Phys Notes

The world communicates

1.1.1) Describe the energy transformations required in one of the following: mobile telephone, fax/modem, radio and television

Radio Sound signal is converted to an electrical (audio) signal Audio signal is then superimposed on carrier waves Superimposed through AM- amplitude modulation (amplitude is affected) or FM- frequency modulation

(frequency is affected) Radio’s receiving antenna detects modulated wave and ‘subtracts’ carrier wave, leaving the audio signal

Sound -> electrical -> radio waves -> electrical -> sound + heat

1.1.2) Describe waves as a transfer of energy disturbance that may occur in one, two or three dimensions, depending on the nature of the wave and the medium.

A wave is a disturbance which transfers energy without transporting matter.

- In terms of a medium, there are mechanical and electromagnetic waves.- In terms of particle oscillation, there are transverse and longitudinal waves.

Waves can be classified as:

- Mechanical or electromagnetic- Transverse of longitudinal

Waves can transmit energy in one, two or three dimensions depending on the type of medium through which the waves move. For example:

One dimension- A transverse or longitudinal wave travelling down stretched strings or springs Two dimension- A transverse wave travelling from a point source of disturbance in still water (Circular

wavefronts) Three dimension- A sound wave transmitting energy produces a spherical wavefront

Mechanical Waves Electromagnetic WavesRequire a medium to propagate through Does not require a medium to propagateInvolve the transfer of energy through a medium by the motion of particles of the medium itself

Travel at a speed of 3x108 ms1

Particles move as oscillations or vibrations around a fixed point

E.g. are light, infra red, x-rays, gamma rays, radio waves, micro waves

E.g. are sound waves, water waves, waves in strings

SA

1.1.3) Identify that mechanical waves require a medium for propagation while electromagnetic waves do not.

Mechanical waves: Require a medium to propagate e.g. sound waves

Electromagnetic waves: Do not require a medium to propagate e.g. radio waves

1.1.4) Define and apply the following terms to the wave model: medium, displacement, amplitude, period, compression, rarefaction, crest, trough, transverse waves, longitudinal waves, frequency, wavelength, velocity.

Medium: a substance through which mechanical waves require to propagate

Displacement: the distance by which a particle is moved from its normal position

Amplitude (A): the maximum displacement of the particles from the undisturbed state (equilibrium)

Period (T): Time taken for a single wavelength pass through a fixed point or the time taken for a particle of a

medium to make one complete oscillation. Period is measured in seconds. T= 1/f

Compression: zones where the particles are spaced further apart than in their undisturbed state (higher pressure)

Rarefaction: zones where the particles are spaced further apart than in their undisturbed state (lower pressure)

Crest: the highest point of a wave.

Trough: the lowest point of a wave.

Transverse waves: A wave in which the particles vibrate perpendicular to the direction of wave propagation.

Longitudinal waves: A wave in which the particles oscillate back and forth in the direction parallel to the direction of propagation.

Frequency (f): the number of waves passing through a fixed point per second or the number of complete oscillations of a medium particle in a second.

Wavelength (λ): the distance between 2 successive identical points on a wave. Note: distance is not limited to crests and troughs

Velocity: the speed of a wave

1.1.5) Describe the relationship between particle motion and the direction of energy propagation in transverse and longitudinal waves.

In transverse waves, the particles oscillate perpendicularly to the direction of energy propagation.

In longitudinal waves, the particles oscillate back and forth parallel to the direction of energy propagation.

1.1.6) Quantify the relationship between velocity, frequency and wavelength for a wave

Velocity, frequency and wavelength of a wave are all related as V = f λWhere V = velocity (m/s) f = frequency (Hz) and λ = wavelength (m)

The frequency of a wave does not change as it travels through different mediums as frequency depends only on source of the wave

f is constant for a particular wave – if V ↑ then λ ↑ , if V ↓ , then λ ↓

E.g. What is the wavelength of a wave travelling at 500m/s at a frequency of 100Hz?

