Energy

Forms of Energy:

• Potential - PE = mgz (mass*gravitational acceleration*vertical distance)
  
• Kinetic - KE = .5mV^2 (.5*mass*velocity squared)
  
• Thermal
• Mechanical
• Electrical
• Magnetic
• Chemical
• Nuclear

Total Energy equals the sum of internal energy + potential energy + kinetic energy.
E = U + PE + KE

Energy flow rate (time rate of energy) is equal to the mass flow rate multiplied by the mechanical energy per unit mass.
E(dot) = m(dot) * e

• m(dot) is the mass flow rate. m(dot) = (rho) * V(dot) where V(dot) is the volume flow rate
• e is the mechanical energy per unit mass. e = (P/rho) + (V^2 / 2) + (gz) where P is pressure, rho is density, V is velocity, g is gravitational acceleration, and z is vertical distance.

For a closed system energy can only cross the boundary in 2 ways:

1. Heat: temperature difference.
   
2. Work: a force that acts on the system through a distance. Forms of work:

• Shaft Work = 2(pi)nT where n = revolutions, and T = torque
• Spring Work = .5k(x2^2 - x1^1) where k = spring constant, x is the final and initial displacements of the spring.
• Electrical Work = VIt where V = volts, I = current, and t = time.

For Open systems energy can be transferred via mass flow.


Energy Balance (First Law of Thermodynamics): the net energy transfer to/from the system is equal to the change in energy for that system. Ein - Eout = Echange

Efficiency = output / input
example:

• Lighting Efficiency is the amount of light output per watts of electricity used.

Basics

Thermodynamics studies energy

First Law of Thermodynamics: Conservation of Energy, which states that energy can't be created or destroyed. Energy can only change form. The total amount of energy is constant.

Primary Dimensions:

• Mass (m, kilogram)
  
• Length (L, meter)
  
• Time (t, seconds)
  
• Temperature (T, kelvin)
• Electric Current (A, ampere)
  
• Amount of Light (cd, candela)
  
• Amount of Matter (mol, mole)
  

Secondary Dimensions: Formed by combining primary dimensions.

• Velocity
• Energy
• Volume
• Specific Weight
• Specific Gravity
• Specific Energy
• Specific Internal Energy
• Specific Enthalpy
• Density
• etc.

Force = Mass x Acceleration
The Force needed to accelerate an object of 1 kilogram at a rate of 1 meter per second squared.

• Force: measured in Newtons(N) or Pound-Force(lbf)

Mass and weight are often confused. Weight equals mass multiplied by the gravitational acceleration (W = mg) and is a force. The gravitational acceleration (at sea level) is 9.81 meters per second squared, or in english units it's 32.2 feet per second squared.


Specific Weight (symbol is gamma): This is the weight of a unit of volume. This equals the density multiplied by the gravitational acceleration.

Work: A type of energy, with units of newton(N) multiplied by meter(m) which equals a joule (J) from the units you can see that work equals a force over a distance.

Watt (W): This is the time rate of work(energy) and has units of watts. A watt is equal to a joule per second (W = J/s). This is commonly known as power.

System: an area of study surrounded by a boundary. Three Types:

• Closed/Control Mass: Mass can't cross the boundary, so it is constant. However, energy may cross the boundary such as heat or work. The volume can change.
• Open/Control Volume: A steady flow process where mass and energy can cross the boundary, called mass flow. The volume is fixed.
• Isolated: Mass and energy can't cross the boundary.
  

Boundary: real/imaginary line that separates the system and surroundings.
Surroundings: anything outside of the system.
Property: a characteristic of a system. Two types:

• Intensive: independent of the mass in the system such as temperature, density, and pressure. Generally lowercase letters are used except for temperature(T) and pressure(P).
  
• Extensive: properties that depend on the size of the system such as mass and volume. Uppercase letters are used except for mass(m).

Specific properties: extensive properties per unit mass.
Density: mass per unit volume. Temperature and pressure can affect density.
State: condition of a system. Defined by two independent intensive properties.
Equilibrium: balanced state, no changes.
Thermal Equilibrium: constant temperature throughout the system.
Mechanical Equilibrium: constant pressure.
Phase Equilibrium: constant mass.
Chemical Equilibrium: constant chemical composition.
Quasi-Equilibrium: slow process where the system remains close to equilibrium.
Simple compressible system: no effects from electrical, magnetic, gravitational, motion, and surface tension.
Path: series of states.
Process: change between equilibrium states. Several types:

• Isothermal: constant temperature.
• Isobaric: constant pressure.
• Isochoric/Isometric: constant specific volume.

Cycle: a system returning to the initial state after the final state.
Steady: no change over time.
Uniform: no change over a region.

Zeroth Law of Thermodynamics: if two separate objects in thermal equilibrium with another object, then the original two objects are in thermal equilibrium with one another.

Temperature:

• Kelvin (K) T(K) = T(Celsius) + 273
  
• Rankine (R) T(R) = T(Fahrenheit) + 460 and T(R) = 1.8 T(K)
• Celsius (degree C)
  
• Fahrenheit (degree F) and T(Fahrenheit) = 1.8 T(Celsius) +32

Pressure: force per unit area. Units - Pascal(Pa) = one newton per meter squared.
Absolute Pressure: actual pressure.
Gauge Pressure: the difference between absolute and local atmospheric pressures, many devices read zero in the atmosphere. Pgauge = Pabs - Patm
Vacuum Pressure: below atmospheric pressure. Pvac = Patm - Pabs

Pressure does not change horizontally when in a fluid at rest. Pressure changes with vertical distance. The pressure between two points in a constant density fluid equals density multiplied by the gravitational acceleration multiplied by the vertical distance between the two points.
P = density x gravitational acceleration x vertical distance

Standard atmospheric pressure is 760mmHg at 0 (degrees C), 760 torr, or 1 atm.



More engineering topics here