Ch. 6 Thermal Energy

6.1 Temperature and Heat

Temperature

Something is hot: the temperature is high

The stove is heated: the temperature increases

Matter in Motion

Matter around you is made up of tiny particles called atoms and molecules which are in constant random motion at all different speeds. Due to them being in motion they then have kinetic energy; the faster they move, the more kinetic energy they have

Temperature

Temperature and kinetic energy of an object are related

Temperature - measure of the average kinetic energy of the particles in the object

As the temperature of an object increases, the average speed of the particles in random motion increases

In SI units, temperature is measured in Kelvins (K); more commonly on the Celsius scale; 1 K = 1°C

Thermal Energy

Thermal energy - the sum of all the kinetic and potential energy of all the particles

Thermal Energy and Temperature

The thermal energy of an object increases as its temperature increases

Thermal Energy and Mass

If the temperature doesn’t change, the thermal energy in an object increases if the mass of the object increases

Heat

Heat - thermal energy that flows from something at a higher temperature to something at a lower temperature

• Heat is a form of energy

• Heat always flows from warmer to cooler materials

Specific Heat

As a substance absorbs heat, its temperature change depends on the nature of the substance, as well as the amount of heat that is added

Specific heat - the material is the amount of heat that is needed to raise the temperature of 1 kg of some material by 1°C

Water as a Coolant

Water has the highest specific heat compared to wood, carbon, glass, and iron

Water is useful as a coolant since it can absorb heat without a large change in temperature

• A coolant is a substance that is used to absorb heat

Changes in Thermal Energy

The thermal energy of an object changes when heat flows into or out of the object

Thermal Energy Equation: Q = m(Tf−Ti)C

Measuring Specific Heat

A calorimeter is a device that measures the specific heat of a material

The specific heat can be determined by the mass, change in temperature, and the amount of heat absorbed or released are known

Using a Calorimeter

Measure the sample of the material, then heat the material and place it into the inner chamber of the calorimeter

6.1 Summary

Temperature

• The temperature of an object is a measure of the average kinetic energy of the particles that make up the object

Thermal Energy and Heat

• Thermal energy is the sum of the kinetic and potential energy of all particles in an object
• If temperature is constant, the thermal energy increases when the mass increases
• Heat is thermal energy that is transferred from an object at a higher temperature to an object at a lower temperature

Specific Heat

• The specific heat of a material is the amount of heat needed to raise the temperature of 1 kg of the material 1°C
• The change in thermal energy of an object can be calculated from this equation: Q = m(T1 − Ti)C

6.2 Transferring Thermal Energy 

Conduction

Thermal energy is transferred from place to place by conduction, convection, and radiation

Conduction - transfer of thermal energy by collisions between particles in matter; conduction occurs because particles in matter are in constant motion

Collisions Transfer Thermal Energy

Thermal energy can be transferred when one end of a metal spoon is heated by a Bunsen burner

• the kinetic energy near the flame then increases. Thermal energy is then transferred when these particles collide with neighboring particles

Heat Conductors

Heat moves faster by conduction in solids and liquids than gases

The best conductors of heat are metals; the best metals include silver copper, and aluminum

Convection

Thermal energy can be transferred by convection in fluids

Convection - the transfer of thermal energy in a fluid by the movement of warmer and cooler fluid from place to place

• During conduction, more energetic particles collide with less energetic particles and transfer thermal energy; while during convection, more energetic particles move from one place to another

As particles move faster, they are usually further apart; as a result a fluid expands as its temperature increases

Heat Transfer by Currents

In a lava lamp, convection occurs when the heat from the light at the bottom of the lamp causes one fluid to expand more than the other which then creates the convection currents in the lamp

Desert and Rain Forests

Earth’s atmosphere is warmer at the equator than it is at the north and south poles

The atmosphere is also warmer at earth’s surface than it is at higher altitudes; these temperature differences create convection currents that carry heat to cooler regions

Radiation

Since there is almost no matter that exists in the space between the Earth and the Sun, heat cannot be transferred by conduction or convection which is why the Sun’s heat reaches the Earth by radiation

Radiation is the transfer of energy by electromagnetic waves

• these waves travel through space even when no matter is present

Energy that is transferred by radiation is called radiant energy

Radiant Energy and Matter

When radiation strikes a material, some of the energy is absorbed, some is reflected, and some may be transmitted through the material

• The amount of energy that is absorbed, reflected, and transmitted is determined by the type of material; materials that are light colored reflect more radiant energy while dark colors absorb more

When radiant energy is absorbed by a material, the thermal energy of the material increases

