Thermo Lecture 4
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Energy boundary crossing
- closed system - form of heat or work
- open system - (control surface) heat, work, and mass transfer
1st Law of Thermo / conservation of energy principle
- (total energy entinering the system) - (total energy leaving the system) = (change in total energy of the system)
- or delta (E = U + KE + PE)
- E = U + KE + PE
- internal + kinetic + potential
U - dependent on the state and mass of the system
- System not moving with velocity/changing elevation
- delta E = delta U
Q - caused by temperature difference between system and surroundings. Heat transfer into system is energy increase, out of system is decrease. Adiabatic system, Q = 0.
W - energy transfer at a system boundary, the force acting through a distance or energy to lift a weight. Transfered into system energy increases, out of system energy decreases.
m - mass flowing into system, energy increases by the amount of energy carried with the mass. m = 0 for closed systems.
Heat and Work similarities
- 1. Boundary phenomena
- 2. systems posses energy, not heat/work
- 3. Associated with process, not state
- 4. path function
Total heat transfer/work
- Calculated by following the process path and adding differential amounts along the way
- δQ = Q12 or δW = W12
classical sign convention
- positive - heat transfer to a system/work done by a system
- negative - heat transfer from a system/done done on a system
heat transfer in progressive exchange of energy btwn the molecules of a substance
Fourier's law of heat conduction
- k - thermal conductivity [W/mK]
- dT/dx - temp gradient, direction of heat flow
heat transfer of the energy transfer btwn a solid surface and adj liquid or gas in motion, and involves combined effects of conduction and fluid motion.
Newton's law of cooling
- h - convection heat transfer coe [W/m2K]
- Ts - surface temp, Tf - bulk fluid temp
Radiative Heat Transfer
energy in transition from the surface of one body to the surface of another due to electromagnetic radiation.
Radiative Heat Transfer Eq.
- sigma - Stefan-Boltzmann constant, 5.57x10-8 W/m2K4
- epsilon - emissivity
- rate of electrical work done by electrons crossing a system boundary
- P = VI
Mechanical vs. thermo work
- force of a distance vs. energy to lift weight
- angle bten force and displacement vector
- delta means it is path dependent
Types of Mechanical Work
shaft, spring, elastic solid bars, stretching liquid film, raise/accelerate a body
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