Chapter 6: Thermal Energy

Section 1: Temperature and Heat

• Temperature: a measure of the average kinetic energy of the particles in the object
  • You use the words hot and cold to describe temperature.
  • The matter around you is made of tiny particles—atoms and molecules.
  • In all materials these particles are in constant, random motion; moving in all directions at different speeds.
  • 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).
• Thermal Energy
  • Atoms and particles that exert attractive forces on each other have potential energy when they are separated.
  • Thermal Energy: The sum of the kinetic and potential energy of all the particles in an object
  • The thermal energy of a substance is the sum of the kinetic and potential energy of its molecules.
  • Thermal energy and temperature are related.
  • When the temperature of an object increases, the average kinetic energy of the particles in the object increases
  • If the temperature doesn’t change, the thermal energy in an object increases if the mass of the object increases.
• Heat: thermal energy that flows from something at a higher temperature to something at a lower temperature. Heat is a form of energy, so it is measured
  • Heat is a form of energy, so it is measured in joules—the same units that energy is measured in.
  • Heat always flows from warmer to cooler materials.
• Specific Heat: he amount of heat that is needed to raise the temperature of 1 kg of some material by 1°C
  • 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.
  • A coolant is a substance that is used to absorb heat.
  • The specific heat of water is high because water molecules form strong bonds with each other.
  • The thermal energy of an object changes when heat flows into or out of the object
• Measuring Specific Heat
  • The specific heat of a material can be measured using a device called a calorimeter.
  • The specific heat of a material can be determined if the mass of the material, its change in temperature, and the amount of heat absorbed or released are known.
  • To measure the specific heat of a material, the mass of a sample of the material is measured, as is the initial temperature of the water in the calorimeter.

Section 2: Transferring Thermal Energy

• Conduction: the transfer of thermal energy by collisions between particles in matter.

  • Conduction occurs because particles in matter are in constant motion.
  • Thermal energy is transferred from place to place by conduction, convection, and radiation.
  • Conduction occurs within a material as faster-moving particles transfer thermal energy by colliding with slower-moving particles.
  • Thermal energy is transferred by collisions between particles with more kinetic energy and particles with less kinetic energy.
  • Although heat can be transferred by conduction in all materials, the rate at which heat moves depends on the material.
  • Heat moves faster by conduction in solids and liquids than in gases.
  • The best conductors of heat are metals.

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

  • In fluids, thermal energy can be transferred by convection.
  • When conduction occurs, more energetic particles collide with less energetic particles and transfer thermal energy.
  • When convection occurs, more energetic particles move from one place to another.
  • When a fluid expands, its volume increases, but its mass doesn’t change. As a result, its density decreases.
  • Convection currents transfer heat from warmer to cooler parts of the fluid.
  • In a convection current, both conduction and convection transfer thermal energy.
  • Earth’s atmosphere is made of various gases and is a fluid.

• Radiation: the transfer of energy by electromagnetic waves.

  • Energy that is transferred by radiation often is called radiant energy.
  • When radiation strikes a material, some of the energy is absorbed, some is reflected, and some may be transmitted through the material.
  • Not all of the Sun’s radiation reaches Earth. Some of it is reflected by the atmosphere. Some of the radiation that does reach the surface is also reflected.
  • The amount of energy absorbed, reflected, and transmitted depends on the type of material.
  • Materials that are light-colored reflect more radiant energy, while dark-colored materials absorb more radiant energy.
  • When radiant energy is absorbed by a material, the thermal energy of the material increases.
  • The transfer of energy by radiation is most important in gases.
  • Because molecules are much farther apart in gases than in solids or liquids, radiation usually passes more easily through gases than through solids or liquids.

• Controlling Heat Flow

  • Animals have different features that help them control heat flow.

• Insulator: A material in which heat flows slowly

  • Materials, such as metals, that are good conductors of heat are poor insulators.
  • Gases, such as air, are usually much better insulators than solids or liquids.
  • Insulation, or materials that are insulators, 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, saving energy.
  • A thermos bottle uses a vacuum and reflective surfaces 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.

Section 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.
  • The most common type of heating system in use today is the forced-air system
  • In forced-air systems, air heated by the furnace gets blown through ducts that usually lead to every room.
  • Before forced-air systems were widely used, many homes and buildings were heated by radiators.
  • A radiator is a closed metal container that contains hot water or steam.
  • An electric heating system has no central furnace. Instead, electrically heated coils placed in floors and in walls heat the surrounding air by conduction.

• Solar Heating

  • The Sun emits an enormous amount of radiant energy that strikes Earth every day. The radiant energy from the Sun can be used to help heat homes and buildings.
  • In passive solar heating systems, materials inside a building absorb radiant energy from the Sun during the day and heat up.
  • In a passive solar heating system, radiant energy from the Sun is transferred to the room through windows. Windows also prevent air inside from mixing with cooler air outside.
  • Solar Collectors: absorbs radiant energy from the Sun.

• Thermodynamics: Study of the relationship among thermal, energy, and heat

  • A system is anything you can draw a boundary around.
  • The heat transferred to a system is the amount of heat flowing into the system that crosses the boundary.
  • The work done on a system is the work done by something outside the system’s boundary.
  • First Law of Thermodynamics: the increase in thermal energy of a system equals the work done on the system plus the heat transferred to the system.
  • Doing work on a system is a way of adding energy to a system.
  • The increase in energy of a system equals the energy added to the system.
  • A system is an open system if heat flows across the boundary or if work is done across the boundary.
  • According to the first law of thermodynamics, the thermal energy of a closed system doesn’t change.
  • When heat flows from a warm object to a cool object the thermal energy of the warm object decreases and the thermal energy of the cool object increases.
  • Second Law of Thermodynamics: It is impossible for heat to flow from a cool object to a warmer object unless work is done.

• Converting Heat to Work

  • Heat Engine: A device that converts heat into work
  • Internal Combustion Engine: The heat engine in a car
  • Almost three fourths of the heat produced in an internal combustion engine is not converted into useful work.

• Heat Movers

  • The second law of thermodynamics allows heat to move from a cold to a warm object if work is done in the process.
  • A refrigerator does work as it moves heat from inside the refrigerator to the warmer room. The energy to do the work comes from the electrical energy the refrigerator obtains from an electrical outlet.
  • An air conditioner is another type of heat mover. It operates like a refrigerator, except that warm air from the room is forced to pass over tubes containing the coolant.
  • A heat pump is a two-way heat mover. In warm weather, it operates as an air conditioner. In cold weather, a heat pump operates like an air conditioner in reverse.
  • As perspiration evaporates from your skin, it carries heat away, cooling your body.
  • Every day many energy transformations occur around you that convert one form of energy into a more useful form.

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