Chapter 6

For many years scienctist thought heat was an invisible fluid that was capable of flowing from hotter objects to colder ones. They called this substance caloric. In 1798 Benjamin Thompson (later known as Count Rumford) challenged this theory. He conducted an experiment in which a cannon barrel was to be drilled in a box of water. After several hours of drilling, the water began to boil, and continued to boil as long as the drilling continued. Rumford concluded that it was the action of drilling, not a flow of caloric, that was producing heat. Several years later, James Prescott Joule investigated the relationship between motion and heat. Joule concluded that heat was a form of energy somehow related to the motion of molecules.
HOW IS HEAT TRANSFERRED?

CONDUCTION
One of the ways heat moves is known as conduction. The definition of conduction is the transfer of heat from a region of higher temperature to a region of lower temperature by increased kinetic energy. This increased kinetic energy is passed from molecule to molecule. In more simple terms, heat energy is passed when objects touch each other. Only certain materials allow heat to pass through, these materials are known as conductors. Not all materials allow conduction. Object that do not allow heat to pass through are known as insulators.

CONVECTION
Convection is the way heat is transferred from one area to another when there is a "bulk movement of matter." Warm air rises and cold air replaces it, heat has moved. So it is the transfer of heat by the movement of objects. When an area of hot water rises to the top of a pot and gives off energy, convection has taken place. The thing to remember is that the object, therefore the heat, has moved.

RADIATION
Must matter be present in order for heat to move? No, heat can move without the presence of matter. How is the earth warmed? Energy travels from the sun through the vacuum of space, all the way to the earth. No matter is present for the heat to pass through. Radiation is the ability of energy to travel through empty space.

HOW IS HEAT MEASURED?
There are 3 different temperature scales that are used.

The zero point on the Kelvin scale is absolute zero, the lowest possible temperature. This is the point at which all molecular motion stops.
The zero point on the Celsius scale equals the melting/freezing point. It is equal to 273 Kelvin units and 32 degrees Fahrenheit.
The zero point on the Fahrenheit scale is based on the freezing point of a mixture of salt and water and scientifically meaningless.
The boiling point of water is set to 100 degrees on the Celsius scale.
The interval between the melting point and the boiling point of water is divided into 100 intervals, each equal to 1 degree. This interval is the same on the Kelvin scale. 1 Kelvin unit = 1 degree Celsius.
When working with the Kelvin scale, the term "degrees" is not used. Zero degrees Celsius is written as 273 K and read as "273 Kelvins".
The Fahrenheit scale is not used in science.

Calorie: The unit of heat.

The amount of heat needed to raise the temperature of one gram of water 1 ^oC.
A device known as a caloriemeter is used to determine how much heat is found in a substance.
A food calorie is 1000 calories, or a Kilocalorie.

Specific Heat: The ability of a substance to absorb heat.

The number of calories needed to raise the temperature of one gram of a substance 1 C^o.

The units of Specific Heat are - calories per gram Celsius degree.

Heat gained or lost = (mass) (change in temperature) (specific heat)

Most substances-solids, liquids, and gases-expand when their temperature is increased. As heat is added, the kinetic energy of the molecules increases and their motions speed up. Therefore, the molecules move farther away from their fixed positions and farther away from each other. The increased distance between the molecules accounts for the expansion of the solid. Take a look at the diagram below and notice the thermal expansion of each:

USES OF HEAT

Controlling temperature is an important aspect of our daily lives. If you have ever been in a building that was either too hot or too cold, you know the importance of a good heating or cooling system. Based on how the heat is delivered, central heating systems are divided into two main groups: direct or indirect systems. A direct system circulates warm air thorughout the area to be heated. An indirect system circulates hot water or steam through pipes that lead to a radiator. The most common heating system used today is the heat pump. A heat pump is an air conditioner that contains a valve that lets it switch between "air conditioner" and "heater." Think about the word "pump." A heat pump absorbs heat out of outside air. The heat is either kept outside, which allows cold air to flow into the building, or the heat is pumped into the building. When the valve is switched one way, the heat pump acts like an air conditioner, and when it is switched the other way it reverses the flow and acts like a heater. There are five basic parts to any refrigerator or air-conditioning system:

Compressor
Heat-exchanging pipes - serpentine or coiled set of pipes outside the unit
Expansion valve
Heat-exchanging pipes - serpentine or coiled set of pipes inside the unit
Refrigerant - liquid that evaporates inside the refrigerator to create the cold temperatures
Many industrial installations use pure ammonia as the refrigerant. Pure ammonia evaporates at -27 degrees Fahrenheit (-32 degrees Celsius).

