Active Physics1

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Active Physics

• Materials are or may be copyrighted. These
  should only be used for educational purposes
  (Fair Use Policy).

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Parts of a Wave

• A wave is a disturbance that travels through a
  substance
• The high points are called crests.
• The low points are called troughs.
• The amplitude is the distance from the midpoint
  to the crest (or trough) of the wave.
• The wavelength is the distance between two points
  of the same phase in consecutive cycles of a wave.

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Practice
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Transverse Waves

• A transverse wave oscillates (moves)
  perpendicular (at right angles) to the direction
  of travel
• Ex
• Light
• Stretched strings of musical instruments
• radio waves
• Seismic S waves

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Longitudinal Waves

• A longitudinal wave oscillates (moves) parallel
  to (or along) the direction of travel.
• Ex
• Springs (like a slinky)
• Sound (acoustic) waves
• Seismic P waves

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Longitudinal Waves

• Longitudinal waves causes compression (squeezing)
  and rarefaction (expansion) of the substance
  carrying the wave.

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Waves and Energy

• A wave transports energy.
• Forms of energy are movement, light, heat, etc.
• Waves are transmitted through a medium
  (substance).
• Ex, sound waves travel through air

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Waves and Energy

• Waves do not transport matter

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Sound Waves and Medium
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Waves and Medium

• Waves travel at the same speed in the medium
  (p.17) through which they propagate (travel)
• Ex
• All sound waves in air travel at the same speed
  (340 m/s)
• All sound waves in water travel at the same speed
  (1497 m/s)
• All electromagnetic waves in water travel at the
  same speed (140,000 mi./s)

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Frequency

• The number of back-and-forth movements an object
  makes in a unit of time is an objects frequency
  measured in hertz (Hz) or cycles per second.
• A complete back-and-forth movement is one cycle
  or period measured in seconds.

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Frequency

• One period is one back-and-forth motion

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Frequency
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Practice

• The Sears Tower in Chicago sways back and forth
  at a frequency of about 0.1 Hz. What is its
  period of vibration?

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Practice

• The Sears Tower in Chicago sways back and forth
  at a frequency of about 0.1 Hz. What is its
  period of vibration?
• Answer The period is

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Practice

1. Suppose, for example, that a pendulum makes two
   vibrations in one second in other words, its
   frequency is 2 Hz. What is its period?
2. A waves period is 5 s, what is its frequency?
3. A pendulum frequency is 8 Hz. What is its period?
4. A waves period is 9 s, what is its frequency?
5. A pendulums frequency is 5 Hz. What is its
   period?
6. A waves period is 11 s, what is its frequency?

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Wave Speed

• The energy transferred by a wave is measured by
  the wave speed, whose units is in meters per
  second (m/s).

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Wave Speed
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Wave Practice Problems

• wave speed wavelength x wave frequency
• v ?f
• v wave speed
• wavelength
• f wave frequency

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Practice

• If the wavelength is 10 meters, and 2 wavelengths
  per second passes the pole, then what is the
  speed of the wave?
• v ?f
• 10 m/wavelength
• f 2 wavelength/s
• v (10 m/wavelength) x (2 wavelength/s)
• v 20 m/s

10 m
?
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Practice

• If the wavelength is 7 m/wavelength and if the
  frequency is 5 wavelengths/s, what is the wave
  speed?
• What is the frequency of a wave when the
  wavelength is 2 m and the speed of the wave is 10
  m/s?
• If the frequency is 3-Hz and the speed of the
  wave is 9 m/s, what is the wavelength?

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Sound Waves

• Acoustic (sound) waves are produced when a
  vibrating object is in contact with a medium
  (p.17)
• Ex, speaker in contact with air
• The larger the amplitude (p.11), the louder the
  sound
• The higher the frequency (p. 21), the higher the
  sound pitch

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Sound Waves
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Sound Waves

• Sound waves cannot travel through a vacuum (empty
  space)
• There are no particles in a vacuum, so the sound
  has no medium (p. 17) to travel through

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Sound Waves

• The speed of sound is faster in solid and liquids
  than it is in gases.
• The molecules in solids and liquids are closer to
  each other than in gases.
• The closer the molecules are to each other, the
  less time it takes for them to pass the sound to
  each other, creating faster sound waves.

