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16-6
what is convection?
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Describe how heat travels through gases and
liquids.
Key Ttzrms
convection (kuhn-VEHK-shuhn): transfer of heat
in gases and liquids
Convection currents are found in 1iquids as well
as in air. In bodies of water, warmer water rises
to the surface and carries heat with it. Cooler water
sinks to the bottom. Convection currents caltse
heat to be transferred through bodies of water.
When water is heated in a pot, convection currents
form. Figure 16-20 shows convection currents in a
pot of boiling water.
convection current: movement of gases or
liquids caused by drfferences in density
Heat Transfen in Gases and Liquids The
transfer of heat in gases and liquids is called
convection. \i\4ren gases and liquids are heated, the
particles gain energy and move faster and farther
apart. Because these particles are farther apart, the
heated gas or liquid becomes less dense than the
cooler, surrounding gas or liquid. This makes the
less-dense gas or liquid rise and the cooler, denser
gas or liquid moves in to take its place. As it rises, the
gas or liquid carries heat with it.
ffi
orrtrur: What is convection?
eonveetion Culrrents Heat is carried in gases
and liquids by means of convection currents.
Convection currents are movements of gases or
liquids caused by differences in density.
Convection currents transfer heat. Air is a gas. Air
at Earth's surface is warmed by the Sun and then
rises. Cool air abor.e the North Pole and the South
Pole sinks. The rising of warm air and the sinking
of cool air cause convection currents. Certain birds,
such as eagles and hawks, use rising warm air in
convection currents to soar above Earth's surface.
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Figure 16-19 An eagle uses convection currents to soar.
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Figure 16-20
Convection currenls
transfer heat in a
liquid.
Just below Earth's crust is a layer of rock called
the mantle. The mantle is heated from below by
heat from Earth's core. Rock in the upper part of
the mantle behaves like a very thick liqr_rid.
Scientists believe that convection currents
traveling through the upper mantle are responsible
for the very slow morrement of large sections of
Earth's crust. This concept is known as the theory
of plate tectonics.
Hp rxpUtl: How do convection currents
transfer
heat in liquids?
Uses
of €onveetion
is used in
Heat transfer by convection
some home heating systems. In a
hot-water heating system, water is heated in a
hot-water heater. The hot water is then pumped
through pipes to each room in the house. The hot
water flows through heaters near the floor of each
room. The hot water warms the air near the floor
by conduction. The warm air rises, carrying heat
through each room by means of convection
currents. Heat circulates through each room in this
way. After the water loses some of its heat, it returns
to the hot-water heater to repeat the process.
3.
Warm water rises and cool watet
4.
Convection takes place in gases and
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room, should an air
conditior-rer be placed at the bottom of a wall
or at the top of a r,t all? Explain your answer.
ANALYZE:To cool a
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Expansion
tank
Smoke
Pump
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heat transferred through solids by
convection? Explain.
6. INFER: Is
outlet
tliALTH
Figure 16-21 ln a holwater heating system, heated water
Smoke and gases from the burners of hot-water
heaters contain pollutants. These pollutants are
sent through an exhaust pipe that runs out
through a chimney or vent in the roof. Hot air
containing the pollutants rises up the chimney.
Cool, clean air moves in from outside to take its
place. Why do you think these pollutants need to
be removed from a house?
travels through a system of pipes to heat a home,
f,p
torrurtrY: What method of heat transfer is
used in a hot-water heating system?
CHECKING CONCEPTS
1. Heat travels tl-rrough gases by
dense than cold air.
2. Warm air is
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Inteyating Earth,
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Convection currenl
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T*FIC: plate teetonies
CONVECTION CURRENTS INSIDE THE MANTLE
Differences in density within a gas or liquid can cause
convection currents within the gas or liquid. Under Earth's
crust is a thick layer of rock called the mantle. The part of
the mantle closest to Earth's core acts like a thick, hot
liquid. The part of the mantle that is farthest from the core
,r, Figure 16-22 Convection currents in
has the properties of a thick, cooler liquid. Scientists
mantle
the
believe that the differences in the density in the mantle
cause convection currents. The thick, hot liquid of the
mantle rises and cools. The cooler liquid of the mantle sinks. As the cooler liquid gets
closer to the core, it heats up and rises. The process repeats over and over again.
Scientists also believe that these convection currents cause the plates that make
up the crust to continuously move. The crust floats on top of the mantle. The
convection currents in the mantle carry the plates along so that the continents that
sit on top of the plates slowly move, too. This concept supports the theory of plate
tectonics. The theory of plate tectonics explains how Earth has changed over time.
Thinking Critically How do convection currents support the theory of plate
tecton ics?
CHAPTER 16:
Heat
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