unit 2

A ball at rest at the top of a hill does not have kinetic

energy. As it rolls down the hill, it gains kinetic

energy. Where does the kinetic energy gained by

the ball come from? When the ball is at rest at the

top of the hill, its position gives it the potential to

begin rolling and gain kinetic energy. The energy

stored in an object due to its position or condition is

called potential energy. For example, increasing the

height of this ball's position will increase its potential

energy. As the ball rolls down the hill, its potential

energy decreases. Its potential energy is as low as

possible at the bottom.

The potential energy of an object due to its height, or its position relative to Earth's

surface, is called gravitational potential energy. The higher an object is, the greater its

gravitational potential energy. For example, water at the top of a waterfall will have

its greatest gravitational potential energy because its height above Earth's surface

is greater than it is at the bottom of the waterfall.

Mechanical energy describes an object's ability to

move-or d o work o n - o t h e r objects. It is t h e s u m

of the potential energy and kinetic energy of an

object or a system. For example, a person swinging a

hammer is providing kinetic energy to the hammer.

The hammer does work on a nail. An object's

mechanical energy can be all potential energy. It can

be all kinetic energy. It can also be a combination of

the two.

You use many forms of energy every dav. In fact, you are using several forms of energy

as you explore this lesson! Electrical eneray is a flow of negatively charged particles

that creates the electric current used to power computers, lamps, toasters, and other

technologies. Chemical energy is the form of energy involved in chemical reactions. The

batery on your cel ohone uses chemical enerav. Nuclear energy powers the sun. The

sun gives off light energy that reaches Earth.

More than one form of energy can exist in a system at the same time. For example,

fireworks explode because a huge amount of chemical potential energy is released. This

energy becomes sound, light, and thermal energy.

. The passing of energy from one object to another is known as energy transfer.

Because energy cannot be created or destroyed, you can model energy flowing through

a system as inputs and outputs. Think about the bowling bal It needed an input of

energy from the bowler. The bal carried that energy as it roled down the lane. When

the bal colided with a pin. it transferred eneray to the pin. Energy is transferred from

the object with more kinetic energy to the one with less kinetic energy. The pin moves

because energy was transferred to it.

Energy transfers in other types of collisions can also be modeled. For example,

energy is transferred within the system when a swinging pendulum hits a pendulum

that is not moving. Transferring kinetic energy to an object can move it to a position with

higher potential energy.

Transfers of kinetic energy are used in devices and

processes that reduce human effort. They may also

improve efficiency. Energy transfer can also be used

to do things that the human body alone would not be

able t o do.

The process of one form of energy

changing to another form is known as energy transformation. It differs from simple

energy transfer in which energy moves from one object to another or from one place to

another while staying in the same form.

Many

alternative energy sources transform kinetic mechanical energy into electrical energy.

In hydroelectric dams, the kinetic energy of flowing water is transferred to a turbine that

transforms that energy into electricity. Windmils work similarly, using the kinetic energy

of wind to generate electricity. Tidal energy provides power by converting the kinetic

energy of ocean waves into electrical energy. Solar panels do not use kinetic energy.

Instead, they transform light energy from the sun directly into electrical energy.

Because energy cannot be created or destroyed, transfers of energy away from a

system to its surroundings result in an overall loss of energy from the system. The loss

of energy from a system may seem minor, but over time it adds up.

The process of energy transformation happens all the time and everywhere. In fact,

all of the electronic technologies you use every day need energy transformations to

work. Refrigerators, microwave ovens, lights, batteries, and cars all rely on energy

transformations. They make use of the fact that any form of energy can transform into

any other form of energy. For example, a personal music player transforms electrical

energy to sound energy and thermal energy.

The chemical energy stored in fireworks is transformed into

electromagnetic, sound, and thermal energy.

Electrical energy is transformed into electromagnetic energy

and sound energy in a television or computer monitor.

Batteries power electronic devices by transforming chemical

energy into electrical energy.

People can also generate electrical energy. Wind-up radios

work by converting kinetic mechanical energy from a person

to electrical energy.

