Download Laws of Motion A force is anything that can change the state of

Laws of Motion
A force is anything that can change the state of motion of an object, like a push or a pull. You use
force when you push a letter on the computer keyboard or when you kick a ball. Forces are
everywhere. Gravity acts as a constant force on your body, keeping you secure on planet Earth so you
don't float away. To describe a force we use the direction and strength. For example when you kick a
ball you are exerting force in a specific direction. That is the direction the ball will travel. Also, the
harder you kick the ball the stronger the force you place on it and the farther it will go.
A scientist named Isaac Newton came up with three Laws of Motion to describe how things move
scientifically. He also described how gravity works, which is an important force that affects
everything.
First Law of Motion
The first law says that any object in motion will continue to move in the same direction and speed
unless forces act on it. That means if you kick a ball it will fly forever unless some sort of forces act on
it! As strange as this may sound, it's true. When you kick a ball, forces start to act on it immediately.
These include resistance or friction from the air and gravity. Gravity pulls the ball down to the ground
and the air resistance slows it down.
Second Law of Motion
The second law states that the greater the mass of an object, the more force it will take to accelerate the
object. There is even an equation that says Force = mass x acceleration or F=ma. This also means that
the harder you kick a ball the farther it will go. This seams kind of obvious to us, but having an
equation to figure out the math and science is very helpful to scientists.
Third Law of Motion
The third law states that for every action, there is an equal and opposite reaction. This means that there
are always two forces that are the same. In the example where you kicked the ball there is the force of
your foot on the ball, but there is also the same amount of force that the ball puts on your foot. This
force is in the exact opposite direction.
Fun facts about Forces and Motion
 It is said that Isaac Newton got the idea for gravity when an apple fell off a tree and hit him on
the head.
 Gases and liquids push out in equal forces in all directions. This is called Pascal's Law because
it was discovered by the scientist Blaise Pascal.
 When you go upside down in a roller coaster loop-the-loop, a special kind of force called
"centripetal
force"
keeps
you
in
your
seat
and
from
falling
out.
Energy
What is Energy?
The simplest definition of energy is "the ability to do work". Energy is how things change and move.
It's everywhere around us and takes all sorts of forms. It takes energy to cook food, to drive to school,
and to jump in the air.
Different forms of Energy
Energy can take a number of different forms. Here are some examples:
Chemical - Chemical energy comes from atoms and molecules and how they interact.
Electrical - Electrical energy is generated by the movement of electrons.
Gravitational - Large objects such as the Earth and the Sun create gravity and gravitational energy.
Heat - Heat energy is also called thermal energy. It comes from molecules of different temperatures
interacting.
Light - Light is called radiant energy. The Earth gets a lot of its energy from the light of the Sun.
Motion - Anything that is moving has energy. This is also called kinetic energy.
Nuclear - Huge amounts of nuclear energy can be generated by splitting atoms.
Potential - Potential energy is energy that is stored. One example of this is a spring that is pressed all
the way down. Another example is a book sitting high on a shelf.
Units of Measure for Energy
In physics, the standard unit of measure for energy is the joule which is abbreviated as J. There are
other units of measure for energy that are used throughout the world including kilowatt-hours, calories,
newton-meters, therms, and foot-pounds.
Law of Conservation of Energy
This law states that energy is never created or destroyed, it is only changed from one state to another.
One example is the chemical energy in food that we turn into kinetic energy when we move.
Renewable and Nonrenewable
As humans we use a lot of energy to drive our cars, heat and cool our houses, watch TV, and more.
This energy comes from a variety of places and in a number of forms. Conservationists classify the
energy we use into two types: renewable and nonrenewable. Nonrenewable energy uses up resources
that we cannot recreate. Some examples of this are gas to run our car and coal burned in power plants.
Once they are used, they are gone forever. A renewable energy source is one that can be replenished.
Examples of this include hydropower from turbines in a dam, wind power from windmills, and solar
power from the sun. The more renewable power we use the better for our planet and for future
generations as they won't run out of resources someday.
Fun Facts about Energy
In 2008 about 7% of the energy used in the United States was from renewable sources. A modern
windmill or turbine can generate enough electricity to power around 300 homes. People have used
waterpower to grind grain for over 2,000 years. Geothermal power uses energy from geysers, hot
springs, and volcanoes. The entire world could be powered for a year from the energy from the sun
that falls on the Earth's surface in one hour. We just need to figure out how to harness it!
