Slide 1

5.4 Measuring Thermal (Internal)Energy

The amount of energy in an object depends on:

1.Mass of the object

2.Temperature of the object

3.The nature of the material of the object

Specific Heat Capacity

Different materials have different capacities for holding thermalenergy

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Specific Heat Capacity

The amount of heat needed to raise thetemperature of 1 kg of a substance by 1˚C

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Specific Heat Capacity

The amount of heat needed to raise thetemperature of 1 kg of a substance by 1˚C

Specific Heat Capacity

The amount of heat needed to raise thetemperature of 1 kg of a substance by 1˚C

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Specific Heat Capacity

The amount of heat needed to raise thetemperature of 1 kg of a substance by 1˚C

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If 25 kJ of heat is transferred to 50.0 kg of water initially at 20.0˚C,what will be the final temperature of water?

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HOMEWORK

Textbook: Page 161

Problems: 5.4.1

The rate of thermal conduction depends on thesubstance

The metal tray feels colder than the package because metalconducts energy more easily and more rapidly thancardboard does.

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Thermal Conductors

Substances that rapidly transfer energy as heat

In general, metals are good thermal conductors.

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Substances that slowly transfer energy as heat

Materials such as asbestos, cork, ceramic, cardboard, and fibreglass arepoor thermal conductors (and therefore good thermal insulators).

Thermal Insulators

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Heat Transfer by Convection

• Involves the movement of cold and hot matter, such as hot air or hot water

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Heat Transfer by Convection

• Thermal energy is not easily transferred by CONDUCTION in fluids (liquids andgases)

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Heat Transfer by Convection

• The thermal energy “flows” with a particle from one place to another as asubstance is being heated = the thermal energy is being carried around

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Heat Transfer by Convection

This movement is called a CONVECTION CURRENT

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Convection in Everyday Life

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Heat Transfer by Radiation

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Heat Transfer by Radiation

• another way that heat can be transmitted

• occurs where energy CAN’T be transferred by CONDUCTION or CONVECTION

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Heat Transfer by Radiation

• Energy transmitted via RADIATION will be in the form of ELECTROMANETIC

WAVES and will travel at the SPEED OF LIGHT

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Heat Transfer by Radiation

The various forms of RADIANT ENERGY (which originate in the SUN)

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Convection in Everyday Life

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SOLAR FURNACE

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HOMEWORK

Textbook: Page 154

Problems: Quick Check

Vacuum in the bottle prevents Conduction and Convection
The metal walls reflect Radiant Energy back into the bottle
The rubber/metal lid slows heat transfer by all three methods

Dirty snow – darker/black objects ABSORB
most of the radiant energy; light colors
reflect most of the radiant energy

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HOMEWORK

Textbook: Page 159

Problems: all

WORKSHEET EXAMPLE

A 6.0 kg cat runs after a mouse at 10m/s.

What is the cat’s kinetic energy?

KE = 300 J

WORKSHEET EXAMPLE

How much net work is needed to accelerate a 1000.0 kg carfrom 20.0 m/s to 30.0 m/s

Can You Have a Negative PE?

You could bring a textbook from a table heightto a zero – level (ground) – performing anegative work

If yes, give an example

If not, explain why.

Can you have an object with a positivePE relative to one point and negativePE to another point at the same time?

A textbook which is 1.0 m below a table(negative PE relative to the table)

If yes, give an example

If not, explain why.

A textbook which is 1.0 m above theground (positive PE relative to the table)

WHAT IS THE UNIT OF k?

N/m

WORKSHEET EXAMPLE

When a 2.00 kg mass is attached to a vertical spring, the spring is stretched 10.0 cm suchthat the mass is 50.0 cm above the table.

a.What is the gravitational potential energy associated with this massrelative to the table?

b.What is the spring’s elastic potential energy if the spring constant is 400.0N/m?

c.What is the total potential energy of this system?