lesson 5 internal energy liceo alfano i. review last week: ideal gases pressure vs. volume pressure...
TRANSCRIPT
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Lesson 5
Internal EnergyLiceo Alfano I
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Review
• Last week: Ideal gases
• Pressure vs. volume
• Pressure vs. temperature
• Volume vs. temperature
• Two weeks ago: Heat propagation
• Conduction, convection, radiation
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Reminder Question
What is thermal conductivity?
1)A quantity measured in degrees Celsius
2)How much energy it takes to increase the temperature of a material
3)Energy transfer due to a temperature difference
4)A measure of how well a material conducts heat
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Reminder Question
What is thermal conductivity?
1)A quantity measured in degrees Celsius
2)How much energy it takes to increase the temperature of a material
3)Energy transfer due to a temperature difference
4)A measure of how well a material conducts heat
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Heating Different Materials
During the summer time, when you walk from the sand into the ocean, which is hotter?
Why could this be?
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Sand vs. Water• The sun transfers heat to the sand and water
• What kind of heat propagation is this?
• The sand and water each get the same amount of heat from the sun
• Why does one get hotter than the other?
vs.
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Heat Capacity• The sand and water are at different
temperatures because they have different heat capacities
• Heat capacity tells us how quickly a material’s temperature will increase for a given amount of heat transfer
• Think of a pie: the crust and filling get the same amount of heat from the oven, but which part do you burn your mouth on?
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Heat Capacity vs. Thermal Conductivity
• Thermal conductivity = how well a material will conduct heat (heat passing, heat transfer)
• Heat capacity = how quickly something will heat up (heat staying, temperature)
• Heat going through the walls vs. temperature of the walls
vs.
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Heat Capacity vs. Thermal Conductivity
PracticeAre each of these examples of heat capacity (1) or thermal conductivity (2)?
•I make a pizza and the tomato sauce gets hotter than the cheese
•I burn my mouth when I eat the pizza
•A pot doesn’t feel hot if I use a glove to hold it
•Gold increases its temperature more quickly than silver
•Walking barefoot across the street feels hotter than walking on the sidewalk
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Specific Heat CapacityIf we look at the heat capacity per mass, we call this specific heat capacity.
A high specific heat capacity means that something takes a long time to heat up, while a low specific heat capacity means something heats up quickly.
Water has an extremely high specific heat capacity, while metals do not.
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Calculating Specific Heat Capacity
We have a formula that relates:•heat (Q) •mass (m)•specific heat capacity (c)•change in temperature (ΔT)
Q = m c ΔT
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Concept Question
Remember, Q = m c ΔT
If I increase the specific heat capacity, will it take more or less heat to increase the temperature by 1 degree?
1)More heat
2)Less heat
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Concept Question
Remember, Q = m c ΔT
If I increase the specific heat capacity, will it take more or less heat to increase the temperature by 1 degree?
1)More heat
2)Less heat
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Concept Question
Remember, Q = m c ΔT
Remember the beach question. Does sand or water have a higher specific heat capacity?
1)Sand
2)Water
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Concept Question
Remember, Q = m c ΔT
Remember the beach question. Does sand or water have a higher specific heat capacity?
1)Sand
2)Water
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VideoSpecific heat capacity/global warming
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Specific Heat Capacity of Water
The specific heat capacity of a material is usually not a constant - it changes with temperature.
However, we generally only work around 15 deg. C, so we take the specific heat capacity of water as 1 C/g, or 4.184 J/g.
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Internal Energy
• Remember, heat is a kind of ENERGY
• When you add heat energy to an object, where does that energy go?
• The energy is used to increase the temperature, and it is stored in the object
• Since it is stored in the object, we refer to this energy as internal energy
• Change in internal energy = heat added to the object
• ΔU = Q
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Practice Problems
Remember, Q = m c ΔT
•How much heat is absorbed when 500. g of water , c = 4.184 J/goC, goes from 25.0oC to 35.0oC?
•What is the change in internal energy when 500. g of copper, c = 0.385 J/goC , goes from 25.0oC to 35.0oC?
•I have a cup filled with 0.150kg of coffee (c=4187J/kgC) at 70C. I add 0.01kg of milk (c=3800J/(kgC)) at 5.0C. What is the final temperature of the coffee and milk mixture?
•I made a bowl of minestrone soup (c=4187J/kgC). I then put 0.6kg of soup at 90C in a 0.2kg glass bowl (c=840J/kgC) that is initially at 20C, what will be the temperature of the soup and the bowl when they have reached equilibrium?