Radiation can pass through solids, liquids, and gases

Radiation in Solids, Liquids, and Gases

Transfer of energy by radiation is more important in gases; it can pass more easily since the molecules are further apart in gases rather than in a solid or liquid

Controlling Heat Flow

All living things have special features that control the flow of heat

An animal’s color can also play a role in keeping it warm or cool

Insulators

An insulator is a material in which heat flows slowly

• Ex. Wood, plastics, fiberglass, and air

Gases are usually much better insulators than solids or liquids

Some types of insulators contain many pockets of trapped air; these air pockets conduct heat poorly and keep convection currents from forming

Insulating Buildings

Insulation helps keep warm air from flowing out of buildings in cold weather and from flowing into buildings in warm weather

Insulation helps furnaces and air conditioners work more effectively by saving energy

• in the US about 55% of the energy used in homes is for heating and cooling

Reducing Heat Flow in a Thermos

A thermos bottle uses a vacuum and reflective surface to reduce the flow of heat into and out of the bottle; the vacuum prevents heat flow by conduction and convection.

• The reflective surfaces reduce the heat transfer by radiation

6.2 Summary

Conduction

• Conduction is the transfer of thermal energy by collisions between more energetic and less energetic particles
• Conduction occurs in solids, liquids, and gases. Metals are the best conductors of heat

Convection

• Convection is the transfer of thermal energy by the movement of warmer and cooler material
• Convection occurs in fluids. Rising of warmer fluid and sinking of cooler fluid forms a convection current

Radiation

• Radiation is the transfer of energy by electromagnetic waves

Controlling Heat Flow

• Insulators are used to reduce the rate of heat transfer from one place to another

6.3 Using Heat

Heating Systems

The best heating system for any building depends on the local climate and how the building is constructed

All heating systems require some source of energy; in the simplest and oldest heating system wood or cool is burned in a stove.

• The heat that is produced by the burning fuel is then transferred from the stove to the surrounding air by conduction, convection, and radiation. Disadvantages to this system is heat transfer is very slow

Forced-Air Systems

The most common type of heating system used today is the forced-air system.

Fuel is burned in a furnace and heats a volume of air which then is blown by a fan through a series of ducts which then lead to vents in each room

Radiator Systems

Before forced-air systems, radiator systems were very popular

A radiator is a closed metal container that contains hot water or steam. The thermal energy from it then is transferred to the air surrounding the radiator by conduction then the warm air moves through the room by convection

Fuel burned in a central furnace heats a tank of water; after the water cools, it flows through the pipes back to the radiators. As the steam cools, it condenses into water and flows back to the tank

Electric Heating Systems

An electric heating system has no central furnace, it has electrically heated coils that are placed in floors and walls that heat the surrounding air by conduction; heat is then distributed by convection

Solar Heating

The radiant energy from the Sun can be used to help heat homes and buildings

2 types of systems that use the Sun’s energy for heating

• Passive solar heating
• Active solar heating

Passive Solar Heating

Materials inside a building absorb radiant energy from the Sun during the day and heat up. A night when it begins to cool, thermal energy absorbed by these materials help keep the room warm

Radiant energy from the Sun is transferred to the room through windows. Windows also prevent air inside from mixing with cooler air outside

Active Solar Heating

Solar collectors - used to absorb radiant energy from the Sun

Solar collectors are installed on the roof, radiant energy from the Sun heats air or water in the solar collectors

Solar collectors are mounted on the roof to absorb solar energy. The absorbed energy heats a liquid that is circulated throughout the house

Thermodynamics

Thermodynamics - the study of the relationship of thermal energy, heat and work

Heat and Work Increase Thermal Energy

You can warm your hands by placing them near a fire, so that heat is added to your hands by radiation

• if you rub your hands and hold them near a fire, the increase in thermal energy of your hands is greater. Both the work you do and the heat transferred from the fire increase the thermal energy of your hands

The First Law of Thermodynamics

First law of thermodynamics - states that the increase in thermal energy of a system equals the work done on the system plus the heat transferred to the system

• The first law of thermodynamics is another way of stating the law of conservation of energy; the increase of energy of a system equals the energy added to the system

Closed and Open Systems

An open system is when heat flows across the boundary or if work is done across the boundary

• then energy is added to the system

A closed system is when no heat flows across the boundary and there is no outside work done

• According to the first law of thermodynamics, the thermal energy of a closed system doesn’t change

Since energy cannot be created or destroyed, the total energy stays constant in a closed system