Heat
Heat is the transfer of energy from a one object to another due to a difference in temperature. Heat can
be measured in joules, BTUs (British thermal unit), or calories. Heat and temperature are closely
related, but they are not the same thing. The temperature of an object is determined by how fast its
molecules are moving. The faster the molecules are moving the higher the temperature. We say objects
that have a high temperature are hot and objects with a low temperature are cold.
Transferring of Heat
When two items are combined or touching each other, their molecules will transfer energy called heat.
They will try to come to a point where they both have the same temperature. This is called equilibrium.
Heat will flow from the hotter object to the colder. The molecules in the hotter object will slow down
and the molecules in the colder object will speed up. Eventually they will get to the point where they
have the same temperature. This happens all the time around you. For example, when you take an ice
cube and put it into a warm soda. The ice cube will become warmer and melt, while the soda will cool
down.
Hot Objects Expand
When something gets hotter it will expand, or get bigger. At the same time, when something gets
colder it will shrink. This property is used to make mercury thermometers. The line in the thermometer
is actually liquid mercury. As the liquid gets hotter, it will expand and rise in the thermometer to show
a higher temperature. It's the expansion and contraction due to temperature that allows the
thermometer to work.
Heat Conduction
When heat transfers from one object to another, this is called conduction. Some materials conduct heat
better than others. Metal, for example, is a good conductor of heat. We use metal in pots and pans to
cook because it will move the heat from the flame to our food quickly. Cloth, like a blanket, isn't a
good conductor of heat. Because it's not a good conductor, a blanket works well to keep us warm at
night as it won't conduct the heat from our bodies out to the cold air.
Matter Changing State
Heat has an impact on the state of matter. Matter can change state based on heat or temperature. There
are three states that matter can take depending on its temperature: solid, liquid, and gas. For example,
if water is cold and its molecules are moving very slow, it will be a solid (ice). If it warms up some,
the ice will melt and water becomes a liquid. If you add a lot of heat to water, the molecules will move
very fast and it will become a gas (steam).
Waves
What is a wave?
When we think of the word "wave" we usually picture someone moving their hand back and forth to
say hello or maybe we think of a tall curling wall of water moving in from the ocean to crash on the
beach. In physics, a wave is a traveling disturbance that travels through space and matter transferring
energy from one place to another. When studying waves it's important to remember that they transfer
energy, not matter.
Waves in Everyday Life
There are lots of waves all around us in everyday life. Sound is a type of wave that moves through
matter and then vibrates our eardrums so we can hear. Light is a special kind of wave that is made up
of photons that helps us to see. You can drop a rock into a pond and see waves form in the water. We
even use waves (microwaves) to cook our food really fast.
Types of Waves
Waves can be divided into various categories depending on their characteristics. Below we describe
some of the different terms that scientists use to describe waves.
Mechanical Waves and Electromagnetic Waves
All waves can be categorized as either mechanical or electromagnetic.
Mechanical waves are waves that require a medium. This means that they have to have some sort of
matter to travel through. These waves travel when molecules in the medium collide with each other
passing on energy. One example of a mechanical wave is sound. Sound can travel through air, water,
or solids, but it can't travel through a vacuum. It needs the medium to help it travel. Other examples
include water waves, seismic waves, and waves traveling through a spring. Electromagnetic waves are
waves that can travel through a vacuum (empty space). They don't need a medium or matter. They
travel through electrical and magnetic fields that are generated by charged particles. Examples of
electromagnetic waves include light, microwaves, radio waves, and X-rays.
Transverse Waves and Longitudinal Waves
Another way to describe a wave is by the direction that its disturbance is traveling.
Transverse waves are waves where the disturbance moves perpendicular to the direction of the wave.
You can think of the wave moving left to right, while the disturbance moves up and down. One
example of a transverse wave is a water wave where the water moves up and down as the wave passes
through the ocean. Other examples include an oscillating string and a wave of fans in a stadium (the
people move up and down while the wave moves around the stadium). Longitudinal waves are waves
where the disturbance moves in the same direction as the wave. One example of this is a wave moving
through a stretched out slinky or spring. If you compress one portion of the slinky and let go, the wave
will move left to right. At the same time, the disturbance (which is the coils of the springs moving),
will also move left to right. Another classic example of a longitudinal wave is sound. As sound waves
propagate through a medium, the molecules collide with each other in the same direction as the sound
is moving. In the above picture the top wave is transverse and the bottom wave is longitudinal.
Interesting Facts about Waves
 Waves in the ocean are mostly generated by the wind moving across the ocean surface.
