Year 9 Physics - Waves & Electricity Notes

Waves

• Transfer of energy without transferring matter.
• Caused by vibrations where particles oscillate (rhythmic movement).

Transverse Waves

• Example: light.
• Oscillations are perpendicular to the direction of energy transfer (up and down).
• Transmits energy through matter.

Longitudinal Waves

• Example: sound.
• The oscillations are parallel to the direction of energy transfer (back and forth).
• Require matter for transmission; cannot pass through a vacuum.
• Include compressions (regions where particles are closest together) and rarefactions (regions where particles are furthest apart).

Parts of a Wave

• Origin: Resting position.
• Crest: Highest point away from the origin.
• Trough: Lowest point away from the origin.
• Wavelength: Distance from crest to crest or trough to trough. Reflects wave intensity.
• Amplitude: Distance from the origin to either crest or trough. Shows the amount of energy a wave has.

Amplitude and Sound

• The louder the noise, the higher the amplitude.
• The quieter the noise, the lower the amplitude.

Frequency

• The number of waves per second.
• If one wave passes in one second, it has a frequency of one Hertz (Hz).
• Different from wave speed (how fast the wave fronts pass a stationary point).
• The higher the pitch, the more waves per second (short wavelength). The lower the pitch, the fewer waves per second (low frequency).

Soundwaves

• Travel fastest and furthest through solids because the particles are closest together (compared to liquids and gases), meaning vibrations are more easily passed from particle to particle.

Wave Equation

• $v = f \times λ$
  • v = speed of wave (m/s)
  • f = frequency of wave (Hz)
  • $λ$ = wavelength (m)

Hearing and the Ear

• Sound waves enter the outer ear (made of cartilage designed to catch sound) and travel down the ear canal to the eardrum.
• The eardrum vibrates and sends sound to three tiny bones in the middle ear.
• Bones amplify vibrations and send them to the cochlea in the inner ear.
• Activates tiny hair cells in the cochlea, which convert vibrations to electrical impulses.
• Impulses are carried to the brain, which interprets them as sound.

Cochlear Implants

• Amplify sounds and bypass damaged portions of the ear, directly stimulating the auditory nerve.
• Signals generated by the implant are sent via the auditory nerve to the brain, which recognizes the signals as sound.

Electromagnetic Spectrum

• The full range of EM radiation arranged in order from:
  • Lowest Frequency/Energy -> Highest Frequency/Energy
  • Longest Wavelength -> Shortest Wavelength
• EM Waves are transverse (perpendicular/up & down oscillations, can travel through a vacuum) moving at light speed of 300,000 km/sec.

Electromagnetic Waves Types

1. Radio: Produced artificially or naturally (e.g., stars), used in communications, can travel large distances.
2. Microwave: Used in radar/communication systems, absorbed by water, fats, and sugars in food.
3. Infrared: Heat transferred to us from the Sun, detected as warmth on our skin.
4. Visible Light: Narrow portion we can see with the human eye, consists of ROGYBIV (Red, Orange, Yellow, Green, Blue, Indigo, Violet).
5. Ultraviolet: We need some exposure to produce Vitamin D; too much can lead to skin cancer.
6. X-Ray: Great penetrating power through soft solids, can damage cells, tissues, and genetic material.
7. Gamma Ray: Only stopped by a thick sheet of lead.

Lenses and Seeing with the Eye

• Convex Lens: Curves outwards, causing light rays to converge (come together) to a focus.
• Concave Lens: Curves inwards, causing light rays to diverge (spread apart).

Seeing Process

• Light enters the eye through the cornea.
• Focused by the convex lens onto the retina at the back of the eye.
• Creates a clear inverted (upside down) image.
• The retina turns the image into electrical signals that travel up the optic nerve to the brain.
• The brain interprets the signals to make the image upright.
• Lenses in your eyes focus on objects at different distances by changing their focal length (distance between the focus and the optical center - f); being thinner for far-off objects and fatter for close-up objects.

Reflection

• Regular Reflection: Light reflects off a very smooth surface, producing a clear image.
• Diffuse Reflection: Light reflects off a rough surface, scattering in many directions and not forming an image.

