Chapter 9: Heat and States of Matter

Section 1: Temperature and Thermal Energy

• Kinetic Theory of Matter
  • The temperature of an object is related to the motion of the particles in that object.
  • Kinetic Theory: The motion of the particles in matter.
  • According to the kinetic theory, matter is composed of particles that are atoms, molecules, or ions that always are in random motion.
  • When particles collide, kinetic energy can be transferred from one particle to another and the direction of particle motion changes.
• Temperature: a measure of the average kinetic energy of its particles.
  • The particles in matter are moving at various speeds and have a range of kinetic energies.
  • As the average speed of the particles in random motion increases, the temperature of the substance increases.
  • Kelvin: The SI unit for temperature
• Thermal Energy: The sum of the kinetic and potential energies of all the particles in an object
  • According to the kinetic theory, atoms and molecules that make up matter are in constant motion.
  • A ball rolling across the floor has greater kinetic energy than if it were sitting still.
• 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 in joules.
  • Thermal energy always flows from a warmer to a cooler material
• Specific Heat: The amount of thermal energy needed to raise the temperature of 1 kg of some material by 1°C
  • As a substance is heated, its temperature change depends both on the amount of thermal energy added and the nature of the substance.
  • Specific heat is measured in joules per kilogram per degree Celsius, which is written as J/(kg °C).
  • Change in Thermal Energy Equation: mass (kg) X change in temperature (°C) X specific heat (J/kg°C)
• 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 thermal energy 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.
  • When the initial and final temperatures of the water are known, and the thermal energy gained by the water can be calculated.

Section 2: States of Matter

• Four States of Matter
  • Every day you encounter different states of matter.
  • The differences between solids, liquids and gases are due to differences in the attraction between particles.
  • The particles of a solid are packed closely together and are constantly vibrating in place. Because the attractions between particles are strong, solids have a fixed volume and shape.
  • In solids, strong attractive forces hold particles in place. Particles in liquids and gases do not have fixed arrangements. Particles are farther apart in gases than in solids and liquids.
  • In a liquid the attractive forces between particles are weaker than in a solid.
  • Liquids have a definite volume, but not a definite shape.
  • In a gas, the particles are much farther apart than in a liquid or a solid.
  • Gases do not have a definite shape or volume.
  • A gas contains mostly empty space.
  • The density of gases is much less than the density of solids and liquids.
  • Plasma: matter consisting of positively and negatively charged particles and does not have a definite shape or volume.
  • It results from collisions between particles moving at such high speeds that electrons are knocked from the atoms, leaving only charged particles.
  • The most common state of matter in the universe is the plasma state.
  • The Sun and other stars contain matter that is in the plasma state. Plasma usually exists where the temperature is extremely high.
• Changing States
  • What causes an ice cube to melt into liquid water?
  • As the temperature of the ice increases, its particles move faster, and the attractive forces between them aren’t strong enough to keep them in place. The particles can slip out of their ordered arrangement, and the ice melts.
  • Heat of Fusion: The amount of energy required to change 1 kg of a substance from a solid to a liquid at its melting point
  • The heat of fusion is also the energy released when a liquid freezes, or changes to a solid.
  • The heat of vaporization is also the amount of energy released during condensation, when a gas changes into a liquid.
  • Vaporization occurs as liquid changes into a gas.
  • Vaporization can occur within the liquid or at the surface of the liquid.
  • Evaporation causes the temperature of the liquid to decrease.
  • Evaporation: Vaporization that occurs at the surface of a liquid
  • The boiling point of a liquid is the temperature at which the pressure of the vapor in the liquid is equal to the external pressure acting on its surface.
  • Heat of Vaporization: the amount of energy required for 1 kg of the liquid at its boiling point to become a gas.
  • Boiling occurs throughout a liquid when the pressure of the pockets of gas forming in the liquid equals the pressure of the vapor at the surface of the liquid.
  • The formation of dew is an example of condensation. Dew forms when the air around matter, such as spider webs, cools enough so that water vapor in the air changes to a liquid.
• Thermal Expansion
  • Expansion joints provide space for the materials to expand on hot days without causing the materials to become warped or cracked.
  • According to the kinetic theory, the particles in an object are in constant motion. The speed of these particles increases as the temperature of the object increases.
  • The forces between the particles in liquids are usually weaker than the forces between the particles in a solid.
  • In a gas, the forces between particles are much weaker than they are in liquids.
  • The air inside the hot-air balloon expands as it is heated. Its density decreases and the balloon rises.

