Chapter 27
27.2 Convection
Another type of heat transfer that is due to temperature differences is called convection. This type of
heat transfer is responsible for global weather patterns, the heating of our homes and the circulation of
waters in the oceans.
What is convection?
What is Have you ever warmed up your hands by putting them over an open flame? You
convection? can do this because the air right above the flame heats up and expands. Because
the expanded air is less dense, it rises, bringing the heat to your hand (figure 27.6).
This heat transfer process is called convection. Unlike conduction, which occurs
mostly in solids, convection occurs only in liquids and gases. Convection comes
from a Latin word meaning to carry together.
Figure 27.6: The air right above
the flame heats up and expands,
transferring heat to your hand.
Convection is the transfer of heat by the actual
motion of a fluid (liquid or gas) in the form of
currents.
Convection can occur in all fluids, whether liquids or gases. Convection occurs
because warmer fluids are less dense, and rise. Cooler fluids are more dense, and
sink. This motion of fluids causes currents.
Convection causes The currents caused by convection occur constantly in our atmosphere and are
the weather responsible for much of our weather. On a global scale, hot air near the equator
patterns on Earth rises and is forced toward the poles as shown in figure 27.7. The sinking air forces
cold air at the poles toward the equator. Combined with forces due to the rotation
of the Earth, convection and unequal heating are the primary causes of weather.
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Figure 27.7: Currents caused by
convection are responsible for much
of our weather. Warm air rises off the
surface of the Earth. As it cools, it
sinks back down and replaces warmer
air.
Chapter 27
Natural convection
Why does warmer There is a natural upward force called buoyancy. This force occurs whenever you
air rise? have an object submerged in a denser medium. An example is an inflated ball
under water. The ball is less dense than the water. There is an upward force equal
to the weight of the displaced medium that pushes the ball out of the water. But
this force not only applies to solids, it also applies to fluids and gases. As heated
air or a fluid rises, there are density differences, which act with gravitational
forces to produce natural or buoyant convection.
Sea breezes are On a smaller scale near coastlines, convection is responsible for sea breezes.
due to convection During the daytime, land is much hotter than the ocean. A sea breeze is created
when hot air over the land rises due to convection and is replaced by cooler air
from the ocean. In the evening, the ground cools rapidly but the ocean remains
warm, due to water’s high specific heat. Warm air rises over the water and is
replaced with cooler air from over the land. This is known as the land breeze.
Heating a room As a clear example of natural convection, we can analyze how a room radiator
heats a room during winter (figure 27.8). As the temperature of the air around the
radiator is increased by conduction, it becomes less dense than the cold air in the
room. This warmer air rises and cooler air from the far side of the room replaces it.
This air circulation transfers heat from the radiator to the cooler parts of the room.
Figure 27.8: During the day, a sea
breeze is created when hot air over
the land rises due to convection and is
replaced by cooler air from the ocean.
At night, temperatures reverse and a
land breeze occurs. This happens
because the land cools more rapidly
than the ocean.
27.2 Convection
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Chapter 27
Convection in the Another application of natural convection is cooking on a gas stovetop. As with a
kitchen candle, the heat from the burning gas rises to boil the water in the pot above it.
Even the water in the cooking pot itself experiences convection. The hot water at
the bottom of the pot rises to the top of the pot, replaced by the cooler water. Next
the cooler water is heated. If this did not happen, we would have to rely on the
slower method of conduction to boil a pot of water.
Why wearing a Through the process of convection, air carries heat away from your body. A wool
sweater keeps you sweater prevents this from happening by trapping air in many small pockets so
warm that it cannot flow and carry the heat away. Similarly, in cold weather birds trap
pockets of air by fluffing their feathers.
Wind chill If you expose bare skin to cold temperatures, natural convection can quickly
become dangerous. As the air surrounding your body warms up, it rises and carries
heat away. The effect of air carrying heat away is greatly increased on a windy day
when a steady stream of air flows. The faster the wind speed, the more effectively
heat is carried away. Antarctic explorers created a commonly used method for
“measuring” the chilling effects of the wind, called the Wind Chill Equivalent
Temperature or wind chill factor. The wind chill factor was originally based on the
temperature at which plastic jugs of water placed on top of a high pole would
freeze, given a certain wind speed.
Figure 27.9: Convection in water.
The hot water at the bottom of the pot
rises to the top and replaces the cold
water.
Why does smoke Convection causes the smoke from the fire in a fireplace to rise up the chimney
rise up the instead of entering your home. This is because convection is extremely efficient in
chimney? focusing the heat in one direction: up! Smoke particles are carried upward by the
rising hot air. If you have ever toasted marshmallows on a campfire, you may have
noticed that if you hold the marshmallow on the stick right next to the fire, it gets
nice and toasty. However, if you hold the marshmallow directly above the fire, it
quickly catches fire and burns to a crisp. The air directly above the fire carries
much more heat by convection. At the edge of the fire, the heat is mostly carried
by a different kind of heat transfer called radiation.
Figure 27.10: Convection is
extremely efficient at focusing heat in
one direction: up.
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Forced convection
What is forced Another type of convection is the one in which a mechanical device is used to
convection? force the fluid or gas to move, as opposed to the buoyant force. This is called
forced convection. Air or liquids can be forced with fans or pumps. Warm fluids
can carry heat to cooler regions and cool fluids can take heat away from hot
regions.
Applications
Most heating systems use a combination of forced and natural convection. Let’s
of forced return to the example of a radiator for home heating. Water is heated in the
convection basement and pumped into the rooms of the house. The process of pumping the
hot water through the house is forced convection. In a room, the hot water releases
heat to the air surrounding the radiator through conduction. The heat is then
carried away from the radiator by natural convection (figure 27.11).
The opposite occurs in an air-conditioning system, where cool air is blown
through a room with a fan. This forces the cooler air to replace the warmer air in
the room.
Figure 27.11: Both natural and
forced convection help to heat a
house.
Forced convection In the radiator of a car, there are two examples of forced convection. In the cooling
in a car system of a car, heat is transferred from the engine to the water by conduction.
Then the heated water is pumped to the radiator by forced convection. After the
water is inside the radiator, heat from the water is conducted to the radiator fins.
The radiator fins are cooled by air blowing over the radiator.
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