The Physics of Waves, Sound, Light, and Magnetism
Waves and Sound
Waves
Waves: Disturbances that carry energy. They can travel through solids, liquids, gases, and space.
Two Main Waveforms:
- Mechanical Waves: Require a medium (solid, liquid, or gas) to travel.
- Electromagnetic Waves: Do not require a medium; light is an example.
Larger Wave = More Energy: Waves transfer energy, NOT particles. Energy spreads out as the wave travels.
Sound
Sound: A mechanical wave created by vibrations.
Vibrations: The medium is any material that contains particles or molecules.
Longitudinal Wave: Sound waves are longitudinal, meaning the vibrations occur parallel to the direction of wave travel.
Sound in Different Mediums:
- Sound moves best in liquids.
- Sound moves fastest in solids.
- Sound moves slowest in gases.
- Sound waves cannot travel in space (no medium).
How We Hear Sound
Our ears can feel vibrations and interpret them as sounds.
Amplitude:
- Bigger amplitude = more energy = louder sound.
- Amplitude dissipates as sound travels.
Intensity: Measured in Watts/meter2 (dB). Intensity depends on the area over which the sound wave is spread.
Frequency:
- How fast the medium is vibrating.
- Measured in Hertz (Hz), which represents the number of waves per second.
Higher Pitch = Higher Hz
Range of Human Hearing: 20 Hz to 20,000 Hz. Younger people can typically hear higher frequencies.
Ultrasound: Frequencies above 20,000 Hz.
Infrasound: Frequencies below 20 Hz.
Threshold of Hearing: 0 dB
Threshold of Pain: 120-130 dB
Instant Damage: 160 dB
Period (T): How many seconds per wave (for slower waves). T = 1/f
Frequency (f): How many waves per second (for quicker waves). f = 1/T
Wavelength: How long a wave is, measured in meters.
Doppler Effect
Doppler Effect: The frequency of a sound wave shifts due to the movement of the observer or the source of the sound. This affects the pitch (high or low) but NOT the amplitude.
Sonic Boom: The breaking of the sound barrier.
Sound at Boundaries
When sound hits boundaries, interference occurs.
Reflection: The sound wave bounces back, creating an echo.
Diffraction: The sound wave bends around obstacles, allowing us to hear around corners.
Refraction: The sound wave changes speed as it passes from one medium to another. Sound travels faster in hot air and slower in cold air.
Interference
Interference: When two or more sound waves in the same medium interact.
Beats: When interfering waves have different frequencies, interesting patterns can form. The beat frequency can be found by subtracting the two frequencies being played.
Standing Wave: Created when sound waves interfere in a perfectly constructive/destructive way.
Node: Where perfect destructive interference happens (no motion).
Antinode: Where perfect constructive interference happens (maximum motion).
Resonance
Resonance: Certain frequencies create standing waves. Each frequency that creates a standing wave is a harmonic.
Open Pipe: Antinodes on both ends. Multiply the length by 2 to find the wavelength.
Closed Pipe: Node on one end, antinode on the other. Multiply the length by 4 to find the wavelength.
Chladni Plate: A vibrating plate with sand on it. The sand settles at the nodes, revealing the patterns of the standing waves.
Magnetism and Electricity
Magnets
Permanent Magnets: Once magnetized, they remain magnetized.
Ferromagnet: Easily magnetized (iron, nickel, cobalt, some rare earth metals, magnetite).
Paramagnet: Magnetized with more difficulty.
Diamagnetic: Repelled by magnetic fields (slightly) – not magnetic.
All magnets have two poles (North and South). The poles are where the magnet is strongest.
The force of magnets can be felt over a distance.
Field Lines: Used to visualize the magnetic force. Always drawn from North to South. Closer lines indicate a stronger field.
Compasses: Used to “see” magnetic field lines. Earth has geographic and magnetic North. Geographic North does NOT equal Magnetic North (declination). The south pole of a bar magnet points towards magnetic north (near Canada).
Magnetic systems contain energy. Opposite poles naturally attract; being close together is a low-energy state. It takes an outside force to pull them apart.
Gauss Gun: An application of magnetic force.
Electricity
Atoms: The building blocks of the universe. All atoms are made of some combination of neutrons, protons, and electrons.
Protons do not typically move from one atom to another (too large). Instead, electrons move from one object to another when energy is added to the system (typically through friction).
Charges can be positive (protons) or negative (electrons). They may also be neutral (to start). Neutral does NOT mean no charge, but that the + and – charges add up to 0.
Electroscope: A device used to qualitatively demonstrate the effect of charged objects.
Coulomb’s Law (Electric Force): F = Kq x q/d²
Universal Law of Gravitation (Gravitational Force): Similar to Coulomb’s Law but for gravitational force.
Electric force can act over a distance. Electric field lines help visualize the force. Lines point INTO negative charges and OUT OF positive charges. More lines = stronger charge.
Materials can be broken into smaller “chunks” called domains. Each domain has its own magnetic field.
Factors Affecting Magnetic Strength: Material, temperature, condition, size and shape, distance.
A magnet needs to be moving to cause a current in a wire. Increase the current by using a bigger magnet or moving the magnet quickly.
Light
Light as a Particle and a Wave
Light is a Particle: The energy of light is contained in small packets called photons. This explains why light can travel through space. This is demonstrated by the “Photoelectric Effect.”
Light as a Wave: Light can also be described as a transverse electromagnetic wave. Light reflects, refracts, and diffracts.
Wave-Particle Duality: Light can behave as both a wave and a particle. This was proven by Einstein.
The Electromagnetic Spectrum
EM Spectrum: Particles create changing electric (E) and magnetic (B) fields as they move.
Light Moving: Velocity/speed can change depending on the situation. The speed of light in a vacuum is a fundamental constant (c = 3 x 108 m/s or 300,000,000 m/s). This can be substituted for v in the equation: c = d/t and c = fλ.
Energy: E = hν, where h = Planck’s Constant (6.626 x 10-34 Js) and ν is the frequency.
Energy Levels:
- Low Energy: Radio Waves (Microwaves)
- Medium Energy: Infrared, Visible Light, Ultraviolet
- High Energy: X-Rays and Gamma Rays (ionizing radiation)
Superposition of Light: Multiple waves interact with each other in superposition. Two waves in the same space will add together.
Resolving Light: Light coming from two sources will interact.
Reflection: Waves bounce back.
Diffraction: Waves hit a corner of a boundary and bend around.
Speed of Light (c): 3 x 108 m/s.