Chapter 14

Section 1

Temperature

•Temperature is a measure of the average kinetic energy of the particles in an object.

• Kinetic energy is the energy of motion. All moving objects have kinetic energy.

• The amount of kinetic energy that an object has depends on the object’s mass and speed.

•Temperature and Kinetic Energy All matter is made of atoms or molecules that are always moving.

• The faster the particles are moving, the more kinetic energy they have. The more kinetic energy the particles of an object have, the higher the temperature of the object is.

•The gas particles on the right have a higher average kinetic energy than those on the left. So, the gas on the right is at a higher temperature.

•Average Kinetic Energy of Particles The motion of particles in matter is random, so individual particles have different amounts of kinetic energy.

• When you measure an object’s temperature, you are measuring the average kinetic energy of the particles in the object.

•Using a Thermometer Thermometers can measure temperature because of a property called thermal expansion.

• Thermal expansion is the increase in volume of a substance in response to an increase in temperature. As a substance’s temperature increases, its particles move faster and spread out.

•Temperature Scales Three common temperature scales are the Celsius scale, the Fahrenheit scale, and the Kelvin scale.

• The Kelvin scale is the official SI temperature scale. The lowest temperature on the Kelvin scale is 0 K, which is called absolute zero.

•Temperature Conversions A given temperature is represented by different numbers on the three temperature scales.

• You can convert from one temperature scale to another using the equations shown on the next slide.

To convert Use the equation

°C °F

°F °C

°C K K °C 273

K °F °C K 273

F 95

C

32

C 59

F 32

•Expansion Joints on Highways If the weather is very hot, a bridge can heat up enough to expand. As the bridge expands, it is in danger of breaking.•Expansion joints keep segments of the bridge apart so that they have room to expand without the bridge breaking.

•Bimetallic Strips in Thermostats are made of two metals stacked in thin strips. Because the metals expand at different rates, a strip coils and uncoils in response to changes in temperature, as shown on the next slide.

•Materials and Energy Transfer Some materials transfer, or conduct, energy better than other materials do. In general, metals are good conductors of energy.•So, the steel in bridge expansion joints, the metals in bimetallic strips, and cast iron in skillets are used because they are suited to the jobs that they are supposed to do.

•Solar Heating systems use energy from the sun to heat buildings. Each part of a solar heating system is made of materials that suit its function.

•The parts of a solar heating system include clear windows or covers to let in solar energy, dark surfaces that soak up energy, and materials that prevent energy from escaping.

•Sometimes, materials that do not conduct energy are required. For example, fiberglass insulation does not conduct energy and is used to keep buildings warm.

• Poor Conductors In general, nonmetals do not conduct heat very well. Materials such as wood, cardboard, air, cork, and ceramic are poor conductors.

•To remain alive and healthy, the human body must maintain a temperature of about 37°C.

• In hot climates, people wear lightweight clothes that let air circulate next to the skin. Circulating air evaporates sweat, which helps cool the body. In cold climates, clothes are made of materials that trap body heat and prevent energy loss.

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•Heat is the energy transferred between objects that are at different temperatures.

• When two objects at different temperatures come into contact, energy is always transferred from the object that has the higher temperature to the object that has the lower temperature.

•Heat and Thermal Energy Heat is transferred in the form of thermal energy. Thermal energy is the total kinetic energy of the particles that make up a substance.

• Thermal energy depends partly on temperature. Thermal energy also depends on how much of a substance there is.

•Reaching Thermal Equilibrium The point at which two object that are touching reach the same temperature is called thermal equilibrium.

• When objects that are touching have the same temperature, they are at thermal equilibrium and no net change in the thermal energy of either one occurs.

•Thermal Conduction is the transfer of thermal energy from one substance to another through direct contact. Conduction can also occur within a substance.

•The particles of substances as different temperatures have different average kinetic energies. When such substances touch, their particles collide.

•When particles collide, particles with higher kinetic energy transfer energy to those with lower kinetic energy.

•This transfer of energy happens until all particles have the same average kinetic energy. As a result, the substances have the same temperature.

•Conductors and Insulators Substances that conduct thermal energy well are called thermal conductors. Most metals are thermal conductors.

• Substances that do not conduct thermal energy well are called thermal insulators. Wood and plastic are examples of thermal insulators.

•Convection is the transfer of thermal energy by the movement of a liquid or a gas.

• As water is heated, it becomes less dense. The warmer water rises through the cooler water above it. At the surface, the warm water cools and becomes more dense. The cooler water then sinks to the bottom and the cycle repeats.

•Radiation is the transfer of energy by electromagnetic (EM) waves. All objects radiate EM waves.

• Unlike conduction and convection, radiation can involve either a transfer of energy between particles of matter or an energy transfer across empty space.

•Radiation and the Greenhouse Effect Earth’s atmosphere allows the sun’s visible light to pass through it. The atmosphere also traps energy, too.

• This process is called the greenhouse effect. Without the greenhouse effect, Earth would be a cold, lifeless planet.

•The atmosphere traps the sun’s energy because of greenhouse gases, such as water vapor, carbon dioxide, and methane.

• Some scientists are concerned that high levels of greenhouse gases in the atmosphere may trap too much energy and make Earth too warm.

•Thermal Conductivity is the rate at which a substance conducts thermal energy.

• Because of its high thermal conductivity, metal transfers energy more rapidly than cloth does. If a piece of metal and a piece of cloth are left in sunlight and are at the same high temperature, the metal will feel hotter when touched.

•Specific Heat is the amount of energy needed to change the temperature of 1 kg of a substance by 1°C. The higher the specific heat of something is, the more energy it takes to increase its temperature.

• Most metals have very lower specific heats. On the other hand, the specific heat of water is very high.

•Heat, Temperature, and Amount Unlike temperature, energy transferred between objects can not be measured directly. Instead it must be calculated with the following equation:

heat (J) specific heat (J/kg•°C) mass (kg)

change in temperature (°C)

•Calculating Heat When the temperature of an object increases, the value of heat is positive.

• When the temperature of an object decreases, the value of heat is negative.