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Sound Waves and Medium
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Electromagnetic Waves
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Electromagnetic Spectrum
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Electromagnetic Waves

• Electromagnetic waves are all transverse waves
  (p. 13)
• Ex. radio waves, microwaves, infrared, visible
  light, ultraviolet, X-rays, gamma rays
• Electromagnetic waves with larger wavelengths
  have lower frequencies and vice versa.
• But they all travel at the same speed, the speed
  of light 3 x 108 m/s (186,000 miles/s) in a
  vacuum (p. 27).

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Electromagnetic Waves and Medium
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Interference

• Wave interference is when waves collide with each
  other.
• In constructive interference, the waves reinforce
  each other to produce a wave of increased
  amplitude.
• In destructive interference, the waves cancel
  each other and no wave is produced.

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Interference
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Interference (Standing Waves)

• A standing wave is created when 2 interfering
  waves traveling in opposite directions have the
  same frequency (p.21).
• appears to stay in one place
• Nodes (nulls) are the stationary points on a
  standing wave.
• The positions on a standing wave with the largest
  amplitudes (p. 11) are known as antinodes
  (maximas).

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Standing Waves
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Standing Waves

• ½ wavelength
• frequency 1 Hz
• b. 1 wavelength
• frequency 2Hz
• c. 1 ½ wavelengths
• frequency 3Hz

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Interference (Beats)

• A beat is an interference between two sounds with
  slightly different frequencies (p. 21)
• Used in tuning instruments

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Interference (Beats)
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Diffraction

• Diffraction causes waves to bend around an
  obstacle or spread out as they pass through an
  opening
• The diffraction pattern depends on the ratio of
  the size of the obstacle to the wavelength

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Diffraction
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Refraction

• Refraction is the change in the direction of a
  wave due to a change in its medium (p. 17).
• Ex, a light beam passing through both air and
  water will bend because both have different
  indices (pl. of index) of refraction.

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Refraction
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Refractive Index
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Polarization

• Visible light is unpolarized, meaning it vibrates
  in a variety of directions compared to its
  direction of motion.
• In polarized light, the waves oscillate (move) in
  only one direction
• Unpolarized light can become polarized by going
  through a material that allows only waves
  corresponding to one direction to pass through.
• Ex., polarized sunglasses and stretched
  cellophane wrap

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Polarization
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Polarization
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Doppler Effect
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Doppler Effect
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Doppler Effect

• The change in frequency (p. 21) due to the motion
  of the source (or receiver) is called the Doppler
  effect.
• When a source approaches the receiver, the pitch
  sounds increase because the sound wave crests
  arrive more frequently.
• When the source moves away from the receiver, the
  pitch decreases because the wave crests are
  arriving less frequently.

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Doppler Effect
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Doppler Effect
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Charges

• Electrical forces arise from sub-atomic particles
  in atoms.
• Protons are positively charged ()
• Electrons are negatively charged (-)
• Opposite charges attract / same charges repel

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Charges

• The number of protons and electrons determine the
  charge of an atom
• A neutral atom has an equal number of protons and
  electrons, and has a net charge of zero.
• If an atom has more electrons than protons, it is
  an anion, a negatively charged ion.
• If an atom has more protons than electrons, it is
  a cation, a positively charged ion.

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Practice

• If a boron atom has 5 protons and 2 electrons,
  what is its charge?
• If a carbon atom has 6 electrons and 6 protons,
  what is its charge?
• If a radium atom has 88 protons and 89 electrons,
  what is its charge?
• If a sodium atom has 15 electrons and 11 protons,
  what is its charge?
• If a lead atom has 82 protons, and 72 electrons,
  what is its charge?

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Charges

• Objects can also be charged.
• Rubbing one object against another can transfer
  electrons. This makes one positive and one
  negatively charged object, creating
  electrostatic.
• Electrical discharge (static shock) returns
  electrons to charged objects by touch.

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Electricity

• Electricity is the flow of electrons
• For electrons to move or flow, they must travel
  through a substance called a conductor (p. 49).