Law of Reflection

• Angle of incidence (i) = Angle of Reflection (r)
• Light gets reflected from a surface at the same angle it hits it.
• Incident ray (inwards) & Reflected ray (outwards).
• The Normal - Imaginary line 90 degrees to the surface of the mirror, in between i & r (used to measure them).

Refraction

• Light bends as it travels from one transparent substance into another.

Refractive Index (RI)

• Measure of how fast light travels through a substance and how much it bends.
• The smaller the Index, the faster the light travels.

Snell’s Law

• Relationship between i & r of light passing between two different mediums: $n1 sin i = n2 sin r$
  • $n1$ = Refractive index of medium 1
  • $n2$ = Refractive index of medium 2
  • i = Angle of incidence
  • r = Angle of refraction

Snell's Law - Example 1

• Given:
  • Medium 1: Air, $n1 = 1.00$
  • Medium 2: Water, $n2 = 1.33$
  • Angle of incidence, $i = 10°$
• Find: Angle of refraction, r
• $n1 sin i = n2 sin r$
• $1 \times sin(10) = 1.33 \times sin(r)$
• $0.17 = 1.33 \times sin(r)$
• $0.17 / 1.33 = sin(r)$
• $0.13 = sin(r)$
• $sin^{-1}(0.13) = r$
• $r = 7.5°$
• Rearranging the formula to make r the subject (the only thing on one side of the = sign/what you are specifically calculating).
• In order to rearrange something from one side of the equation to the other, you must apply the opposite mathematical operation
• Opposite of multiply (x) is divide (/)
• Opposite of sin is sin-1 – On your calculator, you press the ‘shift’ button before pressing the ‘sin’ button to get sin-1

Snell's Law - Example 2

• A ray of light is traveling through air (n = 1.00) at an angle of incidence of 30°. The light passes into a second material and has and angle of refraction of 15°. What is the refractive index of the second material?
• Given:
  • $n1 = 1$
  • $n2 = ?$
  • $i = 30$
  • $r = 15$
• Find: $n2$
• $n1 \times sin(i) = n2 \times sin(r)$
• $1 \times sin(30) = n2 \times sin(15)$
• $0.5 = n2 \times 0.26$
• $0.5 / 0.26 = n2$
• $1.93 = n2$

Electricity

• Relates to the flow of an electric charge due to the behavior of subatomic particles (i.e., positively charged protons, negatively charged electrons, and neutral neutrons).
• Objects are normally uncharged because they have equal numbers of positively charged protons and negatively charged electrons.
• Electron transfer creates that electrostatic charge.
• An object with fewer electrons becomes positively charged.
• An object with more electrons becomes negatively charged.
• Like charges repel ($++, --$), Unlike charges attract ($+-,-+$). Charged objects attract neutral objects ($+0,-0$).
• Static Electricity: Build-up of an electric charge on a surface due to electron transfer between another surface.
• Current Electricity: Electrons moving along a wire.

Electrical Circuits

• Path for electrons to travel around so they can deliver their energy (+ to -).
• Need an energy source, an energy user, a connective wire, and a switch.
• Made from a combination of conductors and insulators
  • C – Charged electrons can move because electrons are held more weakly
  • I – Charged electrons held tightly, preventing them from moving

Circuit Components & Diagram

• [Diagram of circuit components and symbols]

Series and Parallel Circuits

• Series Circuits: A series circuit is made by connecting the end of one device to the beginning of another.
• Parallel Circuits: In parallel circuits, the same terminals of both devices are connected together.

Measuring Electricity

• Voltage: What makes current move (the 'push' that makes a charge move in a wire). Potential difference in electrical force between two points. Measured in volts (V) using a voltmeter, which needs to be connected to circuits in parallel.
• Current: Formed whenever charge flows from one spot to another (e.g., electrons along wires in a circuit). Measured in amperes (A or 'amps') by an ammeter (measures the amount of charge that flows every second), which needs to be connected to circuits in series.
• Resistance: Measures how difficult it is for an electric current to flow through a material or component due to collisions with atoms.

Ohm’s Law

• $Voltage (V) = Current (I) \times Resistance (R)$