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.