2.1.1) Identify that sound waves are vibrations or oscillations of particles in a medium

A sound wave is categorised as a mechanical longitudinal wave Therefore sound waves are oscillations of particles in a medium – when the wave propagates, the

vibrations of the particles create pressure variations within that medium Particles in the medium oscillate and create areas of compression and rarefaction

2.1.2) Relate compressions and rarefactions of sound waves to the crests and troughs of transverse waves used to represent them

Compressions and rarefactions can be represented in the form of a transverse wave:

2.1.3) Explain qualitatively that pitch is related to frequency and volume to amplitude of sound waves

The pitch of a sound is directly related to the frequency of a sound wave as the higher the frequency of the sound is, the more vibrations per second and the higher the pitch

High frequency Low frequency

Period and frequency are also related. As frequency increases, period decreases. As frequency decreases, period increases T= 1/f

Pitch does not change when a sound wave travels from one medium to another since the frequency doesn’t change.

The amplitude of a sound wave determines the volume of the sound

High amplitude = high volume (loud sound) Low amplitude = low volume (soft sound)

2.1.4) Explain an echo as a reflection of a sound wave

Echoes are formed when sound wave reflects of a hard surface and bounces back to its original source Echoes are used in SONAR (Sound Navigation And Ranging) to find depth of water and detect animals and

other objects in water

E.g. If a sound signal (1400 m/s) returns in 10 seconds due to a shipwreck, how high is the shipwreck?

2.1.5) Describe the principle of superposition and compare the resulting waves to the original waves in sound

Superposition (aka wave interference) : when two or more waves of the same type pass through the same medium at the same time and interfere with each other

Resultant wave: produced when individual component waves interfere and combination wave is formed The position of any point on the resultant wave is the sum of the amplitudes of the component waves

In phase waves add together 180⁰ out of phase waves cancel Different waves = new wave created each other

Curve + curve = curve Line+ line = line Curve+ line = curve

Destructive interference occurs if amplitude of the crest of one wave is exactly equal to the amplitude of the trough of the other wave and the second is exactly 180o out of phase from the first.

Complete loss of amplitude occurs as the waves cancel out each other It is possible to add sound together to produce no sound

4.1.1) Describe and apply the law of reflection and explain the effect of reflection from a plane surface on waves.

The Law of Reflection is comprised of 2 components:

1. The angle of incidence and the angle of reflection are equal2. The incident ray, the reflected ray and the normal are on the same plane.

When light waves hit a reflective surface (i.e. mirrors), they bounce (reflect) off the surface of the plane, with the reflected angle equal to the incident angle, which are defined as the angle the ray makes with the normal.

4.1.4) Explain that refraction is related to the velocities of a wave in different media and outline how this may result in the bending of a wavefront.

Waves travel faster in less dense medium - bends away from the normal Waves travel slower in more dense medium - bends toward the normal When a wave travels from one medium into another, it changes velocity and direction- bending of a

wavefront

When the angle of refraction = 90o, the angle of incidence is called the critical angle

4.1.5) Define refractive index in terms of changes in the velocity of a wave passing from one medium to another.

Absolute refractive index (RI or nx) is the ratio of speed of light in a vacuum (C), where C is 3 x 108ms-1 to the speed of light in that particular medium (Vm)

Denser mediums have a higher refractive index. I.e. Between RIglass= 1.5 and RIair = 1 , GLASS IS MORE DENSE

4.1.6) Define Snell’s Law V1/V2 = sini/sinr

Snell’s Law expresses the relationship between the angles and the velocities of the waves as:

Sin i V1

Sin r V2

4.1.7) Identify the conditions necessary for total internal reflection with reference to the critical angle.

For Total Internal Reflection to occur, the angle of incidence must be larger than the critical angle

4.1.8) Outline how total internal reflection is used in optical fibres.

Total internal reflection is used in optical fibres for communication purposes:

The encoded data is converted from electrical signals to optical light pulses and then transmitted through the optical fibre to its destination, where it is then converted back.

