

Potential Energy (PE)

• Energy stored within a system due to its position or configuration.

Work

• The force applied to an object multiplied by the distance the object travels in the direction of the force.
• Work is not a property of the system or the surroundings, but rather a measure of energy transfer between them.

Power

• The rate at which work is done, or the rate at which energy is transferred.
• Power is denoted by "W" with a dot above it.
• Energy and work share the same units.

Conservation of Energy

• Energy cannot be created or destroyed, only transferred or transformed from one form to another.
• The total energy of a closed system remains constant.

Quasiequilibrium Process (or Quasistatic) Process

• A process that occurs slowly enough that the system is always in (or very close to) thermodynamic equilibrium.
• The departure from thermodynamic equilibrium is infinitesimal.

Polytropic process

• A quasiequilibrium process that can be described by the equation PV^n = constant, where 'n' is a constant.

Change in the total energy of a system

• The change in the total energy of a system is equal to the heat added to the system minus the work done by the system.

Heat Flux (q)

• The heat transfer rate per unit of system surface area.
• Represented by the symbol 'q'.
• Units for heat flux: J/(m^2 * s).

Internal Energy (U)

• An extensive property of the system that represents the sum of the kinetic and potential energies of all the molecules within the system.
• Internal energy does not include energy associated with the system's motion or position as a whole.

Energy Transfer by Heat

• Energy transfer due to a temperature difference between the system and its surroundings.
• No work is done in this process.

Adiabatic Process

• A process that occurs without heat transfer.

Heat (Q)

• The amount of energy transferred across the boundary of a system in a heat interaction with the system's surroundings.
• Heat is not a property of the system, but rather a measure of energy transfer.
• It is an empirical parameter incorporating the flow pattern, fluid properties, and geometry.

Net rate of heat transfer (Q)

• The rate at which heat is transferred across a boundary.

Conduction

• Energy transfer through direct contact between molecules.
• Occurs in solids, liquids, and gases.

Convection

• Energy transfer through the movement of fluids (liquids or gases).

Thermal Radiation

• Energy transfer through electromagnetic waves.
• Radiation does not require a medium to propagate.

Fourier’s Law

• States that the rate of heat transfer across a plane is proportional to the area of the plane and the temperature gradient across the plane.

First Law of Thermodynamics

• A statement of the conservation of energy principle as it applies to thermodynamic processes.

Energy Balance

• A statement of the conservation of energy principle as it applies to a system.
• The change in internal energy of a system is equal to the heat added to the system minus the work done by the system.

Symbols

• ΔU: The change in internal energy
• Q: Heat added to the system
• W: Work done by the system

Differential Form

• dU = dQ - dW

Instantaneous Time Rate Form

• dU/dt = dQ/dt - dW/dt

Rate Form

• ΔU/Δt = Q/Δt - W/Δt