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Temperature, Heat, and the Phases of Matter

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Chap 7 Energy and Chemical Reactions


Temperature Scales

From CPO Physical Science, Chapter 11

There are two common temperature scales. On the Fahrenheit scale, water freezes at 32 degrees and boils at 212 degrees (Figure 11.1). There are 180 Fahrenheit degrees between the freezing point and the boiling point of water. Temperature in the United States is commonly measured in Fahrenheit; 72 °F is a comfortable room temperature.

The Celsius scale divides the interval between the freezing and boiling points of water into 100 degrees (instead of 180). Water freezes at 0 °C and boils at 100 °C. Most scientists and engineers use Celsius because 0 and 100 are easier to work with than 32 and 212.

A weather report that says 21 °C in London, England, predicts a pleasant day, good for shorts and a T-shirt. A weather report predicting 21 °F in Minneapolis, Minnesota, means a heavy winter coat, gloves, and a hat. Because the U.S. is one of a few countries that use the Fahrenheit scale, it is useful to know how to convert between Fahrenheit and Celsius.







We can sense hot and cold, but not very accurately. A thermometer is an instrument that measures temperature. A type of thermometer you have likely seen uses colored liquid alcohol to sense temperature. As the temperature increases, the alcohol expands and rises up a long, thin tube. You tell the temperature by the height the alcohol rises. The tube is long and thin so a small change in volume makes a large change in the height (Figure 11.2).

How thermometers work: All thermometers are based on a physical property (such as color or volume) that changes with temperature. A thermistor is a device that changes its electrical resistance as the temperature changes. Some electronic thermometers sense temperature by measuring the resistance of a thermistor. There are some chemicals that change color at different temperatures. These are used for aquarium “sticker” thermometers that are placed on the outside of a fish tank.

What temperature really is:

Imagine you had a microscope powerful enough to see individual atoms. You would see that atoms are in constant motion, even in a solid object. The atoms are not fixed in place, but act like they are connected by springs (Figure 11.3).

Each atom stays in the same average place, but constantly jiggles back and forth in all directions. As you might guess, the “jiggling” means motion and motion means energy. The back-and-forth jiggling of atoms is caused by thermal energy, which is a kind of kinetic energy.

Temperature and energy:

 Thermal energy is proportional to temperature. When the temperature goes up, the energy of motion increases. That means the atoms jiggle around more vigorously. The higher the temperature, the more thermal energy atoms have and the faster they move around. Temperature measures a particular kind of kinetic energy per atom.

Random versus average motion: If you throw a rock, the rock gets more kinetic energy, but the temperature of the rock does not go up. How can temperature measure kinetic energy then? The answer is the difference between random and average.

For a collection of many atoms (like a rock), the
kinetic energy of has two parts. The kinetic energy of the thrown rock comes from the average motion of the whole collection; the whole rock. This kinetic energy is not what temperature measures.

Random motion: Each atom in the rock is also jiggling back and forth independently of the other atoms in the rock. This jiggling motion is random. Random motion is motion that is scattered equally in all directions. On average, there are as many atoms moving one way as there are moving the opposite way. Temperature measures the kinetic energy in the random motion. Temperature is not affected by any kinetic energy associated with average motion. That is why throwing a rock does not make it hotter (Figure 11.4)

There is a limit to how cold matter can get. As the temperature is reduced. molecules move more and more slowly. When the temperature gets down to absolute zero, molecules have the lowest energy they can have and the temperature cannot get any lower.

You can think of absolute zero as the temperature where molecules are completely frozen, with no motion. Technically, molecules never become absolutely motionless, but the kinetic energy is so small it might as well be zero. Absolute zero occurs at minus 273 °C (–459 °F). You cannot have a temperature lower than absolute zero.

The Kelvin scale

A temperature in Celsius measures only relative thermal energy, relative to zero Celsius. The Kelvin temperature scale is useful in science because it starts at absolute zero. A temperature in Kelvins measures the actual energy of atoms relative to zero energy.

The Kelvin (K) unit of temperature is the same size as the Celsius degree. However, water freezes at 273K and boils at 373K.

Most of the outer planets and moons have temperatures closer to absolute zero than to the freezing point of water. To convert from Celsius to Kelvins you add 273 to the temperature in Celsius. For example, a temperature of 21 °C is equal to 294K (21 + 273).

High temperatures: While absolute zero is the lower limit for temperature, there is no practical upper limit. Temperature can go up almost indefinitely. As the temperature increases, exotic forms of matter appear. For example, at 10,000 °C, atoms start to come apart and become a plasma. In a plasma, atoms are broken apart into separate positive ions and negative electrons. Plasma conducts electricity and is formed in lightning and inside stars.

The Heat Scale of the Universe

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