Matter and Energy
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Matter and Energy
Some basic terms
Matter material objects
Energy the ability to do work; what makes matter move
Can be measured in Calories, Joules, or other units
Three basic categories of energy:
Kinetic energy energy of motion
Potential energy energy being stored for later use
Radioactive energy energy carried by light or radiation
Scientific View of Energy
Kinetic energy = mv2, where m is mass and v is speed
Gives energy in Joules
We can take a group of objects and determine the average kinetic energy of the group
This is defined as the temperature of the group
The sum of all the kinetic energies of the group is its thermal energy
Temperature and thermal energy (or heats) are not the same thing
Both boxes have the same temperature, but the right box has more thermal energy due to having more particles
Gravitational potential energy of an object released in a fall depends on its mass, the strength of gravity, and the distance it falls
As interstellar material was attracted to each other to form a star, the gravitational potential energy was converted into thermal energy.
Mass-energy is the potential energy stored inside matter
Predicted by Einstein's equation, E = mc2
Law of Conservation of Energy energy cannot be created or destroyed, only converted into other forms of energy
Matter can exist and change in phases: solid, liquid, or gas
All matter can be broken down into atoms, which come in different varieties called elements
Atoms can combine to form molecules for even more variety, creating compounds
Each atom is composed of a nucleus of particles called protons and usually neutrons, and surrounded by electrons
Each particle has an electrical charge. Protons have a charge of +1, electrons a charge of 1, and neutrons are electrically neutral
Atomic number number of protons in the atom's nucleus
Atomic mass combined number of protons and neutrons
Isotopes atoms of the same elements with different numbers of neutrons
Phases of Matter
When ice is heated, the molecules in the solid vibrate faster and faster
At 0 degrees Celsius, the molecules have enough energy to break the solid bonds, but they're still bound somewhat as a liquid
As energy continues to increase, the water molecules vibrate at increasing rates until they break free into a gas
A gas can be heated until the molecules separate into atoms. This is molecular dissociation
Going from solid to liquid or gas is sublimation
Going from liquid to gas is evaporation
When atoms have enough energy, they separate into ions and electrons as plasma
Energy in Atoms
Electrons surround nucleus in a "smeared out" cloud
When the electron is smeared out to the minimum extent, it is in the ground state
As the electron gains energy, the cloud is smeared out over a greater volume, into an excited state
When the electron gains enough energy to escape the atom completely, the atom is ionized
Only very specific energy values are possible for excited states
Electrons must gain exact amount of energy to jump levels
When electrons drop to lower levels, they either give their energy to other particles, or else emit the energy as light
These little "packets" of light are called photons
The electron energy levels are said to be quantized, as they only exist at specific values
Different elements have different electron energy levels, so the light they give off carries a "fingerprint" of the element
Universal Motion
Describing Motion
Velocity is speed in a specific direction
Acceleration is a change in velocity
You can be accelerating without changing your speed, if you are only changing direction
Can be positive or negative
Constant velocity feels like standing still, while acceleration can be felt easily
One common acceleration is the acceleration due to gravity
All objects are accelerated at the same rate, regardless of mass
The acceleration due to gravity on Earth is abbreviated g
On Earth, the acceleration due to gravity causes objects increase their velocity by 9.8 m/s every second
Massive, faster moving objects are said to have much more momentum than lighter, slower objects
Momentum is defined as mass x velocity
Anything that causes a change in momentum is called a force
There are many types of forces: friction, gravity, air resistance, electric force, etc.
Momentum is not changed, however, if there is no net force
Mass the amount of material in an object
Weight the force that gravity exerts on a mass
Not the same thing
Weight of something on Earth would be very different than its weight on the moon, but the mass would be same
Acceleration is what produces a sense of weight
In a free fall, you would feel weightless
When accelerating, person feels lighter or heavier
At constant velocity, person feels normal weight
The weightlessness you would feel in a falling elevator is the same weightlessness that the astronauts experience in orbit
Orbiting spacecraft are in a constant state of freefall
Imagine shooting a cannonball; at faster speeds, the ball lands further away
Objects in orbit are not in zero gravity
At escape velocity, an object breaks free of the Earth's gravity completely
Understanding Motion
The way forces affect motion are defined by Newton's Laws of Motion
Newton's 1st law of motion: In the absence of a net force, an object moves with constant velocity
Any object in motion will continue in a straight line at constant speed unless some force works to change it
Newton's 2nd law of motion states what happens when we do have a net force
Force = mass x acceleration
Objects moving in a circle must experience a force inward to keep them curving around
If the force were removed, the object would follow Newton's 1st Law of motion
Newton's 3rd Law of Motion: For any force, there is always an equal and opposite reaction force.
