Heat and Temperature
Physical Science II Bishop State Community College
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Kinetic Molecular Theory
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!
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.
Conduction
You already know that metals are good conductors of heat. You also know they conduct electricity as well. Metals are able to conduct thermal energy and electricity for the same reason-mobile electrons! The valence electrons in a metal are in motion (unlike nonmetals) and that motion can conduct heat and electricity.
Convection
This picture shows the difference between conduction, convection and insulators. Conduction is through two surfaces touching and convection is a fluid phenomenon. You can think of gases as fluids too as in a hot air balloon. Convection is also related to the density of a substance which changes with temperature.
Radiation
Radiant energy is light and we will spend more time a little later learning more about light. The figure above shows the part of the electromagnetic spectrum (radiation = light= electromagnetic spectrum) we can see-the visible portion. A substance has to be pretty hot to emit visible light like the element on your electric range, etc. Other types of radiant energy include the infrared which is often referred to heat radiation. The mechanism for transfer of heat through radiation is different than for conduction and convection, no matter is required!
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
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. This concept is the same as vector analysis we have done previously. If a system absorbs 100 J of heat (+) but then does 40 J of work (-), then the change in energy is 60 J.
Why is it cooler near the water in summer and warmer near the water in winter?
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?