© 2001-2005 Shannon W. Helzer. All Rights Reserved. Unit 9 Temperature and Heat.

© 2001-2005 Shannon W. Helzer. All Rights Reserved.

Unit 9Temperature and Heat


Warming & Cooling

Suppose we have two beakers containing water.

What would happen to the temperature of the water if we added some ice?

What would happen if we turned on the hotplate?

Why? By what means?

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Heat

What is heat? Heat is the energy that flows from one object to another where

there is a temperature difference between them. Temperature difference means that one is “hotter” and one is

“colder.” Relative temperature between two bodies allows us to tell in

which direction heat will flow. Heat flows from hot to cold always. Heat and temperature are NOT the same thing.

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Temperature

The sum of the average kinetic energies of the atoms or molecules of a body.

The less kinetic energy they have, the lower the body’s temperatures. The more kinetic energy they have, the higher the body’s temperatures. Temperature always exist. Heat only exist when the bodies in thermal contact have different

temperatures.

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Heat Flow

Let us take a block from the refrigerator and one from a hotplate.

Place them in thermal contact side-by-side.In which direction does the heat flow?In what state are the blocks in once they have the same

temperature?

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Heat FlowThermal Equilibrium


Cooling

Let us put two beakers on hot plates and heat the water to the same temperatures.

Now suppose we want to cool the water in the beakers.

Which would cool the water fastest, 100.0 g of ice or 100.0 g of steel ball bearings?

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Heat flows from the “hotter” water to the “colder” ice or steel.

Therefore, they are “warmed up.”


Specific Heat

Recall that heat is the ENERGY that flows from one object to another where there is a temperature difference between them.

In which direction does heat flow?On the previous slide, we saw that 100.0 g of ice cooled the water more

than 100.0 g of iron ball bearings.This faster cooling occurs because ice has a higher specific heat

capacity than steel.Specific heat capacity (c) of a material is the quantity of heat needed to

change a unit mass of that material by a unit amount in temperature.

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Specific Heat Equation

For ice, the specific heat capacity is 2060 J/kg C.For iron, the specific heat capacity is 450 J/kg C..It takes more heat transfer from the “hot” water to the ice

to warm the ice; therefore, the “hot” water cools more for the ice than it does for the steel.

The equation for heat transfer is below where T is temperature in Celsius (C), Q is heat in Joules (J), m is mass in kilograms (kg), and c is specific heat.

Q mC T

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WS 43 #7 & 8

A iron pan full of water is placed on a stove as shown in the figure to the right. The mass of the water is 3.5 kg (C = 4180 J/kg C), and the mass of the iron pan is 3.5 kg (C = 450.0 J/kgC). The initial temperature of both the pot and the water is 18C. They are heated to 90C. How much heat transferred from the burner to the water in the pan? How much heat transferred from the burner to iron pan?

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TmcQ


Conduction

Conduction – heat flow through a solid material placed between two other objects that are at different temperatures.

What is an example of conduction you experience every day?

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Convection

Convection – the transfer of heat by the motion of a liquid or fluid containing thermal energy.

What is an example of convection you experience every day?

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Radiation

Energy can also be transferred in the absence of a solid or liquid medium.

The energy is said to radiate and is called radiation. Consider a common thermos bottle. The thermos not only has insulation but also an evacuated area that

prevents heat transfer via either convection or conduction. However, the contents of the thermos will eventually reach the

environment’s temperature. By what mechanism does this energy transfer take place?

Glass

VacuumInsulation

Radiation

Infrared

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Calorimetry

Calorimeter Cup – a device used to minimize energy loss or gain with the outside environment.

The dead air space ensures that energy is exchanged primarily between the bodies inside the cup.

9-12

A

B

C

D

E

F

G


WS 42 # 1 - Calorimetry A calorimeter contains 500.0 g of water (C = 4180 J/kg

K) at 73.8F. 150.0 g of copper (C = 385 J/kg K) at 0.0C is added to the water. Recalling that heat lost is equal to the heat gained in a system, answer the following questions.

a. Given that a calorimeter cup prevents the loss of energy to the outside environment, what body(ies) make up the “system” in this problem?

b. Which body looses energy? Why? c. Which body gains energy? Why? d. What is the final temperature of the system?

TmcQ

9-13

l gQ Q


WS 42 # 2

In this problem there is not calorimeter cup. As a result, energy is not only gained by the coffee

can, it is also lost to the outside environment. 1500.0 g of water (C = 4180 J/kg K) at 99.0C is

dumped into an aluminum coffee can (m = 150.0 g, C = 903.0 J/kg K) at room temperature (37C). A 250.0 g lead weight (C = 130.0 J/kg K) at 10.0C is also dropped into the water.

Given that a 1200.0 J of energy is lost to the outside environment, what is the final temperature of the system?

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Heat Flow

TmcQ

l gQ Q


WS 45 #1

Hot coffee at 100.0 C (m = 200.0 g) is poured into a glass coffee cup at 44.0 C (m = 175.0 g). The coffee is stirred by a silver spoon taken from the refrigerator at 5.2 C (m = 15.0 g). Given that coffee is mostly water, determine the final temperature of the mixture.

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TmcQ

l gQ Q


WS 45 #2

250.0 g of water at 98.6 C and 300.0 g of water at 0.6 C are dumped into a 2.5 kg iron skillet at 43.3F.

Determine the final temperature of the system.

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98.6 °C 0.6 °C43.3 °C

TmcQ

l gQ Q


WS 46 #2

A large Iron nut (m = 0.25 kg, T1 = 67.2 C) is dropped into an insulated container containing water (m = 2.2 kg, T1 = 55.0 C).

What is the final temperature of the water in the insulated container?

9-17

TmcQ

l gQ Q


WS 46 #3

A rare and very valuable silver coin (m = 0.30 kg, T1 = 12.2 C) is gripped by a pair of iron tweezers (m = 0.10 kg, T1 = 45.0 C).

1000.0 J of energy is lost to the outside environment. Calculate the final temperature of the most precious

coin.

9-18

TmcQ

l gQ Q


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© 2001-2005 Shannon W. Helzer. All Rights Reserved. Unit 9 Temperature and Heat.

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Transcript of © 2001-2005 Shannon W. Helzer. All Rights Reserved. Unit 9 Temperature and Heat.