Topic 2 Flashcards
Energy and the First Law of Thermodynamics
Kinetic energy is
Scalar Quantity
What is an extensive property?
An extensive property is one that depends on the amount of matter in a system or the size of the system
Why is kinetic energy an extensive property?
it changes in proportion to the amount of matter in a system
What is the difference between a scalar and a vector quantity?
A scalar quantity has only magnitude, but no direction. Vector quantity has both magnitude and direction
Why is potential energy also considered an extensive property?
Because it depends on the amount of matter in the system—in other words, it scales with the size or quantity of the system.
Work is equal to
Force (distance)
Conversation of Energy
states that the total work of all forces acting on the body from the surroundings, with the exception of the gravitational force, equals the sum of the changes in the kinetic and potential energies of the body.
Thermodynamic definition of work
“Work is done by a system on its surroundings if the sole effect
on everything external to the system could have been the
raising of a weight.
W > 0
positive (+)
W < 0:
negative (-)
work done BY the system
positive (+)
work done ON the system
negative (-)
The rate of energy transfer by work
Power
is one in which the departure from thermodynamic equilibrium is at most infinitesimal.
quasiequilibrium process (or quasistatic) process
a change that a gas undergoes under general conditions
Polytropic Process
When n = 0 in a polytropic process
process is an isobaric (constant-pressure) process
When n = +- infinity in a polytropic process
process is an isometric (constant-volume) process
For ideal gas n = 1 in a polytropic process
isothermal (constant-temperature) process
Internal energy is what property
extensive property of the system
amount of energy transferred across the boundary of a system in a heat interaction with the system’s surroundings.
Heat
Q > 0
positive (+)
Q < 0
negative (-)
heat transfer from the system
negative (-)
heat transfer to the system
positive (+)
Work and Heat is a property?
Both are not a property
lowercase delta (δ)
denotes an infinitesimal or differential amount of heat added to or removed from a system. This symbol highlights that heat (𝑄) is a path-dependent quantity, meaning it depends on the specific path taken during a process rather than the end states alone.
units for heat transfer Q and heat transfer rate Q (.)
same as those introduced
previously for W and W (.) respectively.
adiabatic
means without heat transfer.
if a system undergoes a process involving no heat transfer with its surroundings
adiabatic process
Heat Transfer Modes
Three basic transfer mechanisms:
1. Conduction
2. Convection
3. Thermal radiation
Energy transfer can take place in solids, liquids, and gases.
Conduction
Transfer of energy from the more energetic particles of a substance to adjacent particles that are less energetic due to interactions between particles.
Conduction
rate of heat transfer across
any plane normal to the x direction, Qx, is proportional to the wall area, A, and the temperature gradient in the x direction, dT/dx:
Fourier’s law
the negative gradient of temperature and the time rate of heat transfer is proportional to the area at right angles of that gradient through which the heat flows
Fourier’s law
Fourier’s law is also known as
law of heat conduction
Substances with large values of thermal conductivity
good conductors
Substances with small values of thermal conductivity
good insulators
is emitted by matter as a result of changes in the electronic configurations of the atoms or molecules within it.
Thermal radiation
The energy is transported by
electromagnetic waves (or photons)
requires no intervening medium
thermal radiation
Stefan–Boltzmann law
shows that the thermal radiation is associated with the fourth power of the absolute temperature of the surface, Tb
Stefan–Boltzmann law states that
the magnitude of the radiation output of an object is directly proportional to the fourth power of its surface temperature
property of the surface that indicates how effectively the surface radiates (0≤ ε ≤ 1.0)
Emissivity (ε)
Stefan– Boltzmann constant (σ )
5.67 x 10^-8 W/m^2 K^4
Stefan– Boltzmann constant (σ )
0.1714 x 10^-8 Btu/h . ft^2°R^4
energy transfer between a solid surface
Convection
adjacent gas or liquid at another temperature
Convection
heat transfer coefficient
not a thermodynamic property
Similarities of Heat and Work
- both transient phenomena
- Systems never possess heat or work, but either or both cross
- boundary phenomena
- Both are observed only at the boundary of the system, and both represent energy crossing the boundary
- Both heat and work are path functions and inexact differentials.
What is Transient
the evaluation of how a system responds to fixed and varying boundary conditions over time
APPLICATION of the conservation of energy principle to heat and thermodynamic
First law of themodynamics
The change in internal energy of a system is equal to the heat added to the system minus the work done by the system
Energy Balance for Closed Systems
Energy balance in differential form
dE = δQ (.) - W (.)
The instantaneous time rate form of the energy
balance is
dE/dt = Q (.) - W (.)
Rate form of the energy balance
Time rate of change of the energy contained within the system at time t
=
Net rate at which energy is being transferred in by heat transfer at time t
-
Net rate at which energy is being transferred out by work at time t