Explains how rockets function:
When hot gas is driven out of the back of the rocket, the reaction is that the rocket must be pushed with an equal force in the opposite direction
The rocket does not push off the ground
All three Newton's laws are expressions of conservation of momentum
1st law: If you don't do anything to an object, its momentum doesn't change at all
2nd law: If you exert a force on an object, its momentum does change
3rd law: When the momentum changes, an equal and opposite change of momentum happens to some other object
So total momentum is always unchanged
On a pool table, momentum conservation more obvious
Even a collision on the edge of the ball still conserves the momentum between the two balls
Understanding Motion
Something spinning or curving has angular momentum
Just as force causes a change in momentum, torque causes a change in angular momentum
Torque depends not just on hard the force is, but where the force is applied
Just like regular momentum, angular momentum is conserved, too
Law of Conservation of Angular Momentum: In the absence of net torque, the total amount of angular momentum of a system remains constant
Planetary Motion
Copernicus: First theorized a Sun-centered solar system
Brahe: Maintained an extensive collection of data obtained by naked-eye observation
Kepler: Used Brahe's data to correctly calculate the orbits of the planets
Summarized his findings in three laws of planetary motion
Kepler's second law states as a planet moves around its orbit, it sweeps out equal areas in equal times
This means a planet travels faster nearer the sun, and slower farther away
Kepler's third law of planetary motion states that a planet's orbital period is related to its average distance from the Sun by the equation:
(orbital period in years) 2 = (average distance in AU) 3
This means more distant planets move at slower speeds
Note that the period doesn't depend on the mass of the planet
Galileo used a telescope and managed to refute several arguments against a Sun-centered model
His discoveries included:
Mountains and valleys on the moon
Resolving the Milky Way into countless individual stars
Moons around Jupiter
Venus going through phases like the moon
Kepler's first law states that the planet's orbits are ellipses with the sun at one focus
Gravity
Newton developed his universal law of gravitation to describe the force of gravity
Can be summarized in three statements:
Every mass attracts every other mass in the universe
The force of attraction is directly proportional to the product of their masses
The force of attraction decreases with the square of the distance between the centers of the objects
Mathematically expressed as F = GM1M2/d2
Tides are caused by the pull of the moon's gravity
The difference in the attraction at different distances creates two bulges on the earth
The sun's gravity also causes some variations in the tides
Gravity causes tidal friction that slows the Earth's rotation very gradually
Also lets the moon move slightly farther from the Earth
The ultimate outcome of tidal friction is synchronous rotation, in which objects rotate at the same rate that they orbit
Pluto and Charon are an ultimate example of this
Because objects in orbit must conserve their total orbital energy, gravitational potential and kinetic
Orbits cannot change spontaneously
Must gain energy to change orbit or escape from gravity completely
One surprising result of the universal law of gravitation was that objects fall at the same acceleration regardless of mass
Light
Facts of Light
Light is a form of radioactive energy
Energy of light can interact with matter in four ways:
Emission When light is sent out from an object
Absorption When an object absorbs the energy of the light
Transmission When a substance allows light to pass through it
Reflection When light bounces off a substance without being absorbed or transmitted
Materials that transmit light are transparent, while materials that absorb light are opaque
Light behaves like waves, which are separate from particles
Waves have three basic properties:
Wavelength the distance between adjacent peaks
Frequency the number of peaks passing by any point each second
Speed the rate at which the wave moves forward
Light is an electromagnetic wave composed of vibrating electric and magnetic fields
Light also acts like a particle as well as a wave
Particles are called photons, and possess kinetic energy that is related to its wavelength
The various wavelengths of light all compose a broad spectrum, that represents light of various energies
The shorter the wavelength, the higher the energy
Spectrum ranges from short-wavelength gamma rays, to long-wavelength radio waves
Visible light is only a tiny section on the spectrum
Light and Matter
The interaction of light and matter leaves indications on the matter
A red shirt absorbs all light except the red wavelengths, and reflects those back to your eye
In the same way, we can look at what is emitted or absorbed from a celestial body to determine its composition
Different substances absorb or emit different wavelengths of light
We can observe these wavelengths on a line spectrum
Thermal radiation An opaque object re-emits the energy it has gained as thermal radiation
Depends mainly on the temperature of the body
Two main rules of thermal radiation:
Hotter objects radiate more total energy
Hotter objects emit photons with higher average energy, and thus, shorter wavelengths
A poker in a fire demonstrates both rules of thermal radiation
As the poker is heated, it gets brighter, and its color moves from infrared, to red, to white
The Doppler Effect
The same thing that happens to sound waves in motion also happens for light waves emitted from moving objects
The shift of an object's spectrum because of its motion is the Doppler Effect
Light from objects moving away from us are redshifted
Light from objects moving toward us are blueshifted
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