The basic mechanism of a refrigerator works like this:

The compressor compresses the refrigerant gas. This raises the refrigerant's pressure and temperature (orange), so the heat-exchanging coils outside the refrigerator allow the refrigerant to dissipate the heat of pressurization.
As it cools, the refrigerant condenses into liquid form (dark blue) and flows through the expansion valve.
When it flows through the expansion valve, the liquid refrigerant is allowed to move from a high-pressure zone to a low-pressure zone, so it expands and evaporates (light blue). In evaporating, it absorbs heat.
The coils inside the refrigerator allow the refrigerant to absorb heat, making the inside of the refrigerator cold. The cycle then repeats.

I am not recommending that you do this . . . but say you had a device commonly known as a potato cannon. When you introduce a spark, you can ignite the fuel. What is interesting, and the reason we are talking about such a device, is that a potato cannon can launch a potato about 500 feet through the air!

The potato cannon uses the basic principle behind any reciprocating internal combustion engine: If you put a tiny amount of high-energy fuel (like gasoline) in a small, enclosed space and ignite it, an incredible amount of energy is released in the form of expanding gas. You can use that energy to propel a potato 500 feet. In this case, the energy is translated into potato motion. You can also use it for more interesting purposes. For example, if you can create a cycle that allows you to set off explosions like this hundreds of times per minute, and if you can harness that energy in a useful way, what you have is the core of a car engine!

Almost all cars currently use what is called a four-stroke combustion cycle to convert gasoline into motion. The four-stroke approach is also known as the Otto cycle, in honor of Nikolaus Otto, who invented it in 1867. The four strokes are below. They are:

Intake stroke
Compression stroke
Power (combustion) stroke
Exhaust stroke

The piston starts at the top, the intake valve opens, and the piston moves down to let the engine take in a cylinder-full of air and gasoline. This is the intake stroke. Only the tiniest drop of gasoline needs to be mixed into the air for this to work.
Then the piston moves back up to compress this fuel/air mixture. Compression makes the explosion more powerful.
When the piston reaches the top of its stroke, the spark plug emits a spark to ignite the gasoline. The gasoline charge in the cylinder explodes, driving the piston down.
Once the piston hits the bottom of its stroke, the exhaust valve opens and the exhaust leaves the cylinder to go out the tail pipe.

Now the engine is ready for the next cycle, so it intakes another charge of air and gas. Trace the strokes through the diagram below.

Notice that the fuel burns inside the engine, we call this type of engine an internal combustion engine. There is such a thing as an external combustion engine. A steam engine in old-fashioned trains and steam boats is the best example of an external combustion engine. The fuel (coal, wood, oil, whatever) in a steam engine burns outside the engine to create steam, and the steam creates motion inside the engine. It turns out internal combustion is a lot more efficient (takes less fuel per mile) than external combustion, plus an internal combustion engine is a lot smaller than an equivalent external combustion engine. This explains why we don't see any cars from Ford and GM using steam engines. Notice the steam engine below.

Additional assignments
1. Online Practice Test
2. Interactive Tutor

CONDUCTION One of the ways heat moves is known as conduction. The definition of conduction is the transfer of heat from a region of higher temperature to a region of lower temperature by increased kinetic energy. This increased kinetic energy is passed from molecule to molecule. In more simple terms, heat energy is passed when objects touch each other. Only certain materials allow heat to pass through, these materials are known as conductors. Not all materials allow conduction. Object that do not allow heat to pass through are known as insulators.
CONVECTION Convection is the way heat is transferred from one area to another when there is a "bulk movement of matter." Warm air rises and cold air replaces it, heat has moved. So it is the transfer of heat by the movement of objects. When an area of hot water rises to the top of a pot and gives off energy, convection has taken place. The thing to remember is that the object, therefore the heat, has moved.
RADIATION Must matter be present in order for heat to move? No, heat can move without the presence of matter. How is the earth warmed? Energy travels from the sun through the vacuum of space, all the way to the earth. No matter is present for the heat to pass through. Radiation is the ability of energy to travel through empty space.