Section 3: Transferring Thermal Energy

• Conduction: This transfer of thermal energy between colliding particles
  • When thermal energy is transferred by conduction, it is transferred by the collisions between particles, not by the movement of matter
  • Thermal energy can be transferred by conduction in all materials. However, the rate at which thermal energy is transferred depends on the material.
  • Thermal Conductor: A material in which thermal energy is transferred easily
  • Solids usually are better thermal conductors than liquids or gases. The best thermal conductors are metals.
• Convection: the transfer of thermal energy in a fluid by the movement of fluid from place to place
  • When convection occurs, particles with more energy transfer energy to other particles in the fluid as they move from place to place.
  • When a fluid expands, its volume increases, but its mass doesn’t change.
  • The difference in densities between warmer and cooler fluids can cause convection currents to occur.
  • Convection currents result when heated fluid rises and cooler fluid sinks.
  • Earth’s atmosphere is made of various gases and is a fluid.
  • Not all of the Sun’s radiant energy reaches Earth’s surface. Some of it is reflected or absorbed by the atmosphere. Even some of the radiation that reaches Earth’s surface is reflected.
• Thermal Insulator: A material in which thermal energy moves slowly
  • A jacket and an oven mitt are examples of thermal insulators.
  • Examples of materials that are thermal insulators are wood, some plastics, fiberglass, and air. Materials that are good conductors of thermal energy, such as metals, are poor thermal insulators.

Section 4: Using Thermal Energy

• Heating Systems
  • Heating systems add thermal energy to the interiors of rooms.
  • The most common type of heating system in use today is the forced-air system
  • In a forced-air system, heated air is blown through ducts that lead to rooms in the building.
  • Before forced-air systems were widely used, many homes and buildings were heated by radiators.
  • Electric heaters convert electrical energy to thermal energy. Thermal energy is transferred to the room mainly by radiation and convection.
  • Electric heating systems are not as widely used as forced-air systems.
• Thermodynamics: The study of the relationship among thermal energy, heat, and work
  • The energy transferred to a system is the amount of energy flowing into the system across the boundary.
  • First Law of Thermodynamics: the increase in thermal energy of a system equals the work done on the system plus the thermal energy transferred to the system.
  • A bicycle air pump can be a system. Work is done on the system by pushing down on the handle. This causes the pump to become warm and thermal energy is transferred from the system to the air around it.
  • A system is an open system if thermal energy flows across the boundary or if work is done across the boundary.
  • If no thermal energy flows across the boundary and no outside work is done, the system is a closed system.
  • Second Law of Thermodynamics: States that it is impossible for thermal energy to flow from a cool object to a warmer object unless work is done
• Converting Thermal Energy to Work
  • Heat Engine: A device that converts thermal energy into work
  • When thermal energy is converted into work, some thermal energy always is transferred to the surroundings.
  • The heat engine in a car is an internal combustion engine, in which fuel is burned inside the engine in chambers or cylinders.
  • The up-and-down movement of a piston in an auto- mobile engine consists of four separate strokes. The four strokes form a cycle that is repeated many times each second by each piston.
• Moving Thermal Energy
  • A refrigerator does work on the coolant in order to transfer thermal energy from inside the refrigerator to the warmer air outside. Work is done when the compressor compresses the coolant, causing its temperature to increase.
• Entropy: a measure of how spread out, or dispersed, energy is.
  • Entropy increases when energy becomes more spread out and less concentrated.
  • All events that occur, such as the production of work in an automobile engine, must obey the entropy principle.
  • Entropy Principle: all events that occur cause the entropy of the universe to increase.
  • Any event that occurs causes the amount of useable energy to decrease.

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