 The "medium" is the substance or material that carries a mechanical wave.
 One of the most important things to remember about waves is that they transport energy, not
matter. This makes them different from other phenomenon in physics.
 Many waves cannot be seen such as microwaves and radio waves.
 The tallest ocean wave ever recorded was 1,720 feet tall and occurred in Lituya Bay in Alaska.
Temperature
What is temperature?
Temperature can be a difficult property to define. In our everyday lives we use the word temperature to
describe the hotness or coldness of an object. In physics, the temperature is the average kinetic energy
of the moving particles in a substance.
How is temperature measured?
Temperature is measured using a thermometer. There are different scales and standards for measuring
temperature including Celsius, Fahrenheit, and Kelvin. These are discussed in more detail below.
How does a thermometer work?
Thermometers take advantage of a scientific property called thermal expansion. Most substances will
expand and take up more volume as they get hotter. Liquid thermometers have some sort of substance
(this used to be mercury, but today is generally alcohol) that is enclosed in a small glass tube. As the
temperature rises, the liquid expands and fills up more of the tube. When the temperature drops, the
liquid contracts and takes up less of the tube. The temperature can then be read by the lines calibrated
on the side of the tube.
Temperature Scales
There are three main temperature scales that are used today: Celsius, Fahrenheit, and Kelvin. Celsius The most common temperature scale in the world is Celsius. Celsius uses the unit "degrees" and is
abbreviated as °C. The scale sets the freezing point of water at 0 °C and the boiling point of water at
100 °C. Fahrenheit - The temperature scale most common in the United States is the Fahrenheit scale.
Fahrenheit sets the freezing point of water at 32 °F and the boiling point at 212 °F. Kelvin - The
standard unit of temperature that is most used by scientists is Kelvin. Kelvin doesn't use the ° symbol
like the other two scales. When writing a temperature in Kelvin you just use the letter K. Kelvin uses
absolute zero as the 0 point of its scale. It has the same increments as Celsius in that there are 100
increments between the freezing and boiling points of water.
Temperature and the State of Matter
Temperature has an effect on the state of matter. Each substance of matter will go through different
phases as the temperature increases including solid, liquid, and gas. One example of this is water
which changes from ice (solid) to water (liquid) to vapor (gas) as the temperature increases.
Interesting Facts about Temperature
 Temperature is independent of the size or quantity of an object. This is called an intensive
property.
 The Fahrenheit scale is named after Dutch physicist Daniel Fahrenheit.
 Temperature is a different quantity from the total amount of thermal energy in a substance,
which is dependent on the size of the object.
 Celsius was named after the Swedish astronomer Anders Celsius. Celsius was originally known
as "centigrade."
 As substances approach absolute zero they can achieve some interesting properties such as
superfluidity and superconductivity.
Work
What is work?
We often use the word "work" in our everyday lives. For example, we would say that getting good
grades in school takes a lot of hard "work". In physics, the term "work" has a specific meaning. Work,
in physics, occurs when a force acts on an object to move it some distance from the start point (also
called displacement). Work is calculated as the force times the distance. The following equation is
used to describe work: Work = Force * distance or W = Fd
How to Measure Work
The standard unit for work is the joule (J). The joule is the same as a newton-meter where the newton
is the force and the meter is the distance.
Force and Displacement
The distance (or displacement) in work is the distance from the start point to the end point. The
amount of traveling in between doesn't matter. For example, if you lift a weight off the ground and
then place it back on the ground the distance (or displacement) is zero.
Don't be Tricked
Measuring work can sometimes be tricky. In order for the equation W = Fd to work, the force used in
the equation must be the force used to cause the displacement or distance. Also, remember for work to
have occurred, the object must be displaced by the force. Otherwise, the distance, or "d", in the
formula is 0 and the work will be 0. Here are some examples: If someone is pushing on a wall with all
their might, but the wall doesn't move, no work has occurred. This is because the distance is zero. If
someone is using force to hold a rock over their head while walking eastward across a field, no work
has occurred. This is because the force is not in the same direction (the force is up) as the distance
moved by the rock (eastward). If you do a full push-up, lifting yourself up and then back down, the
total work is zero. This is because the total distance from the starting point to the ending point is zero.
If you drop your pencil, then work has occurred. This is because the displacement of the pencil from
your hand to the ground is greater than zero and is in the same direction as the force acting on the
pencil, which is gravity.
