Science High School Reviewer Physical Science: The Nature of Energy

Using Energy

What are energy and work?
When you feel tired you may say, “I don’t have
any energy.” But what is energy? Energy is the ability
to perform work or to change an object. When you
are low on energy, you probably cannot do much
work. Work is the measurement of the energy used
to perform a task. Work is equal to the force used
times the distance the force was applied. The units
of both work and energy are the units of force times
the unit of distance: newton-meters (N∙m). If you
lift a box that weighs 10 newtons onto a shelf that
is 1.5 meters high, you are performing 15 N∙m of
work. Newton-meters are also known as joules (J).
In describing how you lifted and moved the box, you
could also say that you used 15 J of energy.

Thermal Energy
Sound
Light
Electricity
Magnetism

The Nature of Energy (Introduction to Physical Science)

In other words, energy is the ability to cause
change.

Transferring energy

Energy of Motion

Kinetic energy is the energy an object has due to its
motion. If an object isn’t moving, it doesn’t have kinetic energy.

The faster the ball
goes, the more kinetic energy it has. This is true
for all moving objects. Kinetic energy increases
as an object moves faster.

Kinetic energy also depends on the mass of
a moving object. Kinetic energy increases as the
mass of the object increases.

Energy of Position
Potential energy is the energy stored in an
object because of its position. In this case,
the position is the height of the glass above
the floor. The potential energy of the glass
changes to kinetic energy as the glass falls.
The potential energy of the glass is greater if
it is higher above the floor. Potential energy
also depends on mass. The more mass an object has, the more
potential energy it has.

Forms of Energy

Food, sunlight, and wind have energy, yet they seem different
because they contain different forms of energy. Food and
sunlight contain forms of energy different from the kinetic
energy in the motion of the wind. The warmth you feel from
sunlight is another type of energy that is different from the
energy of motion or position.

Thermal Energy

The feeling of warmth from sunlight signals
that your body is acquiring more thermal energy. All
objects have thermal energy that increases as its temperature
increases.

Chemical Energy

When you eat a meal, you are putting a
source of energy inside your body. Food contains chemical
energy that your body uses to provide energy for your brain, to
power your movements, and to fuel your growth.

Chemical energy is the energy stored in chemical bonds.
When chemicals are broken apart and new chemicals are
formed, some of this energy is released. The flame of a candle is
the result of chemical energy stored in the wax. When the wax
burns, chemical energy is transformed into thermal energy and
light energy.

Light Energy

Light from the candle flame travels through the
air at an incredibly fast speed of 300,000 km/s. This is fast
enough to circle Earth almost eight times in 1 s. When light
strikes something, it can be absorbed, transmitted, or reflected.
When the light is absorbed by an object, the object can become
warmer. The object absorbs energy from the light and this
energy is transformed into thermal energy. Then energy carried
by light is called radiant energy.

Electrical Energy Electrical lighting is
one of the many ways electrical energy is
used. Look around at all the devices that use
electricity. Electric current flows in these
devices when they are connected to batteries or
plugged into an electric outlet. Electrical
energy is the energy that is carried by an electric
current. An electric device uses the electrical
energy provided by the current flowing in
the device.

Nuclear Energy

Nuclear power plants use the energy stored in
the nucleus of an atom to generate electricity. Every atomic
nucleus contains energy—nuclear energy—that can be transformed
into other forms of energy.

Energy Transformations

Changing Forms of Energy
Chemical, thermal, radiant, and electrical are some of the
forms that energy can have. In the world around you, energy is
transforming continually between one form and another.

Thermal energy is almost always produced by an energy transformation.
The energy transformations that occur when people
exercise, when cars run, when living things grow and even when
stars explode, all produce thermal energy.

Law of Conservation of Energy

According to the
law of conservation of energy, energy is never created or
destroyed. The only thing that changes is the form in which
energy appears.

Sources of Energy (Introduction to Physical Science)

Energy Resources
Energy cannot be made, but must come from the natural
world. As you can see in Figure 16, the surface of Earth
receives energy from two sources—the Sun and radioactive
atoms in Earth’s interior. The amount of energy Earth receives
from the Sun is far greater than the amount generated in
Earth’s interior. Nearly all the energy you used today can be
traced to the Sun, even the gasoline used to power the car or
school bus you came to school in.

Fossil fuels

***diagram

Fossil fuels are coal, oil, and natural gas. Oil and natural gas
were made from the remains of microscopic organisms that
lived in Earth’s oceans millions of years ago. Heat and pressure
gradually turned these ancient organisms into oil and natural
gas. Coal was formed by a similar process from the remains of
ancient plants that once lived on land, as shown in Figure 17.
Through the process of photosynthesis, ancient plants converted
the radiant energy in sunlight to chemical energy stored
in various types of molecules. Heat and pressure changed these
molecules into other types of molecules as fossil fuels formed.
Chemical energy stored in these molecules is released when fossil
fuels are burned.

This
means that the supply of oil on Earth will continue to decrease
as oil is used. An energy source that is used up much faster than
it can be replaced is a nonrenewable resource. Fossil fuels are
nonrenewable resources.

Nuclear Energy
Can you imagine running an automobile on 1 kg of fuel that
releases almost 3 million times more energy than 1 L of gas? What
could supply so much energy from so little mass? The answer is
the nuclei of uranium atoms. Some of these nuclei are unstable
and break apart, releasing enormous amounts of energy in the
process.

