Chapter 9 – Heat and States of Matter

Temperature and Thermal Energy Kinetic theory – explains how particles in matter behave. All matter is composed of particles. Particles are in constant random motion. Particles collide with each other and walls of their containers.

Temperature Temperature is related to the average kinetic energy of an object’s molecules. The SI unit for temperature is Kelvin (K). Two other temperature scales are the Celsius scale and the Fahrenheit scale.

Temperature Conversions To convert from oF to oC: oC = 5/9(oF – 32) To convert from oC to oF: oF = 9/5oC + 32 oC = oK – 273 oK = oC + 273

Thermal Energy The sum of the kinetic and potential energy of all atoms in an object. Thermal energy increases as temperature increases At constant temperature, thermal energy increases if mass increases. Thermal energy that flows from higher temperature to lower temperature is heat.

Specific Heat Amount of heat needed to raise the temperature of 1kg of a material by one degree C or K. Specific heat is measured in joules per kilogram per degree Celsius. J/(kgoC)

Changes in Thermal Energy Changes can be calculated as change in thermal energy equals mass (kg) X change in temperature (oC) X specific heat (J/kgoC). Q = m(Tf-Ti)C When heat flows into an object and its temperature rises, the change in temperature is positive. When heat flows out of an object and its temperature decreases, the change in temperature is negative. A calorimeter is used to measure specific heat.

States of Matter States of matter: solid, liquid, gas, plasma.

Solids Solid state: particles are closely packed together in a specific type of geometric arrangement.

Liquids Liquid State: a solid begins to liquify at the melting point as the particles gain enough energy to overcome their ordered arrangement. Energy required to reach the melting point is called the heat of fusion. Liquid particles have more space between them allowing them to flow and take the shape of their container.

Gases Gaseous state: a liquid’s particles have enough energy to escape the attractive forces of the other particles in the liquid. Heat of vaporization is the energy required for a liquid to change to a gas. At the boiling point, the pressure of the liquid’s vapor is equal to the pressure of the atmosphere, and the liquid becomes a gas. Gas particles spread evenly throughout their container in the process of diffusion.

Plasma State of matter consisting of high-temperature gas with balanced positively and negatively charged particles.

Thermal Expansion Increase in size of a substance when the temperature increases. The size of the substance will then decrease when the temperature decreases. Expansion and contraction occur in most solids, liquids, and gases. Water is an exception because it expands as it becomes solid.

Expansion Joint Damage due to thermal expansion

Transferring Thermal Energy Conduction – transfer of thermal energy through matter by direct contact of particles. Kinetic energy is transferred as particles collide. Solids, particularly metals, are good heat conductors.

Convection The transfer of energy by the motion of heated particles in a fluid is called convection. Convection currents transfer heat from warmer to cooler parts of a fluid. Convection currents create rain forests and deserts over different regions of Earth.

Radiation Energy transfer by electromagnetic waves. Some radiation is absorbed and some is reflected when it strikes a material. Heat transfer by radiation is faster in a gas than in a solid or liquid.

Thermal Insulator Material that does not let heat flow though it easily. Gases such as air usually make better insulators than liquids or solids. A jacket with air pockets is a good insulator because the air slows the flow of body heat to the colder outside air.

Using Thermal Energy Heating systems – warm homes and buildings. Forced-air system – fuel heats air, which is blown through ducts and vents; cool air is returned to the furnace to be reheated. Radiator system – hot water or steam in a radiator transfers thermal energy to the air. Electric heating system – electrically heated coils in ceilings or floors heat air by conduction.

Forced air Radiator System

Thermodynamics The study of the relationships among thermal energy, heat, and work. First Law of Thermodynamics – the increase in energy of a system equals the energy added to the system. Second Law of Thermodynamics – the increase in thermal energy of the cool object equals the decrease in the thermal energy of the warm object.

Heat Engine An engine that converts thermal energy into mechanical energy. An internal combustion engine burns fuel inside the engine in chambers or cylinders. Internal combustion engines convert only ~26% of the fuel’s chemical energy to mechanical energy.

Click for motion

Entropy A measure of how dispersed energy is. Energy (like gravity) wants to shrink, entropy wants to grow.

Understanding and Assessment Think of as many ways in which you can keep your house warmer in the winter. Categorize them according to energy source. Create a drawing showing thermal energy flow in any of the heating or cooling systems or engines studied.