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Determine the knowns and convert them to the SI units.
An electric stove has a typical value for the heat transfer.
The difference between the stove and pan is very small.
The inside of the pan is nearly as hot as the stove burner because of its contact with boiling water.
This contact cools the bottom of the pan even though it is close to the stove burner.
It only takes a small temperature difference to produce a heat transfer of 2.26 kilowatts from the pan.
Random motion of atoms and molecule causes conduction.
It is not an effective mechanism for heat transport over long distances.
The temperature on the Earth would be unbearably cold during the night and extremely hot during the day if heat transport in the atmosphere was only through conduction.
If there was a more efficient way to remove excess heat from the pistons, the car engines would be less prone to overheating.
When each dimensions is doubled, the area increases by a factor of four.
Large-scale flow of matter is what drives Convection.
The atmospheric circulation is caused by the flow of hot air from the tropics to the poles and cold air from the poles to the tropics.
Car engines are kept cool by the flow of water in the cooling system, with the water pump maintaining a flow of cool water to the pistons.
When the body is overheated, the blood vessels in the skin expand, which increases the blood flow to the skin where it can be cooled by sweating.
When it is cold outside and hot, these vessels become smaller.
The breathing process reduces the body's heat loss.
We can describe and do realistic calculations of the effects of convection.
Hot air increases in density as temperature increases.
Both are examples of natural movement.
Air heated by the gravity furnace expands and rises, forming a convective loop that transfers energy to other parts of the room.
As the air is cooled at the ceiling and outside walls, it contracts, eventually becoming denser than room air and sinking to the floor.
A properly designed heating system can be quite efficient in heating a home.
The heat transfer inside this pot of water is important.
Convection transfers heat from the inside to other parts of the pot.
The hotter water expands, decreases in density, and rises to transfer heat to other regions of the water, while the colder water sinks to the bottom.
This process continues.
Use an eye dropper to place a drop of food coloring near the bottom of two pots of water.
How long does it take the color to reach the top?
The convective loops form.
Air goes in and out around doors and windows, through cracks and crevices, following wiring to switches and outlets, and so on, in most houses.
In less than an hour, the air in a house is completely replaced.
If a moderately-sized house has inside dimensions high, all air is replaced in 30.0 min.
The temperature of air is raised by heat.
The time for air turnover depends on the rate of heat transfer.
We need to find values for the mass of air and its specific heat before we can calculate.
The specific heat of air is a weighted average of the specific heats of nitrogen and oxygen, which gives from Table 14.
Determine the density of the air and the volume of the house.
The rate of heat transfer is the same as the power used by the light bulbs.
The house of this example has a turnover time of 30 minutes, but new homes are designed for a turnover time of 2 hours or more.
caulking, weather stripping, and improved window seals are used.
In hot climates, more extreme measures are taken to achieve a tight standard of more than 6 hours for one air turnover.
A minimum amount of fresh air is needed to provide oxygen for breathing and to reduce household pollutants.
The rate at which energy is transferred away from the body is increased in a cold wind.
The table below shows the wind-chill factors, which are the temperatures of still air that produce the same rate of cooling as air of a given temperature and speed.
The ability to transfer heat faster than conduction is demonstrated by wind-chill factors.
The chilling equivalent of still air is about 15.0 m/s wind.
Air is a poor conductor and is a good insulator.
The space between the inside and outside walls of a house is about 9 cm.
The addition of wall insulation reduces heat loss.
The gap between the two panes of a double-paned window is about 1 cm and takes advantage of air's low conductivity to prevent greater loss.
Fur, fiber, and fiberglass are able to take advantage of the low conductivity of air by trapping it in small spaces, as shown in the figure.
The lightweight nature of fur and feathers makes them ideal for the protection of animals.
Fur is being broken up into small pockets.
The loops are small, which causes the Convection to be very slow.
Fur is a good lightweight insulator.
There are some interesting phenomena when there is a phase change.
Even if the temperature of the surrounding air exceeds our body temperature, we can still sweat.
Without air flow, sweat can't evaporate from the skin and the air becomes saturated.
The saturated air is replaced by dry air.
The average person produces about 120 W of heat at rest.
A phase change requires energy.
Table 14.2 has the value of the latent heat.
It seems reasonable to expel about 3 g/min.
This would be about 7 ounces per hour.
The sweat may not be noticed if the air is very dry.
In the lungs and breathing passages, there is a lot of evaporation.
The combination of phase change and convection can be seen when water leaves the ocean.
The water in the ocean does not have heat.
As clouds form, heat is released in the atmosphere if the water vapor condenses in liquid droplets.
There is a transfer of heat from the ocean to the atmosphere.
The driving power behind thunderheads is the process behind the great clouds that rise as high as 20.0 km into the sky.
A lot of energy is released by water vapor.
The air expands when it is warmer.
The cloud drives even higher when there is more condensation in the colder regions.
Positive feedback is a mechanism that reinforces the process.
These systems can produce violent storms with lightning and hail.
The water vapor that rises in the clouds is called convection.
Positive feedback drives the rise of clouds.
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