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LPH 105 W15 14:intro

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Richard Datwyler

on 8 July 2016

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Transcript of LPH 105 W15 14:intro

HEAT
Heat is the transfer of energy between
objects due to a temperature difference.

How do you think this transfer happens?
HEAT
The idea of a Calorie had been around for a
while, (amount of energy to raise 1 kg of water
1 degree Celsius)
and Joule made the following relationship (conversion)
4.186 J = 1 cal
This idea of energy being transferred by heat
needs some caution.
Heat is a transfer of energy, not energy itself
It is not a fluid or any substance
It is a change of internal or thermal energy
To be clear, there are 3 ideas that we need to distinguish
Temperature - this is the AVERAGE kinetic energy of the molecules
Internal energy - is the TOTAL energy of ALL the molecules
Heat - is the transfer of energy due to temperature difference.
A B
A and B are both equally dense pieces of iron
A has more mass than B
If these two objects are
in thermal equilibrium then
Which has a greater temperature
A. A does
B. B does
C. they are the same
If these two objects are
in thermal equilibrium then
Which has a greater thermal
energy
A. A does
B. B does
C. they are the same
Carrying on with this idea of internal energy
If I have a substance with N molecules, with each
molecule having an average kinetic energy, I can
add all of this energy up and get an Internal energy





From kinetic theory this parenthetical term is
proportional to temperature (3/2 kT)
This then is the internal energy of an
ideal monatomic gas.
or
Specific Heat
Now that we have an idea of where the energy
can go, (internal energy = motion of molecules)
we can quantify that energy transfer due to
temperature difference = heat flow.
As I'm sure you guessed, (or knew) different substances require more heat to change their internal energy. We define an intrinsic property of a substance that quantifies this change in temperature from heat flow as: Specific Heat.
This is the equation that describes the
amount of heat required to raise a mass, m,
a certain amount of temperature.
Note the term 'c' is the specific heat of an
object.
Things to note:
Temperature can be either C or K because it is a change and they both use the same step size
Mass is in kg
and c needs to be in J/kg C to make Q be in energy units of J.
If water has a c=4186 J/kg c ,
how much energy is needed
to raise 1 kg of water 10 degrees
A. 418.6 J
B. 4186 J
C. 41860 J
Calorimetry is the exchange of energy between
two (or more) objects.
It needs:
An isolated system - no energy at all is transferred out of the system
Some form of heat transfer
be careful of phase changes
Calorimetry
Latent Heat
No temperature change, yet heat flow with phase change
Note: it can be plus or minus +,-
To melt 1 kg of water 333 kJ is needed
to Boil 1 kg of water 2260 kJ is needed
How much heat is removed from 5.0 kg of liquid lead at its melting point of 327 C to cool it down to room temperature (20 C)?
A. 12.5 kJ
B. 187 kJ
C. 200 kJ
D. 212 kJ
E. 225 kJ
Q = -212,050 J
conduction
Quantitatively, we can discuss the rate of heat flow between
two objects.
Q is obviously the heat, and t is the time
k here is conductivity constant ( it is usually a kappa)
A is cross sectional area, l is the distance the heat flows
through.
Example windows
A convection process
is a transfer of heat, by a bulk flow
of molecules that heat up surrounding
ones.


Convection
The temperature of an object really influences the
rate of radiation. Notice the power the temperature
is raised to.
Sigma is a constant (small number)
e is emissivity, it ranges from 0-1
A is area
Radiation
Radiant heat is actually electromagnetic waves. You will get into that, sooner than you think, but not this semester.
That's all we have for this chapter
take away concepts
Internal energy



Calorimetry/latent heat



Modes of heat transfer
A. Conduction
B. Convection
C. Radiation

A range on a stove heats
food by:

A Heat lamp works by
Forced air heater
works by

Heat

Internal Energy
Specific heat
Latent Heat
Calorimetry
Modes of heat transfer
conduction, convection, radiation
(+/-)
"what is latent heat and how is it significant? "
"don't understand question 2 from the pre-quiz. "T/F heat, temp, and internal energy are all one in the same." What is different?"
"can you explain what specific heat is?"
"I'm a little confused by the difference between conduction and convection. Aluminum foil in the oven hardly absorbs any heat, is that convection? Also, why is styrofoam the best type of container to use when eating ice cream? It traps heat or lack of heat very well but is that the same kind of concept as aluminum foil?"
"Could you explain more on conduction and convection"
"When does convection factor into heat and the transfer of it?"
"Can you explain emissivity? "
"the Stefan-Boltzman equation. I'm a little confused about what all the different symbols mean in the equation."
"Does an ice cube make the water cooler or does the water make the ice cube hotter and melt?"
"Need help with recognizing the components of the equations for heat, some new terms like Calorie."
Full transcript