Nuclear wastes

Like all energy sources, nuclear energy has
its advantages and disadvantages. One disadvantage is the
amount of uranium in Earth’s crust is nonrenewable. Another is
that the waste produced by nuclear power plants is radioactive
and can be dangerous to living things. Some of the materials in
the nuclear waste will remain radioactive for many thousands of
years. As a result the waste must be stored so no radioactivity is
released into the environment for a long time.

Hydroelectricity

Hydroelectricity
Currently, transforming the potential energy of water that is
trapped behind dams supplies the world with almost 20 percent
of its electrical energy. Hydroelectricity is the largest renewable
source of energy. A renewable resource is an energy source that
is replenished continually. As long as enough rain and snow fall
to keep rivers flowing, hydroelectric power plants can generate
electrical energy.

Alternative sources of energy

Research is being done to develop new
sources of energy that are safer and cause less harm to the environment.
These sources often are called alternative resources.
These alternative resources include solar energy, wind, and geothermal
energy.

Solar Energy
The Sun is the origin of almost all the energy that is used on
Earth. Because the Sun will go on producing an enormous
amount of energy for billions of years, the Sun is an inexhaustible
source of energy. An inexhaustible resource is an
energy source that can’t be used up by humans.

Geothermal Energy

The heat generated inside Earth is called geothermal
energy. Some of this heat is produced when unstable
radioactive atoms inside Earth decay, converting nuclear
energy to thermal energy.

Tidal Energy

Wind
Conserving energy

Heat and Thermal Energy

Heat is the transfer of thermal
energy from one object to another when the objects are at different
temperatures. The amount of thermal energy that is
transferred when two objects are brought into contact depends
on the difference in temperature between the objects.

Transfer of Thermal Energy

When thermal energy is transferred,
it always moves from warmer to cooler objects. Thermal
energy never flows from a cooler object to a warmer object. The
warmer object loses thermal energy and becomes cooler as the
cooler object gains thermal energy and becomes warmer. This
process of thermal energy transfer can occur in three ways—by
conduction, radiation, or convection.

Conduction
This transfer of
thermal energy by direct contact is called
conduction. Conduction occurs when the
particles in a material collide with neighboring
particles.

Radiation
Radiation
On a clear day, you walk outside and feel the warmth of the
Sun. How does this transfer of thermal energy occur? Thermal
energy transfer does not occur by conduction because almost no
matter exists between the Sun and Earth. Instead, thermal
energy is transferred from the Sun to Earth by radiation.
Thermal energy transfer by radiation occurs when energy is
transferred by electromagnetic waves. These waves carry energy
through empty space, as well as through matter. The transfer of
thermal energy by radiation can occur in empty space, as well as
in solids, liquids, and gases.

Convection
When you heat a pot of water on a stove, thermal energy can
be transferred through the water by a process other than conduction
and radiation. In a gas or liquid, atoms or molecules can
move much more easily than they can in a solid. As a result,
these particles can travel from one place to another, carrying
their energy along with them. This transfer of thermal energy by
the movement of atoms or molecules from one part of a material
to another is called convection.

!!! heating a pot of water on a stove is actually convection???!!!

Convection
Convection is the flow of heat through a bulk, macroscopic movement of matter from a hot region to a cool region, as opposed to the microscopic transfer of heat between atoms involved with conduction. Suppose we consider heating up a local region of air. As this air heats, the molecules spread out, causing this region to become less dense than the surrounding, unheated air. For reasons discussed in the previous section, being less dense than the surrounding cooler air, the hot air will subsequently rise due to buoyant forces – this movement of hot air into a cooler region is then said to transfer heat by convection.
Heating a pot of water on a stove is a good example of the transfer of heat by convection. When the stove is first turned on heat is transferred first by conduction between the element through the bottom of the pot to the water. However, eventually the water starts bubbling – these bubbles are actually local regions of hot water rising to the surface, thereby transferring heat from the hot water at the bottom to the cooler water at the top by convection. At the same time, the cooler, more dense water at the top will sink to the bottom, where it is subsequently heated.

Natural Convection

Natural convection occurs when a
warmer, less dense fluid is pushed away by a cooler, denser fluid.
For example, imagine the shore of a lake. During the day, the
water is cooler than the land.As shown in Figure 7, air above the
warm land is heated by conduction.When the air gets hotter, its
particles move faster and get farther from each other, making
the air less dense. The cooler, denser air from over the lake flows
in over the land, pushing the less dense air upward. You feel this
movement of incoming cool air as wind. The cooler air then is
heated by the land and also begins to rise.

Forced Convection

Sometimes convection can be forced.
Forced convection occurs when an outside force pushes a fluid,
such as air or water, to make it move and transfer thermal energy.
A fan is one type of device that is used to move air. For example,
computers use fans to keep their electronic components from
getting too hot, which can damage them. The fan blows cool air
onto the hot electronic components, as shown in Figure 8.
Thermal energy from the electronic components is transferred to
the air around them by conduction. The warm air is pushed away
as cool air rushes in. The hot components then continue to lose
thermal energy as the fan blows cool air over them.

Thermal conductors

A conductor is any material that easily transfers thermal energy.
Some materials are good conductors because of the types of
atoms or chemical compounds they contain.

Thermal insulators

An insulator is a material
in which thermal energy doesn’t flow easily. Most pans have handles
that are made from insulators. Liquids and gases are usually
better insulators than solids are. Air is a good insulator, and
many insulating materials contain air spaces that reduce the
transfer of thermal energy by conduction within the material.
Materials that are good conductors, such as metals, are poor
insulators, and poor conductors are good insulators.