Chapter 3

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# Chapter 3 - PowerPoint PPT Presentation

Chapter 3. What are Kinetic and Potential Energy?. Energy. Energy is the ability to cause changes in matter. Heat causes ice to change into liquid water- a change in state. Two B asic Kinds of Energy- 1. Energy of Motion 2. Energy of Position or Condition. Two Kinds of Energy.

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### Chapter 3

What are Kinetic and Potential Energy?

Energy

Energy is the ability to cause changes in matter.

Heat causes ice to change into liquid water- a change in state

Two Kinds of Energy

Kinetic

Potential

While bouncing up, the ball gained energy of position. When the ball stopped at the top of a bounce it had potential energy

This is the energy an object has because of where it is or because of its condition.

• Any matter is motion is kinetic energy
• When you let go of the tennis ball, it gained kinetic energy
• When it reached the top of each bounce, it stopped for an instant between rising and falling again. At this point, its kinetic energy was zero.
Forms of Energy

The kinetic energy that moving objects have is also called mechanical energy.

Thermal Energy is another Form of Kinetic Energy

The movement of molecules of matter produces heat.

Electric Energy- Is caused by the movement of electrons

Electric energy produces sound and picture on a television

Light energy from the picture moves to your eyes.Your ears receive vibrations produced by the television speakers as sound energy.

Potential Energy takes several forms. Elastic potential energy is the energy stored in matter, such as stretched rubber bands and bent vaulting poles.

Gravitational potential energy-water behind a dam, a balanced rock, or any matter that can fall.
Most of Earth’s energy comes from the sun

Plants store this energy as chemical energy in the food that they make.

When a pole vaulter starts running, chemical energy stored in her muscles is changed into thermal energy and mechanical energy

It is also energy that produces

It is energy that is produced by the movement of electrons.

You may recall that within an atom, electrons have negative charge and protons have a positive charge. The two types of particles attract each other. Most objects have an equal number. However sometimes electrons are attracted to protons of another object and rub off. When an object gains or loses electrons, it has an electric charge.

When an object gains electrons it has a negative charge. When an object loses electrons, it has a positive charge.
Charged objects have potential electric energy.This is sometimes called static electricity.

When charged objects are close to each other, potential energy can become kinetic.

If the objects touch or come very close to each other, electrons may flows from one object to the other.

When electrons flow from negatively charged objects to positively charged objects it is called electric current.Once electrons have moved form one object to the other, the attraction between the objects is gone. They are balanced and there is no electric force.

Unlike static electricity which does not move, an electric current is a flow of electrons. The shock you get from touching a doorknob is a small electric current. A lightening bolt is a brief but strong electric current.

To light a light bulb or run a computer, a continuous electric current must be produced. A source of electrons is needed. A battery or generator may be a source of electrons.

Electric current flows through many kinds of matter if the electric force is strong enough.Metal conducts electrons more easily than others.This is because it is a conductor!

Electric Circuits

Any path along which electrons can flow.

Copper and aluminum are often used as conductors. Metals are good conductors because their atoms don’t hold electrons tightly.

The conductor in a circuit is wrapped with a material called an insulator. It does not carry electrons. It helps keep wires from touching each other and completing an electric circuit before it reaches a device. If this happens, it is called a short circuit.

Resistors-They are neither conductors nor insulators. They are inside many appliances. They resist the flow of electrons in some way. They are important because they allow electric energy to be changed into other forms. The filament in a light bulb for example resists the flow of electrons. This resistance produces heat.

Magnets and Electricity

Magnets are used to generate, or produce electricity. Spinning a coil wire inside a magnetic field produces an electric force between the ends of the coil.

A current-carrying wire wrapped in a coil of more wire makes a strong magnet.

A coil current-conducting wire wrapped around an iron bar makes an even stronger magnet. Around the coil is a magnetic field

We often tend to think about light as rays that start at a source and travel in a straight line until it strikes something, like the sun striking Earth.

Light rays are a form of energy that can travel through empty space or through some kinds of matter.

Light rays pass through gases in Earth’s atmosphere and through clear glass windows.

Sometimes light energy is absorb when it strikes matter. Most objects absorb some colors of light. Other colors bounce off objects as a reflection. The color of light that objects reflect are the colors we see.

A green leaf absorbs much of the sunlight that strikes it. The rest of the light- the green part- reflects off the leaf.

A mirror reflects all colors.

When light passes through a window, it slows down. This change in speed causes light rays to bend. This bending is called refraction.
Light often refracts when it moves from one substance to another. For example this straw appears to bend at the water’s surface.

This is because light rays traveling from the pencil to your eyes bend as they move from the water through the glass and into the air.

Convex vs. Concave lenses

Thicker around the outside than in the middle.

When light rays pass through they bend away from each other. This makes distant objects seem nearer and smaller.

This helps people who are nearsighted.

They have trouble seeing things in the distant.

Thicker in the middle. Makes nearby objects appear larger.

People who are farsighted have trouble seeing things near them.

Convex lenses magnify those objects or print.

Light energy moves as waves. These waves are called electromagnetic waves, are produced when vibrating electrons inside atoms give off energy.

Microwaves

Infrared waves

ultraviolet waves

Xrays

Electromagnetic waves don’t need a substance to pass through. They move fastest when they have no matter to slow them down.
Sound waves are carried by a vibrating matter. Sound waves travel through air but also through water.
As a sound wave travels through matter, molecules in the matter move back and forth in the direction the sound wave is moving.
As the sound moves forward, the molecules are squeezed together. This is compression.

After the first compression passes, pressure on the molecules drops. This is rarefaction.

The faster the sound waves, the higher the pitch.

In music, pitch is labeled with letters called notes. As you move from left to right, pitch increases on a piano.
Another way to measure sound waves is to measure their strength. The more the molecules are squeezed together the louder.

Sound waves are waves of energy moving through matter. When sound waves travel through the air, they travel about 1100 ft./ second. When sound moves through solids or liquids, they travel faster.

Whales produce sounds that travel for hundreds of miles underwater. Only very loud sounds can travel through the air like that.

You would be able to clearly hear the sounds of a ticking watch by putting your ear on the wooden table.

The average kinetic energy of all the molecules in an object is the object’s temperature. The higher the average kinetic energy, or the faster the molecules move, the higher the temperature.

Large pot is more thermal energy, because more molecules

Thermal Energy can be transferred, or moved, between objects in three ways, conduction, and radiation

Conduction is the direct transfer of heat between objects that touch.

In this picture, conduction transfers heat from the stove burner through the metal frying pan to the sausage links.

Materials that conduct heat are conductors.

Metals are good heat conductors.

Pans are metal.

Materials that do a poor job of conducting heat are called insulators.

Air is a good insulator of heat.

These double paned windows have a layer of air between them, so the air slows down rate of the heat.

In this picture, currents transfer heat from the bottom of the pan all through the boiling water to the egg.

As the hot water rises, it cools and sinks, This process transfers thermal energy throughout the pan.

Radiation is the transfer of energy by electromagnetic waves. Radiation from the glowing coils in the toaster transfers heat to the surfaces of the bread.

Energy is transferred to Earth by electromagnetic waves.

Radiation can transfer heat through space.

Chemical energy is the energy stored in the bonds between atoms when they join together to form molecules.

Chemical energy can be released as several forms of kinetic energy. Batteries, for example, contain chemical energy that can be used to produce electricity.

During cellular respiration, your body changes chemical energy stored in the food you eat into mechanical energy that allows you to carry on your daily activities.