## Thermodynamics (Heat, warmth)

1. Describe the three ways of heat transfer?
2. The ability of material surfaces to absorb thermal radiation
3. Thermal conduction in different materials
4. What happens when a material is heated? What is rail buckling?
5. Thermometer
6. Boiling point
7. Boiling
8. Condensation
9. Melting
10. Solidification
11. Freezing point
12. Evaporation
13. Try using the particle approach (solid – liquid – gaseous)
14. Distillation
15. What did the scientists Celsius, Fahrenheit and Kelvin do?

Describe the three ways of heat transfer?

Thermal conduction: The atoms in the metal material above the candlelight are added heat energy. The atoms start vibrating more and more. The vibrations are spread to neighbor atoms. Heat energy conducts from atom to atom farther to the right.

Thermal convection: Here is shown a saucepan with water on a hot plate. The hot plate transports heat through thermal conduction to the bottom of the saucepan. When the water is heated, the activity increases in the water molecules. The water molecules need more space and the density of the water decreases. This makes the water lighter and it floats upwards. Once the water reaches the surface it cools off and the water once again becomes denser and contracts and sinks towards the bottom. Convection cells are formed.

Thermal radiation: Heat can be transported through radiation, electromagnetic waves. The material hit by radiation absorbs part of the radiation and the material gets hot. Some materials reflect most of the radiation, e.g. a mirror. Dark surfaces absorb much of the radiation energy. The type of radiation also matters. Thermal radiation is included in visible light.

This is the only heat transfer method possible through vacuum or space. Thermal conduction and thermal convection both need a medium, i.e. matter to travel with or through.

The ability of material surfaces to absorb thermal radiation

The rougher the surface is, the greater the ability of the material to absorb thermal radiation and visible light. Black color is a natural radiation trap. When energy is absorbed by the color no radiation is emitted in the visible spectrum, i.e. radiation that we can see with our physical eyes.

No radiation in the visible spectrum leaves the surface. Energy will anyway leave the surface. This energy is emitted as infrared radiation. We cannot see this radiation, but we can experience it as heat.

Thermal conduction in different materials

If the material conducts heat well, like iron or copper, absorbed heat can efficiently be transported through the material. Metal atoms share electrons that quickly transfer energy to neighbor metal atoms. In contrast to iron and copper, wood cannot conduct heat well. The material will instead get warm on the surface – the surface that is heated. Materials such as wood insulate well. You can walk on glowing coal, since it takes relatively long time for the coal to conduct heat into the foot.

What happens when a material is heated? What is rail buckling?

When a material is heated, it takes up more space and expands. A piece of iron that gets warmer thus takes more room. An example is when rail men adds rail to a railway. If they build the railroad in the winter and add rail sections tight together, then in the summer when the rail irons are heated by the sun, the rails will take up more space and expand. The iron-rail-bars thus become longer, and if no space exists between the rail bars, this additional extension will be taken out laterally, allowing the rail bars to curve. The entire railway line will be curved.

Thermometer

A thermometer often consists of a thin glass tube with closed bottom. When the temperature raises the fluid inside expands and rises indicating a higher temperature. Not long ago mercury liquid was used, and nowadays alcohol is the fluid. Both of these liquids are fluent even at temperatures below zero. Water cannot be used in freezing conditions since water freezes to ice at 0 °C. Ice needs more space than its liquid state. This could cause the surrounding glass to break. Water is almost the only substance that has a greater volume in solid state compared to its liquid state.

Boiling point, Boiling, Condensation, Melting, Solidification, Freezing point, Evaporation

Boiling point
The boiling point is the temperature at which the substance changes from liquid to gaseous state. The gas pressure in the water (water molecules) is the same as the atmospheric gas pressure. The boiling point in water is 100 oC.

Boiling
A liquid turns to its gaseous state. See boiling point above.

Condensation
Condensation means that a gas turns to its liquid state. An example is when water vapor encounters a mirror in a bathroom. The mirror takes up heat energy from the water-gas-molecules, which means that the gas turns to liquid. Water droplets accumulate onto the mirror.

Melting
Melting is when the solid phase of a substance turns into its liquid state. An example is when iron is heated and becomes liquid.

Solidification
Solidification is the opposite of melting. A liquid loses energy and turns to its solid state, e.g. when water freezes into ice.

Freezing point
This is the temperature at which solidification occurs. Water solidifies at 0 oC.

Evaporation
For example, in water there are always some water molecules having more energy than the average molecule. A water molecule may have taken up enough energy so it can evaporate out of a water pond. This evaporation occurs at the surface. The water remaining in the pond has thus lost some energy, so it has a slightly lower temperature very locally. This lower temperature will probably not be measurable with a standard thermometer.

Try using particle approach

Particle thinking implies envision atoms or molecules in front of you. When heat or energy is supplied, you envision the atoms e.g. iron atoms vibrating more and more. The transition to liquid phase means that atoms start moving past each other. When even more energy is added the atoms begin to leave the solution and escape as gas.

The melting point of iron is 1538 °C and its boiling point is 2861 oC. If wood is heated to 300 oC, it begins instead to burn. This is because the wood fibers begin to react with oxygen and fires up.

Distillation

Distillation distinguishes substances with different boiling points. If common salt (NaCl) is dissolved in water, these two substances can be separated through distillation. Water has the boiling point 100 ºC. At this temperature water boils away and turns to its gaseous state. The salt left in the beaker has a much higher boiling point, approx. 1473 °C. The water gas (vapor) is then made to condense when its energy is diverted. Below is a set up of a distillation.

What did the scientists Celsius, Fahrenheit and Kelvin do?

Anders Celsius
Lived from 1701 to 1744. Has developed a temperature scale where 0 °C was the water boiling point (also condensation point) and 100 °C was the melting point (also freezing point) of water. His disciple Marten Strömer later changed the scale so it corresponds to the one we use today. Anders Celsius was an astronomer and physicist who worked in Uppsala. He e.g. studied the flattening of the earth’s poles.

Fahrenheit
Lived from 1686 to 1736. He was a German physicist who created a temperature scale used by the older English-speaking generations. 0 F coincides with the temperature of water and ice, when ammonium chloride is poured in. The melting point of ice is +32 F and its boiling point is +212 F. Here is the mathematic formula explaining the relationship between Fahrenheit degrees and Celsius degrees: F = 32 + 8.1C (C = degrees Centigrade).

Kelvin
Lived from 1824 to 1907. He was a British physicist. The unit of absolute temperature (Kelvin - K) is named after him. The scale starts from absolute zero, which is set to 0 K. Here the atoms are close together and locked in their positions not showing especially strong vibrations. All temperatures above 0 K means that the atoms vibrate more and more until they have so much energy that they move past each other and finally escape one another. The water melting point equals 273 K and its boiling point corresponds to 373 K.

• NGU Science
• JAN 1, 2012
• FEB 14, 2012