Example problem:
A baseball player throws a ball with a force of 10 N. The ball travels 20 meters. What is the total
work? W = F * distance W = 10 N* 20 meters W = 200 joules
More Complicated Problems
When the angle between the force and displacement is not 0 degrees or 90 degrees, a more complex
formula for work is used. This formula includes the angle theta (Θ) which is the angle between the
force and displacement. W = F * d * cos Θ In the case where the force and the displacement are in the
exact same direction theta = 0 and the cos Θ = 1. In the case where the force has no impact on the
displacement and theta = 90 degrees, then cos Θ = 0 and, therefore, the work = 0.
Interesting Facts about Work
 Work is a scalar quantity, not a vector quantity. This means that, unlike force and velocity, it
has no direction, only a magnitude.
 Another unit of work is the foot-pound. One foot-pound is equal to 1.35581795 joules.
 The joule is also used as the standard unit of measure for energy.
Negative work is when force acting on an object hinders the object's displacement.
Electricity
Important things to know about electricity?
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Conductors and insulators - Conductors are materials that allow electricity to flow easily. Most
types of metal are good conductors, which is why we use metal for electrical wire. Copper is a
good conductor and isn't too expensive, so it's used a lot for the wiring in homes today.
Insulators are the opposite of conductors. An insulator is a material that doesn't carry
electricity. Insulators are important because they can protect us from electricity. Materials like
rubber, plastic, and paper are good insulators.
Voltage - Voltage is the name for the electric force that causes electrons to flow. It's the
measure of potential difference between two points in the circuit. Voltage may come from a
battery or a power plant.
Current - Current is the measure of the flow of electrons in a circuit. Current is measured in
Amps or Amperes.
Power (Watts) - The power or energy used by a circuit is measured in Watts. You can calculate
the number of Watts by multiplying the Voltage times the Current. When your parents get their
electrical bill it's generally in kilowatt hours. This is the measurement of power over time or
how much power was used that month.
Resistance - Resistance measures how well a material or object conducts electricity. Low
resistance means the object conducts electricity well, high resistance means the object does not
conduct electricity well.
Battery
A battery can act as a source of electricity in circuits. It stores up electric power and then provides a
voltage across a circuit causing power to flow through the circuit. Batteries use chemicals that react to
make electricity. They have a positive connection called the anode and a negative connection called
the cathode. When a circuit with a load is placed across the anode and cathode, the chemicals react
causing electricity to flow through the circuit. The chemicals in batteries only last so long, so batteries
have a limited amount of electricity and eventually will run out.
Alternate and Direct Current
There are two main types of current used in electrical systems today: alternate current (AC) and direct
current (DC). Batteries, and most electronics, use direct current. This is where current always flows in
one direction. Power stations that generate power for our homes generate current that constantly
changes direction (60 times each second). Therefore the power that we get from our wall outlets is AC
current.
Static Electricity
Sometimes electric charges can build up on the surface of objects. This is called static electricity.
When you put on your clothes and they sometimes "stick" to your body or have an attraction to you,
this is static electricity. When your hair sometimes goes straight up for no reason, this can be static
electricity. If you rub a balloon against your clothes, you can build up a static electricity charge on the
balloon that will cause it to stick to a wall. Static electricity can sometimes damage electronic
components. There are anti-static bags and other ways to protect components from getting damaged.
Electric Current
Current is the flow of an electric charge. It is an important quantity in electronic circuits. Current flows
through a circuit when a voltage is placed across two points of a conductor.
Flow of Electrons
In an electronic circuit, the current is the flow of electrons. However, generally current is shown in the
direction of the positive charges. This is actually in the opposite direction of the movement of the
electrons in the circuit.
How is current measured?
The standard unit of measurement for current is the ampere. It is sometimes abbreviated as A or amps.
The symbol used for current is the letter "i." Current is measured as the flow of electric charge over
time past a given point in an electric circuit. One ampere is equal to 1 coulomb over 1 second. A
coulomb is a standard unit of electric charge.
Calculating Current
Current can be calculated using Ohm's Law. It can also be used to figure out the resistance of a circuit
if the voltage is also known or the voltage of a circuit if the resistance is known. I = V/R where I =
current, V = voltage, and R = resistance Current is also used to calculate power using the following
equation: P = I * V where P = power, I = current, and V = voltage.