The light enters the optical fibre at an incidence angle larger than the critical angle, resulting in total internal reflection

The light waves continue reflecting inside the core cladding of the optical fibre until it reaches its destination(core is made of material with higher RI for light than the cladding).

4.1.2) Describe ways in which applications of reflection of light, radio waves and microwaves have assisted in information transfer.

Reflection of light

Optical fibres are used to transfer data around the world through optical cables placed in the earth. Once light waves travel to through the fibres and reach land base, the data is decoded and transferred by land methods.

The use of optical fibres enables fast (3x108 m/s) travelling speeds over long distances.

Radio waves

Used in broadcasting to transfer sound and television signals. Transmitter and receiver antenna) are used in propagation and interception of radio waves. Radio wave acts as a carrier wave and information may be encoded directly on the wave by

superimposition (MODULATION)

Used in radar (SONAR) where radio waves are sent out and reflections are received to:-Produce medical imaging e.g. ultrasounds-Look for submarines -Determine ocean depth-Map the ocean floor

Used for navigation of ships, aircrafts and GPS. Used in space exploration. Long-range radio signals enable astronauts to communicate over far

distances and carry transfer information from space probes because they can travel through a vacuum.

Microwaves

Commonly used in POINT-TO-POINT communication (restricted to one endpoint; contrasts to broadcasting) on the surface of the Earth for:- file sharing- Radios- Radio navigation systems- Sensor systems -radio astronomy.

4.1.3) Describe one application of reflection for each of the following:

- plane surfaces- concave surfaces- convex surfaces- radio waves being reflected by the ionosphere

Plane Surfaces

- Used as mirrors for dressing and shaving

Concave surfaces

Satellite dishes used in microwave repeating stations, airport radar control towers to boost intensity of EM waves by collecting and focusing.

Concave mirrors are used to produce parallel beams of lights in torches and headlights

- Light rays are reflected to a focal point to brighten required regions rather than unwanted light in all directions.

Convex surfaces Convex mirrors used as security mirrors, corner mirrors and side mirrors convex surface allows greater range of sight (i.e. can see around corners)

3.1.1) Describe EM waves in terms of their speed in space and their lack of requirement of a medium for propagation.

All EM waves travel at the speed of light (C = 3 x 108 ms-1) through any medium They can travel through a vacuum i.e. they do not require a medium for propagation - EM waves consist of oscillating electric and magnetic fields to propagate. Therefore, they do not have

medium particles and do not require oscillation of medium particles to propagate.- Because of this nature, EM waves can also ONLY be transverse waves because in longitudinal waves,

medium particles must oscillate parallel to the equilibrium (electric and magnetic fields cannot move like this)

3.1.2) Identify the electromagnetic wavebands filtered out by the atmosphere, especially UV, X-rays and gamma rays

7 EM WAVES: GAMMA, X-RAYS, UV,LIGHT, INRARED, MICROWAVES, RADIOWAVES

Advantages of using EM waves for communication :

- High speed for quick communication- Can travel through a vacuum

Disadvantages of using EM waves for communication:

- limited spectrum- inverse square law

Most EM waves are filtered out by the atmosphere including:

- Gamma rays- X-rays- UV rays- Infrared

EM waves not filtered out by the atmosphere:

- visible light- radio waves- Microwaves

This is why they can be used for communication3.1.3) Identify methods for the detection of various wavebands in the electromagnetic spectrum

3.1.4) Explain that the relationship between the intensity of electromagnetic radiation and the distance from a source is an example of the inverse square law.

Inverse Square Law

The inverse square law is applied to EM waves to calculate their intensity (energy received/sq m/sec) As distance from the source is increased, the intensity decreases because the source is further away

from the point of measurement This process is called attenuation (reduction) Therefore, intensity is inversely related to distance (greater intensity – smaller distance, smaller

intensity – greater distance)

3.1.5) Outline how the modulation of amplitude or frequency of visible light, microwaves and/or radio waves can be used to transmit information.

