# Heat Chapter 9.

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Heat Chapter 9

9.1 Heat Energy Objectives Relate heat energy to moving molecules.
Distinguish between the Celsius and Kelvin temperature scales. Compare temperature, internal energy, and heat energy. Define operationally a temperature change.

9.1 Heat Energy

9.1 Heat Energy You experience heat all the time, and you use words such as hot, cold, and heat often. In science, heat has a different and very specific meaning.

Molecules in Motion Recall that all matter is made of molecules that are in constant motion. Matter that moves has kinetic energy. The measurement of the average kinetic energy of the molecules in a substance is called temperature. If two objects with different temperatures come into contact, energy flows from the object with the higher temperature to the one with the lower temperature. The gas molecules that make up air move freely all around you. Molecules of water move about in a container. The molecules in your chair constantly move back and forth, or vibrate.

Molecules in Motion Energy that is transferred from one substance to another is called heat energy. Heat is energy that flows between objects that have different temperatures. The official SI unit of heat is the joule. When heat energy transfers to a substance, it adds to the internal energy of the substance. Internal energy is the total amount of energy a substance contains. Most of the internal energy of a substance is kinetic energy.

Measuring Temperature
Temperature is a measure of the average movement of the molecules in a substance. Volume doesn't affect temperature. You probably think about how hot or cold something is in terms of its temperature. When the temperature of a substance is high, its molecules are moving rapidly. Imagine that you fill a bucket with water from a bathtub. Which is has the greater temperature? The temperature of the water is the same in each container

Measuring Temperature
How do you measure temperature? You measure temperature with a thermometer. As a thermometer heats up, the molecules in its liquid begin to move faster and farther apart. The liquid expands and rises in the tube. The scale indicates the temperature reading. In SI, two different scales are used for temperature readings; the Celsius scale and the Kelvin scale. Most thermometers are thin glass tubes connected to a reservoir of liquid mercury or colored alcohol. A numbered scale is marked on the outside of the tube.

Celsius Temperature Scale
The Celsius scale is commonly used to measure temperature. The Celsius scale was developed by Anders Celsius, a Swedish astronomer.

Kelvin Temperature Scale
Another SI temperature scale is called the Kelvin scale. The Kelvin scale, also called the absolute scale, is named for its originator, Lord Kelvin. Kelvin thermometers are used primarily in the physical sciences.

Kelvin Temperature Scale
The Kelvin scale is used to measure the temperature of super-cold substances. The Kelvin scale identifies the temperature where molecules in a substance are so cold they don't move. They have no kinetic energy. This temperature is called absolute zero. The Kelvin temperature scale is used in science because it relates directly to energy. For example, I cm3 of a substance at 200 K has twice the kinetic energy of I cm3 of the same substance at 100 K.

SI Temperature Scale Compare the Celsius and Kelvin thermometers. Notice that 0 K equals -273°C. You can convert the Celsius temperature of an object to the Kelvin temperature by adding 273. Since the boiling point of water is 100°C, the boiling point of water on the Kelvin scale is 100 plus 273, or 373 K. What is the freezing point of water on the Kelvin scale?

Check and Explain pg. 212

9.2 Transfer of Heat Objectives
Identify three ways heat is transferred. Explain how heat transfer is measured. Calculate the amount of heat transferred between two substances. Infer how heat transfer affects climate.

Transfer of Heat Heat is transferred by conduction, convection and radiation.

Conduction Heat energy flows from a warm substance to a cool substance. When solid substances are in contact, heat energy transfers by conduction. Conduction is the transfer of heat energy throughout a substance, or when one substance comes in contact with another. When you put the lower par t of a spoon in boiling water, heat transfers from the boiling water to the cool spoon. The spoon's molecules that are in contact with the water move more rapidly and collide with the molecules in the upper portion of the spoon. Through a continuous serles of collisions, heat energy transfers throughout the entire spoon. The hot spoon handle is evidence that heat energy has been transferred.

Conduction Some materials conduct heat better than others.
For example, a cloth pot holder conducts heat poorly. It is an insulator. Wood, plastic, and glass are also insulators. The sun heats the wood door and the metal doorknob on the outside. Soon, the inside doorknob gets hot also. Why doesn't the inside of the door get hot?

Convection Heat can also be transferred by a process called convection. Convection is the transfer of heat energy by the movement of a fluid, such as a liquid or a gas. Air in contact with a flame spreads out to warm an entire room. Water boils when heated in a pan.

Convection Heat energy flows through a fluid in a pattern called a convection current. Convection currents form because heated fluid expands and is less dense than surrounding fluid, which is cooler. The difference in the density causes warm fluid to rise and cooler fluid to sink. The result is a convection current that moves heat energy through the fluid.

Convection If you have ever boiled water, then you have observed convection currents in a liquid. Convection currents cause the rolling motion of gently boiling water. The water at the bottom of the pan heats up. The heated water molecules move faster, spread apart, and become less dense. They rise to the surface. Cooler, denser water at the top moves downward toward the bottom of the pan. This water then heats up and rises. Convection currents continue to form as long as there is a heat source. To understand how convection currents form in a gas, think about how heat moves in an oven.

