Heat Conduction?

Question

3 Questions, an answer to any would be great.

1. How fast does heat conduct in air?
2. How many times faster does heat conduct in water than air?
3. How many times faster does heat conduct in a solid than in air? (eg a plank of wood and a piece of metal)

Answer 1

Conduction occurs when two object at different temperatures are in contact with each other. Heat flows from the warmer to the cooler object until they are both at the same temperature. Conduction is the movement of heat through a substance by the collision of molecules. At the place where the two object touch, the faster-moving molecules of the warmer object collide with the slower moving molecules of the cooler object. As they collide, the faster molecules give up some of their energy to the slower molecules. The slower molecules gain more thermal energy and collide with other molecules in the cooler object. This process continues until heat energy from the warmer object spreads throughout the cooler object. Some substances conduct heat more easily than others. Solids are better conductor than liquids and liquids are better conductor than gases. Metals are very good conductors of heat, while air is very poor conductor of heat. You experience heat transfer by conduction whenever you touch something that is hotter or colder than your skin e.g. when you wash your hands in warm or cold water.
In liquids and gases, convection is usually the most efficient way to transfer heat. Convection occurs when warmer areas of a liquid or gas rise to cooler areas in the liquid or gas. As this happens, cooler liquid or gas takes the place of the warmer areas which have risen higher. This cycle results in a continous circulation pattern and heat is transfered to cooler areas. You see convection when you boil water in a pan. The bubbles of water that rise are the hotter parts of the water rising to the cooler area of water at the top of the pan. You have probably heard the expression "Hot air rises and cool air falls to take its place" - this is a description of convection in our atmosphere. Heat energy is transfered by the circulation of the air.
Both conduction and convection require matter to transfer heat. Radiation is a method of heat transfer that does not rely upon any contact between the heat source and the heated object. For example, we feel heat from the sun even though we are not touching it. Heat can be transmitted though empty space by thermal radiation. Thermal radiation is a type electromagnetic radiation.Radiation is a form of energy transport consisting of electromagnetic waves traveling at the speed of light. No mass is exchanged and no medium is required.
Objects emit radiation when high energy electrons in a higher atomic level fall down to lower energy levels. The energy lost is emitted as light or electromagnetic radiation. Energy that is absorbed by an atom causes its electrons to "jump" up to higher energy levels. All objects absorb and emit radiation.
When the absorption of energy balances the emission of energy, the temperature of an object stays constant. If the absorption of energy is greater than the emission of energy, the temperature of an object rises. If the absorption of energy is less than the emission of energy, the temperature of an object falls.

Answer 2

1...Heating fluids is not 'Conduction'. (Heat transfer by the collision of molecules in a solid).
In air (gases) the molecules are widely spread apart and the heat is transferred by 'Convection' and, the higher the heat input the greater the expansion of the gas, and the faster the Convection rate due to the size of the thermal currents that are set up.
2...Again, in water, it's the same mechanism that transfers heat. The molecules of water are heavier than air and set closer together. The convection currents in a liquid are therefore slower than in a gas. Again, it depends on the heat input. (When you boil water, if the heat input is high, the water really boils hard and the 'rolling' motion of the liquid is very fast).
3...In solids, we have 'Conduction' due to the collision of the molecules. If you hold a metal bar in a flame, for a while, the end in your hand remains cool. As the molecules collide with each other, the collisions, as the bar gains heat, causes the temperature to increase along the bar and eventually, the end in your hand will begin to heat up.
Metals are good heat conductors (copper being one of the best). But, the heat conduction through metals is not very fast due to their being even heavier, and, once again depends on the heat input.
Wood is a poor conductor of heat and, due to its grainy material, it has pockets of air trapped in the grain, it will ignite rather than conduct heat if enough heat is added.

In all cases, the addition of heat energy increases the Kinetic Energy of the molecules.

Answer 3

Thermal conductivity is expressed as the amount of heat energy that passes through a unit surface area of the material per unit of time (W/m^2), for a unit temperature gradient per unit distance (K/m). Dividing the former by the latter, we get (in SI units) watts/(meter*kelvin), or W/(m*K).

Here are some thermal conductivity values, in W/(m*K):
air = 0.024
wood (balsa) = 0.055
wood (pine) = 0.12
wood (oak) = 0.17
water = 0.58
stainless steel = 16
mild steel = 46
aluminum = 250

I think you can work out the rest at this point.

Answer 4

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Answer 5

heat transfer in air is called radiation it is quite fast
heat transfer in a liquid s called convection and its faster
heat in wood is quite slow since it is an insulator
heat in a metal like silver gold and copper would conduct heat fastest bcos they are the best conductors of heat
The only method of heat transfer in opaque solids is conduction. If the temperature at one end of a metal rod is raised by heating, heat is conducted to the colder end. The exact mechanism of heat conduction in solids is not entirely understood, but it is believed to be due in part to the motion of free electrons, which transport energy if a temperature difference is applied. This theory helps to explain why good electrical conductors also tend to be good heat conductors. It was not until 1822 that the French mathematician Jean-Baptiste Joseph Fourier gave precise mathematical expression to what is now called Fourier’s law of heat conduction. This law states that the rate at which heat is conducted through a body per unit cross-sectional area is proportional to the negative of the temperature gradient existing in the body.

The proportionality factor is called the thermal conductivity of the material. Materials such as gold, silver, and copper have high thermal conductivities and conduct heat readily, but materials such as glass and asbestos have values of thermal conductivity hundreds and thousands of times smaller, conduct heat poorly, and are referred to as insulators. In engineering applications it is frequently necessary to know the rate at which heat will be conducted through a solid across which a known temperature difference exists. Sophisticated mathematical techniques are required to establish this, especially if the process varies with time, the phenomenon then being known as transient-heat conduction. With the aid of analogue and digital computers, these problems are now being solved for bodies of complex geometry.


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Answer 6

1. very slowly
2. 0.6/0.025 ≈ 24 times as fast
3. wood ≈ 1.6 to 16 times as fast, depending on the type of wood and its moisture content
aluminum ≈ 9,480 times as fast

A short table of thermal conductivities from Wikipedia:
Material k, [k] = W/(m*K)
Air 0.025
Alcohol or oil 0.15
Aluminum 237
Copper 401
Gold 318
Lead 35.3
Silver 429
Cork 0.05
Diamond 900 – 2320
Glass 1.1
Rubber 0.16
Sandstone 2.4
Soil 0.15
Stainless steel 15
Thermal grease 0.7 – 3
Wood 0.04 – 0.4
Water 0.6

The second reference gives a slightly more extensive list.