Science High School Reviewer Physical Science: Heat

Heat
What is heat?
If you hold a mug of hot cocoa, energy moves
from the mug to your hand. Since the mug is hotter
than your hand, your hand gets warmer as it absorbs
energy from the mug. If you put ice cubes into your
drink, energy flows from the hot liquid into the cold
ice. The drink cools as transferred energy melts the ice.
Heat is the flow of energy from one substance
to another. Heat is a form of kinetic energy caused
by the movement of the molecules that make up
all matter. If you warm something, such as a cup of
soup, you increase the movement of its molecules.
The substance then becomes hotter. As an object is
heated, the total amount of kinetic heat energy, or
thermal energy, within that object increases.
The total amount of thermal energy in a
substance is measured using a calorimeter. The
metric unit used to measure this energy is the calorie.
A calorie is the amount of energy needed to raise the
temperature of 1 gram of water by 1ºC.

Energy and Temperature

The thermometer measures the
temperature of the water. Temperature
is a measurement of the average kinetic
energy of the molecules in a substance.
As heat flows into a substance, the
kinetic energy of the molecules in the
substance increases. Some molecules in
a substance may move faster or slower
than other molecules, but overall the
average speed of the molecules rises.
This causes an increase in temperature.
This increase in energy can also
cause molecules to move farther apart.
As the molecules in a substance spread
out, it usually increases in volume.
An increase in volume that is caused
by an increase in temperature is
called thermal expansion. Scientists,
engineers, and architects consider the
effects of thermal expansion when
choosing materials to build houses
and other structures. Not all materials
respond to changes in temperature in
the same way—some expand more
than others.
Temperature and Mass
The total amount of thermal energy
in a substance depends on temperature
and mass. A thimble filled with boiling
water has a high temperature, but little
mass. It has less thermal energy than a
jug full of water that is warm but not
boiling. Overall there is more energy
in a gallon of warm water than there is
in a thimble of boiling water. Although
the temperature of the jug is lower, the
total amount of energy is much higher.

How does heat travel?
Heat energy always flows from a
higher-temperature material to a lowertemperature
material. That is why, in
the diagram below, the water in the
plastic bag increases in temperature
when it is placed in the beaker of
warmer water. As the warm water
cools, some of its thermal energy is
transferred to its surroundings.
Conduction
Heat energy can move in three ways.
Conduction is the movement of energy
through direct contact. This means that
two materials touch, and energy flows
directly from one material to the other.
Conduction is the only way that heat
energy can travel through solids.
Conductors are materials that absorb
heat and distribute it evenly throughout
an object. Most metals are good
conductors. Insulators are materials that
absorb some heat but do not transfer it
very well. If you placed your hand on
a piece of wood, the area under your
hand would warm, but the temperature
of the rest of the wood would not
change. Insulation (in•suh•LAY•shuhn) is
any material used to prevent heat from
flowing into or out of a substance.
Convection
When you heat a pot of soup, some
heat energy is transferred through direct
contact between the pot and the soup.
However, most of the heat is transferred
as the heated soup located near the
bottom rises and moves around the pot.

Convection is the transfer of
energy by the flow of a liquid or a
gas. Convection occurs in liquids and
gases but not in solids. Convection
occurs because most liquids and gases
become less dense when heated. Their
particles move faster and farther apart.
Convection cycles are responsible for
transferring energy throughout Earth’s
atmosphere and oceans.

Radiation
Energy from the Sun reaches Earth
by radiation. Radiation is the transfer
of energy by electromagnetic waves.
Radiation can travel through gases
and the vacuum of space. Objects
that absorb radiation gain energy.
The Sun is not the only source of
radiation. All objects give off a range
of electromagnetic waves. Objects that
are near or below room temperature
give off infrared radiation, which our
eyes cannot see. However, when objects
are heated to about 600°C (1,112°F),
they give off a great deal of visible light.
We see this visible light as a dull red
glow, like that of a stove-top burner.

Heat Flow and Clothing
When we are cold, we try to trap
heat energy around the body. This is
why winter clothes are often made of
materials that are good insulators,
such as wool. Dark colors absorb more
energy, so cold-weather garments are
often dark, as well. In contrast, summer
clothing is often light in color and
made of thin, lightweight materials.
They absorb less energy and allow heat
to flow away from the body easily.

How do we use heat?
Buildings are heated by systems
designed to transfer heat energy. In
a hot-water heating system, water is
used to transfer energy from a boiler
to the air in a room. The boiler heats
water, which is forced through pipes by
pumps. The pipes lead to convectors
in the rooms, and the air around
the convectors becomes warmer.
Convection currents circulate air
throughout the room, and the room is
warmed as a result. Heated air is less
dense than cooler air, so it rises in the
room. As the air cools, it sinks back
down and is eventually heated again.
In a forced-air heating system, a
room is heated with air alone. Hot air,
forced up by fans from the furnace,
heats the air in the room. Convection
currents circulate the air in the room.

How do we use heat?
Buildings are heated by systems
designed to transfer heat energy. In
a hot-water heating system, water is
used to transfer energy from a boiler
to the air in a room. The boiler heats
water, which is forced through pipes by
pumps. The pipes lead to convectors
in the rooms, and the air around
the convectors becomes warmer.
Convection currents circulate air
throughout the room, and the room is
warmed as a result. Heated air is less
dense than cooler air, so it rises in the
room. As the air cools, it sinks back
down and is eventually heated again.
In a forced-air heating system, a
room is heated with air alone. Hot air,
forced up by fans from the furnace,
heats the air in the room. Convection
currents circulate the air in the room.

How is temperature measured?
Temperature is a measure of the average kinetic energy of
a substance’s molecules. Thermometers measure temperature.
Some thermometers are made of a clear tube containing
a liquid that expands when it warms. Others are made of
coiled bimetallic strips that expand from absorbing heat
energy. There are also different types of instruments that
measure this energy using substances that change color
or another property upon reaching a particular
temperature. Many modern thermometers are
digital. When any type of thermometer is
placed in a warmer material, heat flows
from the material to the thermometer,
causing its temperature to change.
Specific Heat
Physical properties such as shape, size,
color, and texture vary depending on the
type of matter. Another physical property
of matter is the rate at which the substance
warms up upon absorbing heat. The
specific heat of a substance is the amount
of energy, often measured in joules (J),
needed to raise the temperature of 1 gram
of the substance by 1°C. Most metals
have a low specific heat, so little energy is
needed to increase their temperatures. Water
has a high specific heat, so more energy is
needed to raise its temperature. That is why
you can burn your finger by touching the
metal handle of a pot on the stove when
the water in the pot is only lukewarm.