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### Thermodynamics

Day 1 of 9

Today’s Agenda

- What is heat?
- How does heat relate to energy?
- How does heat relate to work?
- Different types of heat transfer methods?
- What is thermal energy and what is temperature?
- Different temperature scales?

Coming up

- Lab this Friday
- Don’t forget: Thursday, January 23rd = test day
- Friday, January 24th = Raytheon Day (glider)

What is thermodynamics?

- It is the branch of science that deals with the relationship between heat and other forms of energy (such as mechanical, electrical, or chemical energy).
summarized:Thermodynamics is the study of heat

What is heat?

- Heat is the energy that flows between two objects as a result of a difference in temperature
- Energy never flows from a colder object to a hotter object
- The symbol Qis used to represent the amount of heat
- If Q is negative = heat has left the object
- If Q is positive = heat has been absorbed by the object
- The unit for heat is Joules (J)

What is temperature?

- Temperature: measures molecules’ averagekinetic energy
- Temperature does not depend on the number of particles in the object

Atomic Theory of Matter

On a microscopic scale, the arrangements of molecules in (a) solids, (b) liquids, and (c) gases are quite different.

TEMPERATURE SCALES

- There are three scales for temperature:
- Celsius, Kelvin, Fahrenheit

- Swedish astronomer and physicist Anders Celsius developed a scale in 1741 based on water. Pure water freezes at 0° Celsius and boils at 100° Celsius

MORE ABOUT TEMPERATURE

- The wide range of temperatures in the universe indicate there is apparently no upper limit.
- (inside the sun is at least 1.5 x 107°C, and other stars are even hotter)

- Temperatures do have a lower limit:
- If you cool a molecule of an ideal gas (one in which the particles occupy a tremendously large volume compared to their own size and don’t interact) it contracts in such a way that it occupies a volume that is onlythe size of the molecules at -273.15°C

- This is called Absolute Zero
- Usually rounded to -273°C

THE KELVIN TEMPERATURE SCALE

- The Kelvin temperature scale is based on absolute zero. At absolute zero (-273°C), the temperature is 0 K.
- Each interval on this scale is called a kelvin
- Equal in “size” of a Celsius degree
- TC + 273 = TK
- Physicists have reached temperatures as low as 2.0x10-9 K (0.000000002 K)

Temperature and Thermometers

- Common thermometers used today include the liquid-in-glass type and the bimetallic strip.

Temperature and Thermometers

Temperature can also be measured using either the Fahrenheit or the Celsius scale.

The freezing point of water is 0°C, or 32°F; the boiling point of water is 100°C, or 212°F.

What is thermal energy?

- The sum totalof all the energy of all the particles/molecules in an object is its thermal energy.
- Not all particles in an object have the same energy
- Some have high energy, some have low, but the average energy of particles is higher in a hot body than in a cooler body

Heat transfer methods

- Conduction
- Convection
- Radiation

Heat Transfer: Conduction

- Heat conduction can be visualized as occurring through molecular collisions.

Heat Transfer: Convection

- Convection occurs when heat flows by the mass movement of molecules from one place to another. It may be natural or forced; both these examples are natural convection.

Heat Transfer: Convection

- Many home heating systems are forced hot-air systems; these have a fan that blows the air out of vents, rather than relying completely on natural convection.
- Our body temperature is regulated by the blood; it runs close to the surface of the skin and transfers heat. Once it reaches the surface of the skin, the heat is released through convection, evaporation, and radiation.

Heat Transfer: Radiation

- If you are sitting in a place that is too cold, your body radiates more heat than it can produce. You will start shivering and your metabolic rate will increase unless you put on warmer clothing.
- Radiation is the emission of energy as electromagnetic waves (which do not need a medium, such as a liquid or gas, to go through)

Heat Transfer: Radiation

- Thermography – the detailed measurement of radiation from the body – can be used in medical imaging. Warmer areas may be a sign of tumors or infection; cooler areas on the skin may be a sign of poor circulation.

2) one Kelvin unit

3) one Fahrenheit degree

4) both one Celsius degree and one Kelvin unit

5) both one Fahrenheit degree and one Celsius degree

Which is the largest unit: one Celsius degree, one Kelvin unit, or one Fahrenheit degree?

2) one Kelvin unit

3) one Fahrenheit degree

4) both one Celsius degree and one Kelvin unit

5) both one Fahrenheit degree and one Celsius degree

Which is the largest unit: one Celsius degree, one Kelvin unit, or one Fahrenheit degree?

The Celsius degree and the Kelvin degree are the same size. The scales only differ by an offset, not by the size of the degree unit. For Fahrenheit, there are 180 degrees between boiling and freezing (212°F–32°F). For Celsius, there are 100 degrees between the same points, so the Celsius (and Kelvin) degrees must be larger.

1) yes, at 0 °C

2) yes, at -273 °C

3) yes, at 0 K

4) no

It turns out that – 40°C is the same temperature as – 40°F. Is there a temperature at which the Kelvin and Celsius scales agree?

1) yes, at 0 °C

2) yes, at -273 °C

3) yes, at 0 K

4) no

It turns out that – 40°C is the same temperature as – 40°F. Is there a temperature at which the Kelvin and Celsius scales agree?

The Celsius and Kelvin scales differ only by an offset, which is 273 degrees. Therefore, a temperature on one scale can never match the same numerical value on the other scale. The reason that such agreement is possible for Celsius and Fahrenheit is the fact that the actual degree units have different sizes (recall the previous question).

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