AC versus DC
There are two main types of current used in most electronic circuits today. They are alternating current
(AC) and direct current (DC). Direct Current (DC) - Direct current is the constant flow of electric
charge in one direction. Batteries generate direct current to power handheld items. Most electronics use
direct current for internal power often converting alternating current (AC) to direct current (DC) using
a transformer. Alternating Current (AC) - Alternating current is current where the flow of electric
charge is constantly changing directions. Alternating current is mostly used today to transmit power on
power lines. In the United States the frequency at which the current alternates is 60 Hertz. Some other
countries use 50 Hertz as the standard frequency.
Electromagnetism
Current also plays an important role in electromagnetism. Ampere's law describes how a magnetic
field is generated by an electric current. This technology is used in electric motors.
Interesting Facts about Current
 The direction of the current flow is often shown with an arrow. In most electronic circuits the
current is shown as flowing towards ground.
 The current in a circuit is measured using a tool called an amperemeter.
 The flowing of electric current through a wire can sometimes be thought of like the flowing of
water through a pipe.
 The electrical conductivity of a material is the measurement of the ability of the material to
allow for the flow of electrical current.
Magnetism
Magnetism is an invisible force or field caused by the unique properties of certain materials. In most
objects, electrons spin in different, random directions. This causes them to cancel each other out over
time. However, magnets are different. In magnets the molecules are uniquely arranged so that their
electrons spin in the same direction. This arrangement of atoms creates two poles in a magnet, a Northseeking pole and a South-seeking pole.
Magnets Have Magnetic Fields
The magnetic force in a magnet flows from the North pole to the South pole. This creates a magnetic
field around a magnet. Have you ever held two magnets close to each other? They don't act like most
objects. If you try to push the South poles together, they repel each other. Two North poles also repel
each other. Turn one magnet around, and the North (N) and the South (S) poles are attracted to each
other. Just like protons and electrons - opposites attract.
Where do we get magnets?
Only a few materials have the right type of structures to allow the electrons to line up just right to
create a magnet. The main material we use in magnets today is iron. Steel has a lot of iron in it, so steel
can be used as well.
The Earth is a giant magnet
At the center of the Earth spins the Earth's core. The core is made up of mostly iron. The outer portion
of the core is liquid iron that spins and makes the earth into a giant magnet. This is where we get the
names for the north and south poles. These poles are actually the positive and negative poles of the
Earth's giant magnet. This is very useful to us here on Earth as it lets us use magnets in compasses to
find our way and make sure we are heading in the right direction. It's also useful to animals such as
birds and whales who use the Earth's magnetic field to find the right direction when migrating. Perhaps
the most important feature of the Earth's magnetic field is that it protects us from the Sun's solar wind
and radiation.
The Electric Magnet and Motor
Magnets can also be created by using electricity. By wrapping a wire around an iron bar and
running current through the wire, very strong magnets can be created. This is called electromagnetism.
The magnetic field created by electromagnets can be used in a variety of applications. One of the most
important is the electric motor.
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Electromagnetism and Electric Motors
Electromagnetism
The word "electromagnetism" in physics is used to describe one of the fundamental forces of nature.
This force is between subatomic particles such as protons and electrons. It helps to hold matter
together. Electromagnetism is also used to describe how a magnetic field is created by the flowing of
electric current. We will be discussing this type of electromagnetism on this page.
Electromagnet
When an electronic current flows through a wire, it generates a magnetic field. This is an important
concept in electricity. The magnetic field can be increased by coiling the wire. This allows more
current to flow through a smaller distance and increases the magnetic field.
Right-Hand Rule
When current is flowing through a wire, the magnetic field rotates around the wire. The direction of
the current determines the direction of the magnetic field. You can figure out the direction of the
magnetic field using the "right-hand rule". To determine the direction of the magnetic field, look at the
picture above. Take your right hand and point your thumb in the direction of the current (I). Now wrap
your fingers around the wire. Your fingers will point in the direction of the rotation of the magnetic
field (B).
Motors
One of the important applications of electromagnetism is the electric motor. An electric motor converts
electrical energy into physical movement. Electric motors generate magnetic fields with electric
current through a coil. The magnetic field then causes a force with a magnet that causes movement or
spinning that runs the motor. Electric motors are used in all sorts of applications. There are even
several electric motors inside your computer including one to turn the fan, one to open and shut the
CDROM drive, and one to operate the hard drive.
Electromagnetic Induction Another important application of electromagnetism is induction. Induction
is when movement is used to create electricity (the opposite of using electricity to create movement).
As a wire is moved through a magnetic field, current will begin to flow through it.
Generators
Electric generators convert mechanical energy into electrical energy using induction. As a coil of wire
is spun between two opposite magnets, an electric current is generated that can be used to power
electronic devices. Generators can get their power from a wide variety of sources. Two popular electric
generators of renewable energy include hydropower and wind power.
