10/04/2015
Thermal Physics
Heat flows from hot to cold
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Net energy flow stops when their temperatures are the same
i.e. They are in thermal equilibrium
The Kelvin temperature scale
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• Tc = Tk - 273
• Tk = Tc +273
• How many times hotter is 100ºC than
33ºC?
• How many times hotter is 10ºC than
0ºC?
• What is your temperature in 0C and
0K
Kinetic theory
10/04/2015
• Why is it that you can
put out a candle flame
with moist fingers
without hurting yourself
although it is at 750 oC
but it is very painful to
put your fingers into a
cup of hot water at 80
oC?
Heat energy and molecular
movement
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• What happens to the molecules when
their temperature increases?
• What is the difference between
internal “heat” energy and
temperature?
• What types of energy do
the molecules have?
Heat energy and temperature
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100mL at 200C
400mL at 200C
Which has the “most heat” energy?
Which has the higher temperature?
400mL at 800C
1mL at 2000C
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Heat capacity
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Click to play
Why are the temperatures different?
Temperature
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The internal energy of a substance is……
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The potential energy of the molecules is….
The thermal energy of a substance is…..
Gas Pressure
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Gas pressure is basically due to gas molecules hitting the sides
of the container they are in:
Hyperlink
Kinetic Theory of Gases
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Let’s make some assumptions:
1) Brownian motion suggests that a pure gas consists of
identical molecules in constant random motion.
2) Molecules never stop so collisions between molecules are,
on average, elastic (no kinetic energy is lost).
3) Because gases can be compressed easily the volume of the
individual molecules is negligible compared to the volume
they occupy when moving around.
4) Therefore the molecules are further apart, so we assume
that there are no forces on the molecules except for when
they collide.
Internal Energy
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Basically, the internal energy of a substance is the sum of the
molecular kinetic and potential energies. Consider a particle:
This particle can store energy by moving
in a number of ways:
1) Moving in three dimensions
2) Vibrating
3) Rotating
These are called “degrees of freedom”,
and each one can store energy.
The Mole and Molar mass
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One mole of carbon
One mole of green atoms
contains 6x1023 atoms
contains 6x1023 atoms
One mole of “anything” contains 6x1023 atoms (or
molecules)
One mole of carbon
weighs 12g
Define the mole
Molar mass
•
•
•
•
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One mole of carbon contains 6x1023 atoms
It weighs 12 grams
How much does one mole of iron weigh?
How many atoms are there in 2 moles of
tungsten?
• How many atoms are the in 0.5 moles of
water?
• Define Avagadro’s constant
• Define the Mole
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Examples
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1. Calculate the number of moles of
oxygen molecules contained in 64.0 g
of oxygen gas, O2.
2. Calculate the number of oxygen
molecules in part of this example.
3. The number of molecules present in
0.5 mol SO3 is
4. Calculate the mass in 0.75 mol of
carbon dioxide gas.
Specific Heat Capacity
10/04/2015
This can be thought of as “the capacity of an object to store
heat”. Consider some water:
If we heat this beaker up it’s fairly clear
that the amount of energy it gains
depends on how much water there is and
how hot it gets…
Energy α mass x temperature rise
Energy = mass x s.h.c x temp
E = mcΔT
Specific Heat Capacity
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How can we do this experimentally?
E = VIt and E = mcΔT
A
12V
V
Possible errors with this experiment:
1) Temperature throughout the liquid
should be the same
Solution:
2) Heat is lost to the surroundings
Solution(s):
Another way…
A
12V
A metal
V
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SHC
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Temperature
Q=mcΔT
Q=Pt, and P=IV
ΔT=IVt/mc
Time
12
V
A
V
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Thermal Physics Questions
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• Specific heat capacities:
• copper400Jkg-1K-1 iron460Jkg-1K1 water4200Jkg-1K-1 ice2100Jkg-1K-1
• Specific latent heat of fusion of ice =
3·3×105Jkg-1
• Molar heat capacities of a diatomic ideal
gas:
Cv = 12·5J(molK)-1 and Cp = 20·8J(molK)-1
10/04/2015
• Question 1
• A piece of metal of mass 0·2kg is heated to a temperature of
200°C. It is then put into 0·2kg of water at 20°C in a container of
negligible heat capacity. The "final" temperature, after stirring, is
40°C. Calculate the specific heat capacity of the metal.