Modulation

The process of adding signal information to an EM wave by applying a carrier wave. The carrier wave carries the information while being transmitted and once it reaches the receiver, it is

“subtracted” or removed and the information remains and is decoded. By modulating either amplitude or frequency of the carrier wave + signal (to make a modulated

wave), the wave can be suited different types of transmissions.

Amplitude modulation (AM)

Used to broadcast AM radio Amplitude varies while frequency remains the same Amplitude of audio signal superimposed onto carrier wave

Frequency modulation (FM)

Used to broadcast FM radio Frequency varies while amplitude remains the same Frequency of audio signals superimposed onto carrier wave

3.1.6) Discuss problems produced by the limited range of the electromagnetic spectrum available for communication.

Because most types of EM waves are absorbed by the atmosphere, only use a limited range of the electromagnetic spectrum can be used for communication

only visible light, microwaves and radio waves are able to be used Considering the limited range of the EM spectrum available for communication:

AM advantages AM disadvantagesIt requires small bandwidth (range of f used for transmitting a signal)

Prone to interference because amplitudes of interfering waves are added (superposition)

More transmissions can be fitted onto the AM band (e.g. more AM stations in the same area)

Amplitude is easily affected by outside disturbances

Can be broadcasted over long distances

FM advantages FM disadvantagesLess prone to interference because FM audio signal is based on frequency

Requires large bandwidth ( not many transmissions allowable in the FM band, otherwise there will be interference)

Frequency is not easily changed by superposition Great demand and competition for limited allowable bandwidth (e.g. competition between radio stations)

Moving about

1.1.1) Identify that a typical journey involves speed changes

A typical journey involves speed changes throughout. The journey usually begins with a speed of zero, so the speed must change in order for the journey to take place

1.1.2) Distinguish between the instantaneous and average speed of vehicles and other bodies

Instantaneous speed is the speed that you are travelling at any given instant.

Average speed is an average speed throughout the whole journey. Average speed is given by the total distance you travelled divided by the time it took you.

1.1.3) Distinguish between scalar and vector quantities in equations

Scalar quantities only specify a magnitude, with no direction. Examples include time, distance, speed, mass, area, age, volume, height.

Vector quantities specify a magnitude and direction. Examples include displacement, velocity and force.

Vector quantities are represented by an arrow, which is known as a vector

When utilising vector representations there are 2 rules which must be obeyed:

I. The direction of the arrowhead represents the direction of the quantityII. The length of the arrow must be proportional to the magnitude of the quantity

e.g. A vector quantity of magnitude 10 towards the east

10Tail Head Note: The arrowhead is known as the head of the vector

Consider... 10 This vector represents the quantity of equal magnitude in the

opposite direction

Mathematically, if we call the first vector +10, we would call this vector -10 because it is in the opposite direction.

1.1.4) Compare instantaneous and average speed with instantaneous and average velocity

INSTANTANEOUS

Instantaneous speed is a scalar quantity

Instantaneous velocity is a vector quantity

AVERAGE

Average speed is a scalar quantity and takes into account your whole journey

Average velocity is a vector quantity and only takes into account the starting and ending

points

1.1.5) Define average velocity as:

Average velocity is change in displacement divided by change in time .

OR

1.2.3) Present information graphically of:

- Displacement vs. Time

-Velocity vs. Time

-Acceleration vs. Time

For objects with uniform and non-uniform linear velocity

Displacement vs. Time

The GRADIENT of a displacement- time graph represents the VELOCITY of the object

Velocity vs. Time

The GRADIENT of a velocity- time graph represents the ACCELERATION of the object

In addition,

The AREA UNDER THE V-T graph represents the DISPLACEMENT during that time

Total displacement = A1 - A2

Total distance= A1 + A2

Acceleration vs. Time

In addition,

The AREA UNDER THE A-T graph represents the CHANGE IN VELOCITY during that time