Fun Facts about Electromagnetism and Electric Motors
 Some electric generators can be driven by human power such as a hand crank or a bicycle to
generate electricity.
 Danish physicist Hans Orsted was the first to discover that a magnetic field was produced by
the flow of electric current.
 American physicist Joseph Henry discovered electromagnetic inductance and built the first
electromagnetic motor.
 Loudspeakers use electromagnets to vibrate the cone and produce sound.
 Using electromagnetism, powerful magnets can be turned on and off using electricity, unlike
permanent magnets.
Electromagnetic Waves
Electromagnetic waves are a form of energy waves that have both an electric and magnetic field.
Electromagnetic waves are different from mechanical waves in that they can transmit energy and travel
through a vacuum. Electromagnetic waves are classified according to their frequency. The different
types of waves have different uses and functions in our everyday lives. The most important of these is
visible light, which enables us to see.
Radio Waves
Radio waves have the longest wavelengths of all the electromagnetic waves. They range from around a
foot long to several miles long. Radio waves are often used to transmit data and have been used for all
sorts of applications including radio, satellites, radar, and computer networks.
Microwaves
Microwaves are shorter than radio waves with wavelengths measured in centimeters. We use
microwaves to cook food, transmit information, and in radar that helps to predict the weather.
Microwaves are useful in communication because they can penetrate clouds, smoke, and light rain.
The universe is filled with cosmic microwave background radiation that scientists believe are clues to
the origin of the universe they call the Big Bang.
Infrared
Between microwaves and visible light are infrared waves. Infrared waves are sometimes classified as
"near" infrared and "far" infrared. Near infrared waves are the waves that are closer to visible light in
wavelength. These are the infrared waves that are used in your TV remote to change channels. Far
infrared waves are further away from visible light in wavelength. Far infrared waves are thermal and
give off heat. Anything that gives off heat radiates infrared waves. This includes the human body!
Visible light
The visible light spectrum covers the wavelengths that can be seen by the human eye. This is the range
of wavelengths from 390 to 700 nm which corresponds to the frequencies 430-790 THz. You can go
here to learn more about the visible spectrum.
Ultraviolet
Ultraviolet waves have the next shortest wavelength after visible light. It is ultraviolet rays from the
Sun that cause sunburns. We are protected from the Sun's ultraviolet rays by the ozone layer. Some
insects, such as bumblebees, can see ultraviolet light. Ultraviolet light is used by powerful telescopes
like the Hubble Space Telescope to see far away stars.
X-rays
X-rays have even shorter wavelengths than ultraviolet rays. At this point in the electromagnetic
spectrum, scientists begin to think of these rays more as particles than waves. X-rays were discovered
by German scientist Wilhelm Roentgen. They can penetrate soft tissue like skin and muscle and are
used to take X-ray pictures of bones in medicine.
Gamma rays
As the wavelengths of electromagnetic waves get shorter, their energy increases. Gamma rays are the
shortest waves in the spectrum and, as a result, have the most energy. Gamma rays are sometimes used
in treating cancer and in taking detailed images for diagnostic medicine. Gamma rays are produced in
high energy nuclear explosions and supernovas.
Optics
What is light made of?
This is not an easy question. Light has no mass and is not really considered matter. So does it even
exist? Of course it does! We couldn't live without light. Today scientists say light is a form of energy
made of photons. Light is unique in that it behaves like both a particle and a wave.
Why does light go through some things and not others?
Depending on the type of matter it comes into contact with, light will behave differently. Sometimes
light will pass directly through the matter, like with air or water. This type of matter is called
transparent. Other objects completely reflect light, like an animal or a book. These objects are called
opaque. A third type of object does some of both and tends to scatter the light. These objects are called
translucent objects.
Light helps us to survive
Without sunlight our world would be a dead dark place. Sunlight does more than just help us see
(which is pretty great, too). Sunlight keeps the Earth warm, so it's not just a frozen ball in outer space.
It also is a major component in photosynthesis which is how most of the plant life on Earth grows and
gets nutrients. Sunlight is a source of energy as well as a source of vitamin D for humans.
The speed of light
Light moves at the fastest known speed in the universe. Nothing moves faster than (or even close to)
the speed of light. In a vacuum, where there is nothing to slow it down, light travels 186,282 miles per
second! Wow, that's fast! When light travels through matter, like air or water, it slows down some, but
it's still pretty fast. To give you and idea as to how fast light is, we'll give you some examples. The sun
is almost 93 million miles from the Earth. It takes around 8 minutes for light to get from the sun to the
Earth. It takes around 1.3 seconds for light to go from the moon to the Earth.
