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Thermal Energy

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Samantha Caramico

on 8 January 2014

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Transcript of Thermal Energy

Thermal Energy
Temperature and Heat
-Heat Transfer
-Thermal Expansion
Ideal Gases
Equal to Boltzmann's Constant times Avogadro's number
Boltzmann's Constant=1.38x10^23 J/K
Avogadro's number=the number of basic particles per mole of a substance NA=6.0221415 × 1023


Atomic Model of Gas
PV Diagrams
Represents ideal gas processes
Each point represents a single unique state of gas
Constant Volume, Vf=Vi
Represented as a vertical line
Isobaric Processes= constant pressure, Pf=Pi
Represented as a horizontal line
Isothermal Processes= constant temperature, Tf=Ti
Represented as a hyperbola due to the inverse relationship between pressure and volume
Adiabatic=an instant change in pressure with no time to change heat
Laws of Thermodynamics
First Law and Second Law
Entropy
The Carnot Cycle
Amanda Hymer, Katie Darrah, and Samantha Caramico.
Processes
Ideal gas- gas where all collisions between atoms and molecules are perfectly elastic
Ideal gas law- PV=nRT
P=pressure
V=volume
n=number of moles
R=ideal gas constant (8.314 J·K−1·mol−1)
T= temperature in kelvins

Ideal Gas Constant
Thermal Energy is equal to the total kinetic energy of the moving atoms in gas
Temperature is a measure of average kinetic energy
measured in kelvins
T(K)=T(C)+273
Kavg=(3/2)KBT
KB=Boltzmann's Constant 1.38x10^23 J/K
Thermal energy of an ideal gas with N atoms is equal to the sum of kinetic energy
Eth=NKavg=(3/2)NKBT
Thermal Energy id directly proportional to temperature
Energy of Ideal Gas
Kinetic Theory= a theory that describes physical properties in terms of motion in atoms and molecules, such as pressure and volume
Molecules in a sample have different speeds
The average speed is called root mean square (rms)
Vrms=√(3Kb/T)
Ideal Gas Processes
Properties:
Quantity is fixed
Defined initial state of pressure, volume and temperature
Defined final state of pressure, volume and temperature
Number of moles and molecules does not change in a sealed container
(PiVi)/Tf=(PfVf)/Tf
First Law: when only the thermal energy changes
Eth= W + Q
Eth= thermal energy; W= work (J); Q= heat (J)
Second Law: the entropy of an isolated system always increases until equilibrium is reached
heat energy spontaneously flows only from hot to cold
the energy transformation is irreversible
no heat engine can be 100% efficient
-Mechanical Equivalent of Heat.

-Methods of Heat Transfer
-occurs when energy is transferred into or out of something, caused by differences in temperature.
A measure of the degree of disorder in a system, and this always increases because no system is perfect
Isovolumetric: a gas process with an unchanging volume. So W=0 and V=0
Eth=Q
Isobaric: a gas process that occurs with an unchanging pressure. So p=0 and W= -p V
Eth= Q + (-p V)
Isothermal: a gas process that occurs with an unchanging temperature. So T=0 and Eth=0
Q=-W
Adiabatic: a gas process that occurs when heat is not transferred. So Q=0 in this case.
Eth= W
-the amount of heat energy depends on three quantities
-change in temperature: T
-mass: m
-heat capacity: c
Q=cm T


H=
Q
L
___
kA(T -T )
_________
2
1
L
=
=
kA T
_______
L
-the rate at which heat conducts depends on:
-temperature difference: T
-cross section area heat is flowing through: A
-thermal conductivity: k
-distance: L
-energy passes from heat to cold
-Conduction: substances in direct contact with one another
*more conduction= more rapid heat transferred
*substance heated, particles gain more energy
-Convection: warmer areas of a liquid or gas rise to cooler areas
*continuous circular pattern
-Radiation: does not rely upon contact between the heat source and the heated object
*infrared radiation: heat transmitted through an empty space by thermal radiation
-electromagnetic radiation
-no mass exchanged

-most substances change in size once applied to a change in temperature
*increase in temperature= expansion
*decrease in temperature= contracts
-temperature is the measure of how much internal energy a substance contains.

-Example: With an increase in heat, the metal plate will increase. The hole in the center will expand instead of getting smaller.
Heat Engine
takes thermal energy and converts it to other forms
efficiency is described as how much useful work is output for a given amount of heat energy input
emax=1- (Tc/Th)
The Carnot Cycle is the most efficient engine possible
Formula: J=W/Q
-Amount of work required to raise the temperature of a substance of unit mass by one degree Kelvin.
-Constant through nature
Example:
An iron ball is dropped onto the pavement from a height of 100 m. If half of the heat generated goes into warming the ball, find the temperature increase of the ball. (The specific heat capacity of iron is 450J/((kg)(degrees Celsius))
Q=
mgh
2
_____
; Q=cm T
_____
mgh
2
=cm T
T
=
gh
___
2c
=
______________
(9.81m/s )(100m)
2

2(450J/((kg)(C )
.
=1.10 C
.
The Physics Hedgehog
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