Heat and Temperature

Physical Science II Bishop State Community College

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Kinetic Molecular Theory

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Heat vs Thermal Energy

These two are not the same thing! Thermal energy is, well, energy, and matter contains thermal energy but not heat. Heat is thermal energy in transit. This transfer of thermal energy only happens in one direction: from a higher temperature substance to a lower energy substance and NEVER in the opposite direction. This is the 2nd law of thermodynamics.

Temperature Scales and Conversions

There are three temperature scales that are currently in use. They all differ by the number of degrees between water freezing and boiling. The one that is most familiar to you is the Fahrenheit scale. This scale has water freezing at 32°F and boiling at 212 °F. The Celsius (same as Centigrade) is a "metric based scale" with water freezing and boiling at 0°C and 100°C respectively. The temperature scale used by many scientists is the Kelvin scale which is a more accurate measure of particle motion which is what temperature really is. Of special interest is the temperature at which all particle motion (kinetic energy) is supposed to stop. The temperature at which KE is the lowest it can be is absolute zero or 0 K. To convert between °C and °F are found below:


°C = 5/9(°F-32) or °F = 9/5°C +32

Thermal Energy and Temperature

The difference here is one of types of motion. Even though the picture here says heat (scientists can be sloppy about using the terms heat and thermal energy interchangeably). Thermal energy (internal energy) represents all types of motion (vibrational, rotational, and translation) particles can have as well as attractions and repulsions between the particles (the latter two represent potential energy) whereas temperature is just translational energy or kinetic energy. Something at a lower temperature can, in fact, have more thermal energy than something at a higher temperature...remember our demo!
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Entropy-a loss of energy!

Thermal energy is the least useful type of energy in terms of being able to do work. Every time energy is transferred, the amount of energy available to do work decreases. This dispersal of energy into "non useful" energy is entropy. It can also be defined as amount of "randomness". Entropy (S) can be expressed mathematically as ∆S = ∆Q/T. It takes energy to decrease entropy. Think about how easy it is for your room to become messy but how hard it is to reverse it! Entropy encompasses two out of the three laws of thermodynamics.

Insulators

The handle on most cookware is made of an insulator-substances like wood, etc that do not conduct heat very well.

Calculating the amount of Heat Transfer-Calorimetry

As you will see in our activities, the amount of thermal energy (Q) transferred, aka heat, depends on three things: amount of substance (mass), the substance itself (specific heat), and the temperature of the substance before and after the transfer of heat (∆T). This equation takes all three into account: Q = mc∆T
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Work and Thermal Energy

When thermal energy is transferred in and out of a system, work may be involved. Conversely, work done on or by the system can influence the amount of thermal energy in a system. The figure to the right shows the relationships between work and thermal energy.

Water is a different substance from what is predicted by the Kinetic Theory!

Water expands when it freezes-most substances do not!

The differences between solids, liquids, and gases is that the volume each phase increases in the order of solid<liquid<gas. Water, however, expands just before it freezes. Between 0°C and 4°C, water takes on a slightly different molecular structure that pushes the molecules farther apart. Kind of a good thing too because what would have happened during colder climate conditions if ice (solid water) sank to the bottom of the pond?

Mechanisms of Heat Transfer