Physics Mod 3 Waves & Thermo Dynamics

Waves & Thermo Dynamics

What is a wave?

A disturbance that travels through a medium or space, transferring energy through matter or space

What causes a wave?

• Created when a source of energy causes a vibration

• Vibration is the up and down, back and forth motion

Examples of waves

Water

Light

Sound

Microwaves

Radio waves

X-rays

Ultraviolet (UV)

Seismic

Electromagnetic waves (EM)

Waves that DO NOT require a medium to transfer energy, travel at the speed of light in a vacuum!

E.g: Radio waves, Microwaves, infrared radiation, light, ultraviolet, X-rays, gamma rays

Mechanical waves

Waves that require a medium to transfer energy

E.g: Sound waves, Water waves, seismic waves

A medium

Something that a wave travels through, this could be solid, liquid or gas

• In a sound wave, the medium is air

• In an earthquake, the medium is the ground

Where light comes from

Comes from the sun, reflected off our eyes.

→If it can travel through a vacuum, it doesn’t need a medium

Transverse waves

The particles of the medium vibrate at 90 degrees or perpendicular to the direction that the wave is travelling

2 Groups of Transverse waves

Electromagnetic waves

Mechanical Waves

Longitudinal Wave

The particles of the medium vibrate parallel to the direction that the wave is travelling

• All longitudinal waves are mechanical

Example: Slinky, Sound

Wave parts

Trough, Crest, Wavelength, Amplitude

Wavelength in Transverse

The distance between two identical consecutive points on a wave, such as crest-to-crest, trough-to-trough

Amplitude in Transverse

The max extent from the resting position of the medium to the crest or trough

Wavelength in Longitudinal

From one compression to another, or from one rarefaction to another

Compression

The space in a medium in which molecules are close together

Rarefaction

The space in a medium where molecules are more spead out

Amplitude in Longitudinal

The biggest move from one molecule to another

Frequency

The number of waves that pass a given point in 1 second, measured in Hertz

Wave number and its equation

The number of wavelengths per unit distance.

k = 2 π / λ

where

K: wavenumber

λ: wavelength (m)

Seismic waves

Caused by the sudden movements of materials within the earth, such as an earthquake

tsunami

a series of ocean waves with very long wavelengths, caused by large scale distrubances of the ocean

Period

Time per wavelength (in seconds)

Relationship between frequency and period

• Indirect relationship

• Frequency increases, period decreases

• Frequency decreases, period increases

Equation for frequency

frequency = velocity / wavelength

Why do mirrors show reflections?

All objects which can be seen either

→Produce their own light (candles, lightbulb)

→ Reflect light (moon, table, you)

A mirror is smooth & shiny, it reflects light rays in one direction

Diffuse scattering

When a surface looks smooth, but in detail it is rough and reflects light rays in all directions.

Law of reflection

angle of incidence = angle of reflection

2 types of curved mirrors

Convex and Concave

Convex mirror

Bend outwards and are dome-shaped, obeys the law of reflection and forms an image smaller than the object

→ produces a virtual image

Concave mirror

Bend inward and are bowl-shaped, obeys the law of reflection and produces a reflected image larger than the object (magnifies)

Refraction

the bending and changing of speed. For example, light beams bend when it transitions from one medium to another (glass, water, air)

Law of Refraction (aka Snell’s law)

Formula: n₁ sin(θ₁) = n₂ sin(θ₂)

When beam first strikes prism → ray bends towards the normal

When beam exits the prism → ray bends away from the normal

Diffraction definition

The spreading out of waves as they pass through a gap

Superposition - constructive interference

When 2 crests/troughs collide, this creates a super crest or super trough, meaning the slinky will double the height

Destructive interference

When a crest and trough with equal size collide, they produce a cancellation

Factors affecting a harmonic

Mass of object, tension

Types of harmonics

1st harmonic (fundamental frequency), 2nd harmonic (first overtone), 3rd harmonic (second overtone)

1st Harmonic

2 nodes, 1 antinode, ½ wavelength

2nd Harmonic

3 nodes, 2 antinodes, 1 wavelength

3rd Harmonic

4 nodes, 3 antinodes, 1.5 wavelength

Damping

A reduction in the amplitude of a wave as a result of energy absorption or destructive interference. For example, a child’s swing will eventually come to rest due to friction and air resistance→ the swing’s motion has been dampened.

Resonance

The tendency for one vibrating object to start another object vibrating if its frequency is close to the second object’s natural frequency (all objects have a natural frequency).

For example: singing at a very high pitch to break a glass

What happens when a building with a natural frequency of 0.02 Hz is disturbed

It would sway back and forth at 0.02 Hz

Critical angle

When the angle of refraction is 90°. The light must travel from a denser medium to a less dense medium. At this point onwards, any further increase in the angle of incidence will cause total internal reflection.

How do optical fibres work?

By trapping light within a plastic core, using the principle of total internal reflection

Total internal reflection

The light ray is reflected back into the denser medium, occurs when the angle of incidence is greater than the critical angle.

Dispersion of light

A phenomenon where white light splits into different colours, because white light is made up of multiple colors, each colour has a different wavelength.

7 colours in the dispersion of light

red, orange, yellow, green, blue, indigo, violet

Inverse Square Law definition

As the distance from the source doubles, the brightness of the field goes down by a quarter

Inverse square law formula

Intensity is proportional to 1/r²

Things that follow the inverse square law

• gravitational fields

• electrical fields

• light intensity

• sound intensity

Thermal energy(aka heat energy) vs Temperature

Temperature measures the average kinetic energy of each particle in a substance, indicating how hot or cold it is, while thermal energy is the total kinetic energy of all particles in the substance, representing the sum of their energies.

A hot bath: high thermal energy, low temperature

A sparkle: low thermal energy, high temperature (1500 degrees Celsius)

What is Thermodynamics?

A study of relationships involving heat, mechanical work, and other aspects of energy and energy transfer

1st Law of thermodynamics

Law of conservation of energy: energy cannot be created nor destroyed

2nd Law of thermodynamics

There is a limit to the availability of energy.

• no such thing as 100% efficient

• follows the concept of entropy (Entropy = energy degradation)

3rd Law of thermodynamics

States that it's impossible to reach absolute zero (0 Kelvin)

0th Law of thermodynamics (4th law)

Thermal Equilibrium.

For example, under the ocean, fish, sand, and corals are the same temperature

Specific Heat Capacity

The amount of heat energy required to raise the temperature of a substance by one unit. That’s why we need to heat water twice the amount of time to get the same temperature as a pot of oil that’s been heating for 1 minute.