Refraction
Normally, light travels in a straight path called a ray, however, when passing through
transparent materials, like water or glass, light bends or turns. This is because different materials or
mediums have different qualities. In each type of medium, whether it is air or water or glass, the
wavelength of the light will change, but not the frequency. As a result, the direction and speed of the
traveling light wave will change and the light will appear to bend or change directions. One example of
refraction is a prism. Prisms are unique in that each color of light is refracted to a different angle. So it
can take white light from the sun and send out light of various colors. Lenses use refraction to help us
see things. Telescopes help us to see things far away and microscopes enable us to see very small
things. Even glasses use refraction so that we can see everyday things more clearly.
Atomic structure
The atom is the basic building block for all matter in the universe. Atoms are extremely small and are
made up of a few even smaller particles. The basic particles that make up an atom are electrons,
protons, and neutrons. Atoms fit together with other atoms to make up matter. It takes a lot of atoms to
make up anything. There are so many atoms in a single human body we won't even try to write the
number here. Suffice it to say that the number is trillions and trillions (and then some more). There are
different kinds of atoms based on the number of electrons, protons, and neutrons each atom contains.
Each different kind of atom makes up an element. There are 92 natural elements and up to 118 when
you count in man-made elements. Atoms last a long time, in most cases forever. They can change and
undergo chemical reactions, sharing electrons with other atoms. But the nucleus is very hard to split,
meaning most atoms are around for a long time.
Structure of the Atom
At the center of the atom is the nucleus. The nucleus is made up of the protons and neutrons. The
electrons spin in orbits around the outside of the nucleus.
The Proton
The proton is a positively charged particle that is located at the center of the atom in the nucleus. The
hydrogen atom is unique in that it only has a single proton and no neutron in its nucleus.
The Electron
The electron is a negatively charged particle that spins around the outside of the nucleus. Electrons
spin so fast around the nucleus, scientists can never be 100% sure where they are located, but scientists
can make estimates of where electrons should be. If there are the same number of electrons and
protons in an atom, then the atom is said to have a neutral charge. Electrons are attracted to the nucleus
by the positive charge of the protons. Electrons are much smaller than neutrons and protons. About
1800 times smaller!
The Neutron
The neutron doesn't have any charge. The number of neutrons affects the mass and the radioactivity of
the atom.
Other (even smaller!) particles
 Quark - The quark is a really small particle that makes up neutrons and protons. Quarks are
nearly impossible to detect and it's only recently that scientists figured out they existed. They
were discovered in 1964 by Murray Gell-Mann. There are 6 types of quarks: up, down, top,
bottom, charm, and strange.
 Neutrino - Neutrinos are formed by nuclear reactions. They are like electrons without any
charge and are usually travelling at the speed of light. Trillions and trillions of neutrinos are
emitted by the sun every second. Neutrinos pass right through most solids including humans!
Nuclear Physics
Nuclear energy is the energy stored inside an atom by the forces that hold together the nucleus of the
atom. Scientists have learned how to capture large amounts of energy from these forces that can then
be used to generate electricity. E = mc2 When working on his theory of relativity, Albert Einstein
discovered the mathematical formula E = mc2. This formula demonstrated that matter could be
converted into energy. Although this sounds like a simple concept, it demonstrated that a large amount
of energy could be generated from a very small amount of matter. This could be done by splitting an
atom in a process called nuclear fission.
Nuclear Fission
Nuclear fission is the process of splitting of a large atom into two or more smaller atoms. When an
atom is split a huge amount of energy is released. When the energy is released in a slow controlled
manner, it can be used to generate electricity to power our homes. When the energy is released all at
once, a chain reaction occurs causing a nuclear explosion.
Nuclear Power Plants
One of the major applications for nuclear fission is nuclear power. Nuclear power plants use nuclear
fission to generate heat. They use this heat to create steam from water which, in turn, powers electrical
generators. Around twenty percent of the electricity in the United States is generated by nuclear power
plants. There are 104 commercial nuclear generating units in the U.S. Nuclear power plants use the
element uranium as fuel. Control rods of uranium are used to make sure that the chain reaction of
atoms splitting proceeds at a controlled pace.
Radioactive Waste
One of the byproducts of nuclear energy is radioactive waste. This is leftover material from the nuclear
reaction. Radioactive material can be dangerous to humans and animal life.