Temperature is a measure of the average kinetic energy of an object. Thermal energy

is the measurement of the total amount of kinetic energy of all the particles in an object

or substance. Thermal energy is measured in joules (I). Al matter has thermal energy.

When an object is hot, its particles are moving faster and it has more thermal energy

than it h a s w h e n it i s c o l d .

Different kinds of matter are made up of different kinds of particles that do

not interact with one another in the same way in each substance. Because of these

differences, the amount of thermal energy in two different substances with equal mass

can be different even if they have the same temperature.

Temperature is a measure of the average kinetic energy of all the particles in

an object or substance. Temperature does not depend on the material or the type of

particles in a substance.

Every object has thermal energy because every object's particles are moving. In the

Hands-On Lab, when you placed a hot object in cold water, the temperature of the water

increased. The change in water temperature was not the same for all of the objects.

The object with the larger mass had more energy than the smaller object of the same

material at the same temperature. Because it had more energy, the larger object warmed

the water to a higher temperature after a certain amount of time.

The brass and aluminum objects that had the same mass warmed the water by

different amounts, so this is evidence that the thermal energy of an object also depends

on the material it is made of. Different materials of the same size and same temperature

can have different amounts of thermal energy.

Heat is the energy that is transferred between two objects that are at

different temperatures.

When thermal energy is transferred to an object as heat, the average kinetic energy

of the particles of the object will increase. And so the temperature of the object will

rise. Heat always flows from an object at a higher temperature to an object at a lower

temperature. Heat will flow as long as there is a temperature difference. If no energy is

added to the system, both objects will eventually have the same temperature.

It is sometimes helpful to know how much thermal energy transfer is needed to

change the temperature of a substance by a certain amount.

Conduction - Thermal energy is

transferred between particles through

conduction. In this example, the candle

is warming one end of the metal bar. The

particles in the metal bar start to move

faster as they gain more thermal energy.

As the particles move faster, they bump

into each other and transfer thermal

energy through the metal rod.

Convection - Thermal energy is

transferred throughout liquids a n d

gases through convection. In this

example, the candle is heating the

box. As air in the box warms, the air

particles begin to move faster and the air

becomes less dense. The colder, denser

air sinks and pushes up the warmer air.

This movement transfers thermal energy

through liquids and gases.

Radiation - Radiation is the transfer of

energy through electromagnetic waves.

In this example, the candle produces

infrared radiation. This radiation travels

through empty space until it hits a

particle. The particle then absorbs t h i s

radiation, and the radiation is converted

into thermal energy. This process i s

how thermal energy is transferred

through space.

A transfer of energy results in a change in the energy in an object when energy is added

to it or removed from it. You can think of the entire radiometer as a system, or a set o f

interacting parts that work together.

The law of conservation of energy states that energy cannot be created or

destroyed. The total energy of a system will increase if the input of energy from outside

the system is greater than its output. By defining a system's boundaries, the inputs and

outputs of energy can be modeled.

The total thermal energy of a particular component

ofa system depends on its temperature, mass,

composition, and physical state. Different parts of a

system can have different temperatures. Differences

in thermal energy and temperature affect the transfer

of energy to and from the system, as well as within

the system.

Geothermal technology uses the transfer of thermal energy to or from the ground

beneath a structure. Just a few feet below the ground's surface, the temperature is

almost constant all year long. Geothermal heat pumps take advantage of the difference

between the above-ground air temperature and the soil temperature below the surface

to warm and cool buildings. A liquid is pumped through underground pipes. To warm

a room, the pump transfers thermal energy from the liquid to the building's heating

system. Then the cooled liquid flows through the pipes underground where thermal

energy flows into it again before returning to the indoor heating system. For cooling,

the heat pump adds thermal energy to the liquid, which is cooled underground.

Hyperthermia, or heat stroke, is a life-threatening condition of elevated body

temperature. Emergency medical providers know that the best way to treat people

suffering from hyperthermia is to cool them very quickly by submerging them in cold

water. This lifesaving process must be performed as quickly as possible to avoid deadly

complications.