The Second Law of Thermodynamics

The flow of heat spontaneously from a cool object to a warm object never happens because it violates the second law of thermodynamics

Second law of thermodynamics - states that it is impossible for heat to flow from a cool object to a warmer object unless work is done

Converting Heat to Work

The second law of thermodynamics makes it impossible to build a device that converts heat completely into heat

A heat engine is a device that converts heat into work

• Ex. Car engine

Internal Combustion Engines

Internal combustion engine - a heat engine that burns fuel inside the engine in chambers or cylinders

Automobile and diesel engines have four different strokes:

• intake
• compression
• power
• exhaust strokes

Intake stroke

The intake valve opens as the piston moves downward, drawing a mixture of gasoline and air into the cylinder

Compression stroke

The intake valve closes as the piston moves upward, compressing the fuel-air mixture

Power stroke

A spark plug ignites the fuel-air mixture. as the mixture burns, hot gases expand, pushing the piston down

Exhaust stroke

As the piston moves up, the exhaust valve opens, and the hot gases are pushed out of the cylinder

Friction and the Efficiency of Heat Engines

Almost ¾ of the heat produced in an internal combustion engine is not converted into useful work

Friction between moving parts causes some of the work done by the engine to be converted into heat

The efficiency of an internal combustion engine depends on the difference in the temperature of the burning gases in the cylinder and the temperature of the air outside the engine

• increasing the temperature of the burning gases makes the engine more efficient

Heat Movers

A refrigerator is a heat mover that does work to move heat from a cooler temperature to a warmer temperature

Refrigerators

A refrigerator contains a coolant that is pumped through pipes on the inside and outside of the refrigerator; the coolant is a special substance that evaporates at a low temperature.

• When the coolant changes to a gas, it cools. The cold gas is then pumped through the refrigerator, where it absorbs thermal energy. As a result, the inside of the refrigerator cools

Air Conditioners and Heat Pumps

An air conditioner is another type of heat mover.

• Acts like a refrigerator except that the warm air has to pass over tubes containing the coolant

A heat pump is two-way heat mover

• In warm weather it operates like an air conditioner
• In cool weather it operates like reverse air conditioner

The Human Coolant

Body generates sweat and as the sweat evaporates, it takes your body heat with it to cool you down

Energy Transformations Produce Heat

Energy transformations occur around you converting one form of energy to a more useful form of energy.

6.3 Summary

Heating Systems

• A forced-air heating system uses a fan to force air heated by a furnace through a system of ducts
• Radiator and electric heating systems transfer heat to rooms by conduction and convection
• Solar heating systems convert radiant energy from the Sun to thermal energy

Thermodynamics

• The first law of thermodynamics states that the increase in thermal energy of a system equals the work done on the system plus the heat added to the system
• One way to state the second law of thermodynamics is that heat will not flow from a hot to a cold object unless work is done

Converting Heat to Work

• The second law of thermodynamics states that heat cannot be converted completely into work
• A heat engine converts heat into work
• A refrigerator moves heat by doing work on the coolant

Ch. 6 Study Guide (Reviewing Main Ideas)

6.1 Temperature and Heat

1. The temperature of a material is a measure of the average kinetic energy of the molecules in the material
2. Heat is thermal energy that flows from a higher to a lower temperature
3. The thermal energy of an object is the total kinetic and potential energy of the molecules in the object
4. The specific heat is the amount of heat needed to raise the temperature of 1 kg of a substance by 1°C

6.2 Transferring Thermal Energy

1. Conduction occurs when thermal energy is transferred by collusions between particles. Matter is not transferred when conduction occurs
2. Convection occurs in a fluid as warmer and cooler fluid move from place to place
3. Radiation is the transfer of energy by electromagnetic waves. Radiation can transfer energy through empty space
4. Heat flows more easily in material that are conductors than in insulators
5. Some insulating materials contain pockets of trapped air that reduce the flow of heat

6.3 Using Heat

1. Conventional heating systems use air, hot water, and steam to transfer thermal energy through a building
2. A solar heating system converts radiant energy from the Sun to thermal energy. Active solar systems use solar collectors to absorb the thermal radiant energy
3. According to the first law of thermodynamics, the increase in the thermal energy of a system equals the work done on the system and the amount of heat added to the system
4. The second law of thermodynamics states that heat cannot flow from a colder to a hotter temperature unless work is done, and that heat cannot be converted completely into work
5. Heat engines convert heat into work. The efficiency of a heat engine can never be 100 percent. Refrigerators transfer heat from a cooler to a warmer temperature by doing work on the coolant