Radiation Radiation differs from conduction and convection because matter isn't needed to transfer energy by radiation. Recall that conduction depends on the collision between the molecules of a substance. Convection depends on the expansion of a fluid when molecules collide. Radiation can occur in a vacuum, where no molecules of matter are present.

Radiation Any form of energy that is transferred by radiation is called radiant energy. Radiant energy and radiation shouldn't be confused with harmful radioactivity, or nuclear radiation. When any object gets hot, it gives off radiant energy. Nuclear radiation - A term used to denote alpha particles, neutrons, electrons, photons, and other particles which emanate from the atomic nucleus as a result of radioactive decay and nuclear reactions.

The earth and its atmosphere reflect and absorb radiant energy from the sun. Most of the sun's radiant energy that reaches the earth is in the form of light.

Measuring Heat Transfer
Which one would you rather stir a hot stew with? Not only is wood a poor conductor of heat, its temperature increases less than many other substances when it absorbs heat energy. Wooden spoons are often used in cooking to stir hot liquids and sauces. Wood is a poor conductor, so the handle of the spoon stays cool while the hot liquid is stirred. There is another reason wooden spoons are used in cooking. Not only is wood a poor conductor, its temperature increases less than many other substances when it absorbs heat energy. For example, when metal and wooden spoons of equal mass absorb the same amount of heat energy, the wooden spoon's temperature increases less. 22 g 22 g

Measuring Heat Transfer
Specific Heat The effect of heat energy on a substance‘s temperature is a physical property of that substance. Each substance requires a different amount of heat to raise its temperature 1°C. The amount of heat 1 g of a substance must absorb to raise its temperature 1°C is called the specific heat of the substance. Specific heat can be used to identify a substance.

Measuring Heat Transfer
Calorie You know that the official SI unit of heat is the joule. Another unit still commonly used to measure heat is the calorie. One calorie equals J. One calorie is the amount of heat needed to raise the temperature of 1 g of water 1°C. Five calories of heat are needed to raise the temperature of 1 g of water soc. Energy is measured in joules. Temperature is measured in C0or F0 Joule - A unit of energy equal to the work done when a force of one newton acts through a distance of one meter. One joule is the equivalent of one watt of power radiated or dissipated for one second. To get an idea of the joule in everyday terms, consider the following examples. A joule is the amount of energy needed to raise the temperature of one gram of cool, dry air by one degree Celsius. A quiet person produces 100 joules of heat energy every second. Finally, the amount of energy required to lift a small apple one meter against the Earth's gravity is roughly equivalent to one joule.

Solar Radiation Heat energy from the sun reaches the earth by radiation. Solar radiation is seen as ‘sunshine’ and felt as ‘heat’.

Heat in the Atmosphere Radiant energy from the sun warms the earth's surface. The earth's surface transfers heat to the air by conduction. Convection currents form as air is heated by the earth's surface.

Heat in the Atmosphere Warm air expands and rises. It displaces the cool, dense air at higher elevations. The dense air sinks. The warm rising air carries moisture to higher elevations. As the air cools, the moisture condenses and falls to the earth as rain or snow. Convection currents in the atmosphere are called convection cells. Convection cells are responsible for the wind and rain patterns over the earth's surface.

Check and Explain pg. 220

9.3 Heat & Matter Objectives
Describe the role of heat in phase changes. Identify three examples of thermal expansion. Generalize about what occurs at the boiling point or melting point of a substance. Interpret data to identify an unknown substance by its boiling point.

Heat and Phase Changes Recall that matter can change phases from solid to liquid to gas. When heat is applied to ice, the water molecules in the ice vibrate faster. The ice melts as it changes from a solid to a liquid. The temperature doesn't change until all the ice melts. When all the ice is melted, additional heat causes the temperature of liquid water to rise again until it reaches 100°C. At 100°C, a phase change occurs and the water evaporates, or changes to its gaseous phase.

Heat and Phase Changes Phase changes also occur when heat leaves matter.

Boiling Point The boiling point is the temperature at which a substance changes from a liquid phase to a gas phase. As a molecule gains heat energy their motion increases. Eventually the molecules move so fast, that they break away from the liquid and move away as a gas. Every substance has its own boiling point.

Melting & Freezing Point
The melting point is the temperature at which a solid becomes a liquid. The melting point of water is 0°c and the freezing of water is also 0°c Melting point is an important physical property of a substance. Scientists use melting-point tables to help identify unknown substances. If heat energy is applied to ice, at O°C the ice will melt. If liquid water is cooled to O°C, it will freeze. - Every substance has its own Melting Point. - For any substance, the melting point and freezing point are always at the same temperature.

Thermal Expansion An increase in the volume of a substance due to heat is called thermal expansion. Thermal expansion occurs in solids, liquids, and gases. As the molecules move away from each other, the substance expands.

Check and Explain pg. 226