Temperature & Heat 2
Temperature vs Heat
What is temperature (degrees)?• A measure of the average kinetic energy of
the particles in an object
What is heat (joules)?• That energy transferred between objects
because of a difference in their temperatures
How does heat flow?• Heat flows from a hotter to a colder body by
transfer of kinetic energy to adjacent molecules
Temperature & Heat 3
Units of Measurement
There are 3 units of temperature
measurement:
• Degrees Celsius (C) (Water: 0 – 100)
• Degrees Fahrenheit (F) (Water: 32 – 212)
• Kelvin (K), referencing absolute zero
Conversion - Celsius & Fahrenheit
• F – 32 = 9/5*C
• K = C + 273
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Measuring Devices
Thermometers
• Analog
• Digital
Thermocouples
• The temperature of the junction of 2 different
wires creates a small voltage, related to the
temperature of the junction
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Expansion
Each material expands, according to its natural property, as its temperature increases by ΔT degrees C.
Options• Linear: ΔL = Lo α ΔT, where
• α = coefficient of linear expansion
• Area: ΔA = Ao (2α) ΔT
• Volume ΔV = Vo β ΔT, where• Β = coefficient of volumetric expansion
Example
A metal bar is 1.6 m long at 21 C. If the
bar is heated to 84 C by how much does
it expand? ( = 1.7 x 10-5 / C)
L = Lo* *(T2 – T1)
• L = 1.6 * 1.7 x 10-5 * (84 – 21)
• L = 1.7 x 10-3 m
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Temperature & Heat 7
Specific Heat – basic property
The amount of heat, in joules, required to raise the temperature of 1 kg of a substance by 1 degree Celsius.• Symbol of the specific heat of a substance is “c”
• Examples of c• Water = 4187 J/kg- C,
• ie 4187 joules of heat energy are required to raise the temperature of 1 kg of water by 1ºC.
• Iron = 448 J/kg- C
• Aluminum = 899 J/kg- C
• Ice = 2090 J/kg- C
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Implications of Specific Heat
The heat energy (ΔQ) transferred into or out of a mass (m) of material with a specific heat (c) is given by:• ΔQ = m*c*ΔT, where ΔT is the change in
temperature
This phenomenon can be used when bodies of different temperatures are brought into contact• Hot bodies lose, while cold bodies gain heat
Example of Heat Transfer
How much heat is required to raise the
temperature of a 2 kg block of copper
from 20C to 250C (cCU=386 J.kg/˚C)?
Solve Q = m*c*ΔT
• Q = 2 * 386 * (250 – 20)
• Q = 177,560 J
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Example
What would be the final temperature of a
5kg iron (c = 448) rod at 20C if it
received 110,000 J of heat?
Solve Q = m*c*ΔT
• 110,000 = 5 * 448 * ΔT
• 49.11 = ΔT
• But ΔT = Tf – To = Tf – 20
• Thus, Tf = ΔT + 20 = 69.11 C
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Temperature & Heat 11
Mixture Examples
What happens when 2 bodies of different temperatures are brought into contact, such as a hot piece of metal being placed into a bucket of cold water?• The hot body (metal) cools while the cold body
(water) warms according to the conservation of energy
• Heat lost by hot body = Heat gained by cold body, or
• (m*c*ΔT)lost = (m*c*ΔT)gained
Heat flows from hot to cold!
Temperature & Heat 13
Phase Change – Change of State
All solids, liquids and gases are made of
particles, and the only difference between
them is how much energy the particles
have. If you give the particles energy, or
take energy away from them, then you can
change their state.
Temperature & Heat 15
Phase Change – Change of State
There are 2 different types of heat
• Sensible (or specific) – the addition of which
causes a rise in temperature of the material,
and
• Latent – the addition of which causes no
temperature change until the current state of
the material has changed completely, from solid
to liquid, liquid to gas or in reverse.
Temperature & Heat 16
Latent Heat
There are 2 different latent heats
• Solid > liquid = latent heat of fusion or melting• ( ex. turning ice to water)
• QF = mass * heat of fusion, or
• QF = m * hf (where hf = 3.35 x 105 J/kg for ice)
• Liquid > gas = latent heat of vaporization• (ex. turning water to vapor)
• QV = mass * heat of fusion, or
• QV = m * hv (where hv = 2.26 x 106 J/kg for water)
• In graphic form it looks like…
Temperature & Heat 17
Change of State
Heat can be applied to an object to change its state from solid, through liquid to gas, according to the 5 steps shown:
1. Heat the solid until it reaches melting point (specific heat of solid)
2. Heat the solid until it changes completely to liquid (latent heat fusion)
3. Heat the liquid until it reaches the boiling point (specific heat of liquid)
4. Heat the liquid until it changes completely to gas (latent heat of vaporization)
5. Heat the gas (specific heat of gas)
For example, H2O…
Temperature & Heat 19
Steps in Solving Specific/Latent
Heat Problems
Identify the temperature range in the problem
Mark the temperature range on the heat graph
On the graph move from the lowest to the highest
temperature identifying the sloped/flat stages
Calculate the heat (Q) for each step as follows:
• Sloped > specific heat stage > Q = mcΔT
• Flat > latent heat stage > Q = mh (h for fusion/vaporiz’n)
Add the Q values for each step to determine the total
heat (Qtotal)
Example
How much heat, in joules, is required to raise the temp of 2 kg of ice from -10˚C to 20˚C?• Mark start and finish temp
• Count slopes and flats• 2 x slopes + 1 flat, meaning:
• 2 x specific heat segments
• 1 x latent heat segment
• Calculate Q for each segment, then total the Q values
Temperature & Heat 20
S
S
F
Calculations
Segment 1: sloped = specific heat for ice being heated from -10 to 0˚C• Q = mcΔT = 2 * 2090 * (0 – (-10)) = 41,800 J
Segment 2: flat = latent heat converting ice @ 0˚C to water @ 0˚C• Q = mhf = 2 * 3.35 x 105 = 670,000 J
Segment 3: sloped = specific heat for water from 0˚C to 20˚C• Q = mcΔT = 2 x 4187 * (20 – 0) = 167,480 J
Total heat (Q) = 879,280 J
Temperature & Heat 21