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Electricity

• Electrons will move from the negative to the
  positive end of a battery.
• To receive a shock, there must be a voltage
  difference (different electrical charges) applied
  to you. (electrons must travel from high to
  low electric charge)

Why dont birds sitting on electrical wires get
electrocuted?
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Electricity
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Conductors

• Materials through which electric charge can flow
  are called conductors.
• Ex metals, salt solutions, acids, bases
• Materials, known as insulators, are poor
  conductors of electricity.
• Electrons are not free to wander about to other
  atoms in the material.
• Ex paper, plastic, glass, rubber, etc.
• Semiconductors are materials that can act
  sometimes as insulators or conductors.
• Ex semi-metals like silicon, germanium

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Conductors vs. Insulators
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Plasma

• Plasma is the 4th state of matter
• Its created when gas is heated or electrified
  until the electrons are pulled free from the
  atoms. The result is a gas with cations (p.
  45) and electrons.
• Examples in nature are
• Stars
• Lightning
• Aurora Borealis

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Plasma
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Plasma

• Unlike gases, plasma can conduct electricity and
  affect electromagnetic (p. 55, 59, 61) fields
• Man-made applications include
• Fluorescent lighting
• Neon signs
• Plasma TVs
• Some computer and cell phone screens

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Transistors

• A transistor is a piece of semiconducting (p. 49)
  material that is used as a switch
• To control the current output
• To turn on or off electric signals
• To amplify waves
• Transistors replaced vacuum tubes used in the
  past
• Transistors are found in
• Cell phones
• TVs
• Computers
• etc.

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Electric Fields

• An electric field is a force field that surrounds
  an electric charge or group of charges.
• Represented by electric field lines
• Electric field lines point from to
• On a charged object, the greater the force, the
  stronger and the field lines are closer together

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Electric Field
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Electric Field Lines
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Electric Field Lines
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Magnetism
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Magnetism

• Magnets have a magnetic field (p. 59) and a north
  and south pole.
• Opposite poles attract / same poles repel
• All materials are magnetic, but most are only
  slightly magnetic.
• This is because atoms themselves are magnets.

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Magnetism

• Only 3 elements are strongly magnetic iron,
  nickel, and cobalt
• These elements atoms have their electrons
  aligned into small areas called magnetic domains
  that behave as small magnets themselves.
• The more the magnetic domains are aligned, the
  stronger the magnetism

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Magnetism
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Magnetism

• If you break a bar magnet in half, each half
  still behaves as a complete magnet with N and S
  poles

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Magnetism
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Magnetic Fields

• The space around a magnet is filled with a
  magnetic field, revealed by magnetic field lines.
  
• A moving charge produces a magnetic field.
• Magnetic field lines point from N to S
• Where the lines are more concentrated is an area
  of higher magnetic flux (strength)
• The magnetic field is stronger at the poles

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Magnetic Fields
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Relationship between Electricity and Magnetism

• A wire carrying an electric current produces
  magnetism.
• In fact, any moving charge creates magnetism (p.
  57)

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Electromagnetism
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Electric Currents and Magnetic Fields

• Iron filings sprinkled on paper reveal the
  magnetic field configurations about
• a current-carrying wire
• a current-carrying loop
• a coil of loops

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Relationship between Electricity and Magnetism

• Just as electricity needs to be moving to create
  a magnetic field, the magnetic field must be
  moving, or changing, to create a current. This
  is called induced current (electromagnetic
  induction).
• When the magnetic field changes, a current flows
• When the magnetic field is constant, no current
  flows

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Electric Currents and Magnetic Fields
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Relationship between Electricity and Magnetism

• When you wrap your right hand around the coil
  with your fingers in the direction of current,
  your thumb points in the direction of the
  magnetic north pole.

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Relationship between Electricity and Magnetism

• In a straight wire, the thumb points in the
  direction of the current (from to -), and the
  fingers point in the direction of the magnetic
  lines of flux.

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Electromagnetic Induction
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Electric Currents and Magnetic Fields
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Electromagnetic Waves
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The Law of Conservation of Energy

• Energy can neither be created nor destroyed
• Energy can be converted to any other form (heat,
  kinetic motion, potential, electrical, light)
• Energy can be transferred from one place to
  another
• Waves transfer energy
• Heat from a boiling pot transfers its energy to
  its surroundings