Other Uses of Nuclear Power
Nuclear power has other applications in addition to power plants. One application is nuclear
propulsion in ships and submarines. Nuclear powered submarines can stay under water and travel at
high speeds for a long time. Nuclear power has also been used in naval ships, ships used for breaking
ice in the polar seas, and space ships. These ships of the U.S. Navy are nuclear powered
Nuclear Fusion
Another form of nuclear energy is nuclear fusion. Fusion occurs when two or more atoms are joined
together to make a larger atom. Stars get their power from nuclear fusion. Deep inside a star, hydrogen
atoms are constantly being converted by fusion into helium atoms. It's this process that generates the
light and heat energy given off by the stars including the Sun. Scientists have not figured out how to
control fusion to create usable energy. If they could it would be great news as fusion produces less
radioactive material and would give us a virtually unlimited supply of energy.
Interesting Facts about Nuclear Energy and Fission
 The top three states for generating nuclear energy are Illinois, Pennsylvania, and South
Carolina.
 The United States generates more nuclear energy than any other nation.
 In the history of nuclear energy there have been three major nuclear power plant disasters
including Chernobyl (Russia), Three Mile Island (United States), and Fukushima Daiichi
(Japan).
 The first nuclear powered submarine was the U.S.S. Nautilus which put out to sea in 1954.
 One uranium pellet can generate the same amount of energy as around 1,000 kilograms of coal.
 The "smoke" you see coming from a nuclear power plant is not pollution, but steam.
Radioactivity
Stable and Unstable Isotopes
Elements can be made up of different isotopes. Isotopes are atoms with the same number of protons
and electrons, but a different number of neutrons. Sometimes isotopes are stable and happy. These are
the elements that we see around us and find in nature. However, some isotopes are unstable. These
isotopes are called radioactive isotopes. You can go here to learn more about isotopes.
What is radioactive decay?
When isotopes are unstable they emit energy in the form of radiation. There are three main types of
radiation or radioactive decay depending on the isotope.
Different Types of Radioactivity
Alpha decay - Alpha decay is caused when there are too many protons in a nucleus. In this case the
element will emit radiation in the form of positively charged particles called alpha particles. Beta
decay - Beta decay is caused when there are too many neutrons in a nucleus. In this case the element
will emit radiation in the form of negatively charged particles called beta particles. Gamma decay Gamma decay occurs when there is too much energy in the nucleus. In this case gamma particles with
no overall charge are emitted from the element.
How is it measured?
Radioactivity is measured using a unit called the "curie". It is abbreviated as "Ci". The curie measures
how many atoms spontaneously decay each second. The curie was named after Marie and Pierre Curie
who discovered the element radium.
What is the half-life of an isotope?
The half-life of an isotope is the time on average that it takes for half of the atoms in a sample to
decay. For example, the half-life of carbon-14 is 5730 years. This means that if you have a sample of
carbon-14 with 1,000 atoms, 500 of these atoms are expected to decay over the course of 5730 years.
Some of the atoms may decay right away, while others will not decay for many thousands more years.
The thing to remember about half-life is that it is a probability. In the example above, 500 atoms are
"expected" to decay. This is not a guarantee for one specific sample. It is just what will happen on
average over the course of billions and billions of atoms.
Radioactive Decay to other Elements
When isotopes decay they can lose some of their atomic particles (i.e. electrons and protons) and turn
from one element into another. Sometimes isotopes decay from one unstable isotope into another
unstable isotope. This can happen continuously in a long radioactive chain. An example of a
radioactive chain is uranium-238. As it decays, it transforms through a number of elements including
thorium, radium, francium, radon, polonium, and bismuth. It finally ends up as a stable isotope as the
element lead.
Why is radiation dangerous? Is some radiation good?
Radiation can alter the structure of cells in our bodies causing mutations which can produce cancer.
The more radiation a person is exposed to, the more dangerous it is.
Despite the risks, there are a number of good ways that science has used radiation. These include Xrays, medicine, carbon dating, energy generation, and to kill germs.
Interesting Facts about Radioactivity
 Uranium in the ground can decay into radon gas which can be very dangerous to humans. It is
thought to be the second leading cause of lung cancer.
 The half-life of carbon-14 is used in carbon dating to determine the age of fossils.
 Bismuth is the heaviest element with at least one stable isotope. All elements heavier than
bismuth are radioactive.
 Radioactivity was discovered by the scientist A. H. Becquerel in 1896.