• Question 2
• A piece of metal of mass 100g, has a temperature of 100°C. It is
put into 100g of water at 20°C in a container of negligible heat
capacity. After stirring, the maximum temperature of the
"mixture" (metal and water) is 27·5°C. Calculate the specific heat
capacity of the metal.
• Question 3
• The specific heat capacity of water is very high. What effect does
this have on the weather conditions experienced by people living on
islands?
• Question 4
• How long will it take to change the temperature of 200kg of water
from 15°C to 40°C, using a heater of power 3kW. Assume that all
the thermal energy remains in the water.
3 Phases of matter
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Solids –fixed positions, many strong bonds (potential energy)
Liquids – Changing positions, many temporary bonds
Gases – Free moving, no bonds
Hyperlink
Changing state
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Change of state (Melting and
boiling)
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Heat energy is going to
potential energy only, therefore
there is no rise in temperature.
Heat energy is
going to
internal EK
Heating ice
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Temp/OC
150 This flat line shows where energy is being
100
used to break the temporary bonds for
boiling. The amount of energy needed to
turn 1kg of a liquid into a gas is called the
Specific Latent Heat of Vaporisation L.
50
0
-50
Time/s
This flat line shows where energy is being
used to break bonds – this has to be done
during melting. The amount of energy
needed to turn 1kg of a solid into a liquid is
called the Specific Latent Heat of Fusion L.
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Latent heat (changes in potential
energy)
Melting Q’s
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• Question 1
• A piece of ice at -20°C is put into a copper calorimeter of
mass 0·2kg which contains 0·15kg of water at 20°C. The
water is stirred until all the ice has melted. At this time the
temperature of the water (and calorimeter) is 15°C.
Calculate the mass of the piece of ice. (8 grammes).
• Question 2
• A refrigerator is capable of removing 50J of heat per
second from a container of water. How long will it take to
change 2kg of water at 10°C into ice at -5°C? Assume that
the rate of removal of heat remains constant and that the
container has negligible heat capacity. Are these
assumptions likely to be valid in practice?
Latent Heat of Fusion
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From the previous slide we can say that the energy needed to
melt water is given by…
Energy = mass x specific latent heat of fusion
To work out L
experimentally you could…
VIt = mL
A
12V
V
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Tsokos page 171 q’s 1-11.
Evaporation
Average speed
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decreases
•High energy
molecules escape
when they reach
the surface.
•The average
kinetic energy of
the remaining
particles drops
•The temperature
of the remaining
liquid is lowered
Hyperlink
Boiling
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Evaporation and Boiling
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• Boiling occurs at a fixed temperature
• Evaporation occurs at any
temperature
• Boiling happens throughout the body
of the liquid
• Evaporation only happens at the
surface of the liquid.
Pressure
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Pressure depends on two things:
1) How much force is applied, and
2) How big (or small) the area on which this force is
applied is.
Pressure can be calculated using the equation:
F
Pressure (in N/m2) = Force (in N)
Area (in m2)
OR in cm2 and N/cm2
P
A
Some pressure questions
Pressure = force
area
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P=F
A
1) Calculate the pressure exerted by a 1000N elephant when
standing on the floor if his feet have a total area of 1m2.
2) A brick is rested on a surface. The brick has an area of
20cm2. Its weight is 10N. Calculate the pressure.
3) A woman exerts a pressure of 100N/cm2 when standing on
the floor. If her weight is 500N what is the area of the
floor she is standing on?
4) (Hard!) The pressure due to the atmosphere is
100,000N/m2. If 10 Newtons are equivalent to 1kg how
much mass is pressing down on every square centimetre of
our body?
Kinetic theory
10/04/2015
•Particles collide with the
walls of the container
•They change their
momentum
•They exert a force on the
wall
•This creates “pressure”.
The collisions of the molecules with the container
(not with each other) create the pressure.
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Assumptions of the kinetic theory
1. Molecules behave as if they were hard, smooth,
elastic spheres. (i.e. the collisions are perfectly
elastic)
2. Molecules are in continuous rapid, random motion.
3. The average kinetic energy of the molecules is
proportional to the absolute temperature of the
gas.
4. The molecules do not exert any appreciable
attraction on each other.
5. The volume of the molecules is infinitesimal when
compared with the volume of the gas.
6. The time spent in collisions is small compared with
the time between collisions.
Macroscopic behaviour of a gas
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• Why does the volume of a gas expand
when heated?
• Why does the pressure of a gas
increase when heated?
• How does the temperature relate to
the molecular behaviour?
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Tsokos page 171 q’s 1-10.