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 CapacityDifferent materials have different capacities for holding thermal

energy

Specific Heat CapacityThe amount of heat needed to raise the

temperature of 1 kg of a substance by 1˚C

Specific Heat CapacityThe amount of heat needed to raise the

temperature of 1 kg of a substance by 1˚C

WHAT DO YOU NOTICE ABOUT THESE NUMBERS?

Specific Heat CapacityThe amount of heat needed to raise the

temperature of 1 kg of a substance by 1˚C

WHAT DO YOU NOTICE ABOUT THESE NUMBERS?

Specific Heat CapacityThe amount of heat needed to raise the

temperature of 1 kg of a substance by 1˚C

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?

HOMEWORKTextbook: Page 161

Problems: 5.4.1

8

The rate of thermal conduction depends on the substance

The metal tray feels colder than the package because metal conducts energy more easily and more

rapidly than cardboard does.

Thermal Conductors

Substances that rapidly transfer energy as heat

In general, metals are good thermal conductors.

Substances that slowly transfer energy as heat

Materials such as asbestos, cork, ceramic, cardboard, and fibreglass are poor thermal conductors (and therefore good

thermal insulators).

Thermal Insulators

Heat Transfer by Convection• Involves the movement of cold and hot matter, such as hot air

or hot water

Heat Transfer by Convection• Thermal energy is not easily transferred by CONDUCTION in fluids (liquids and gases)

Heat Transfer by Convection• The thermal energy “flows” with a particle from one place to

another as a substance is being heated = the thermal energy is being carried around

Heat Transfer by ConvectionThis movement is called a CONVECTION CURRENT

Convection in Everyday Life

Heat Transfer by Radiation

Heat Transfer by Radiation• another way that heat can be transmitted

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

Heat Transfer by Radiation• Energy transmitted via RADIATION will be in the form of

ELECTROMANETIC WAVES and will travel at the SPEED OF LIGHT

Heat Transfer by RadiationThe various forms of RADIANT ENERGY (which originate

in the SUN)

Convection in Everyday Life

SOLAR FURNACE

HOMEWORKTextbook: Page 154

Problems: Quick Check

23

Because of convection

Early in the morning because breeze has a direction towards the sea

1. Vacuum in the bottle prevents Conduction and Convection

2. The metal walls reflect Radiant Energy back into the bottle

3. The rubber/metal lid slows heat transfer by all three methods

Dirty snow – darker/black objects ABSORBmost of the radiant energy; light colorsreflect most of the radiant energy

HOMEWORKTextbook: Page 159

Problems: all

26

ANSWERS TO

WORKSHEET EXAMPLES

WORKSHEET EXAMPLEA 6.0 kg cat runs after a mouse at

10m/s. What is the cat’s kinetic

energy?KE = 300 J

WORKSHEET EXAMPLEHow much net work is needed to

accelerate a 1000.0 kg car from 20.0 m/s to 30.0 m/s

Can You Have a Negative PE?

You could bring a textbook from a table height to a zero – level (ground) –

performing a negative work

If yes, give an example

If not, explain why.

Can you have an object with a positive PE relative to one point and negative PE 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 the ground (positive PE relative to

the table)

WHAT IS THE UNIT OF k?

N/m

WORKSHEET EXAMPLEWhen a 2.00 kg mass is attached to a vertical spring, the

spring is stretched 10.0 cm such that the mass is 50.0 cm above the table.

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

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

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