Unit 4 2014 ppt wave characteristics

WAVE

CHARACTERISTICS

Physics Power Points

Physics I

Mr. Young

www.pedagogics.ca

What is a wave?

What is a wave?

a pattern of disturbances (oscillations, vibrations) caused by the movement of energy through matter or space

What is a wave?

a pattern of disturbances (oscillations, vibrations) caused by the movement of energy through matter or space

What is a wave?

a pattern of disturbances (oscillations, vibrations) caused by the movement of energy through matter or space

Waves can be classified according to their nature.

Mechanical waves

Electromagnetic waves

Waves can be classified according to their nature.

Mechanical waves

Electromagnetic waves

require a medium (matter) to travel through

Sound waves, water waves, ripples in strings or springs are all examples of mechanical waves.

Waves can be classified according to their nature.

Mechanical waves

Electromagnetic waves

require a medium (matter) to travel through

Sound waves, water waves, ripples in strings or springs are all examples of mechanical waves.

do not require a medium (matter) to travel through – they can travel through space (a vacuum) Radio waves, visible light, x rays

Waves can be classified by direction of vibration

Transverse waves

Longitudinal waves

A wave where the medium particles vibrate at right angles to the direction of energy transfer is called a transverse wave.

Look carefully at the motion of ONE of the particles in this transverse wave!

Waves can be classified by direction of vibration

Tranverse waves

Longitudinal waves

direction of energy transfer

direction of vibration

Sound is an example of longitudinal wave

Look carefully at the motion of ONE of the particles in this longitudinal wave!

A tuning fork causes surrounding air molecules to vibrate back and forth.

Direction of sound travel

Waves can be classified by direction of vibration

Transverse waves

Longitudinal waves

direction of energy transfer

direction of vibration

direction of vibration

direction of energy transfer

Wave pulses travel through a medium and therefore transfer energy.

Particles in the medium are moved back and forth by each pulse but are not transferred from one place to another.

direction of energy transfer

Vibration of metal atoms in the spring (the medium)

BIG IDEA!WAVES TRANSFER ENERGY NOT MATTER!

Observing Waves Activity

crest

trough

crest

trough

wave front

crest

trough

wave front

ray

crest

trough

wave front

ray

Adjusting FrequencyWater droplets hitting the surface create disturbances (ripples) which move outward from the source.

What happens when frequency is increased? The rate of water droplets falling increases creates more

disturbances per unit time.

Each disturbance has less time to travel before the next is created – so ripples are closer together

On a plot:

More drops per second, more waves per second, waves are closer together.

What happens when the amplitude of the source (drops) is increased?

This would be caused by the drops hitting the water with more force.

Increasing the height or size of the falling water droplet is a BIGGER disturbance.

What changes? The height of the ripples increases.

What stays the same?The distance between the wavefronts

A cork placed in the dish will bob up and down as the waves passed by.

The relative position of the cork does not change.

You can think of the cork as representing a molecule of water (the medium)

So, the relative position of water molecules does not change (only motion is up and down)

KEY CONCEPT

Waves transfer ENERGY not matter!

Describing WavesWaves are created by vibrating objects

ONE vibration = ONE cycle = ONE oscillation(all describe one complete back and forth motion)

FREQUENCY: the number of vibrations each second. Measured in s-1 (or Hertz). The symbol for frequency is f.

A frequency of 12 Hz means that there are 12 complete waves generated each second!

𝒇 =𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒗𝒊𝒃𝒓𝒂𝒕𝒊𝒐𝒏𝒔

𝒕𝒊𝒎𝒆 (𝒔)

PERIOD: the time required for one vibration. Measured in seconds. The symbol for period is T.

𝑻 =𝒕𝒊𝒎𝒆 (𝒔)

𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒗𝒊𝒃𝒓𝒂𝒕𝒊𝒐𝒏𝒔

𝑻 =𝟏

𝒇 𝒇 =𝟏

𝑻

WAVELENGTH: How far the wave energy travels in one complete vibration.

This is the distance traveled in an amount of time equal to the period of the wave.

The symbol for wavelength is the Greek letter lambda (l).

Wavelength is measured as the distance between identical points on two successive waves.

AMPLITUDE: a measure of the maximum distance the particles in the medium are displaced from their resting position as the wave passes.

It is easier to visualize these characteristics by looking at plots

Direction of Energy transfer

transversevibration

longitudinalvibration

AMPLITUDE: a measure of the maximum distance the particles in the medium are displaced from their resting position as the wave passes.

It is easier to visualize these characteristics by looking at plots

Direction of Energy transfer

transversevibration

longitudinalvibration

Activity 2 – Examining Wave Motion in a Slinky Spring

A continuous wave is produced when a series of pulses are generated by a vibrating source.

One shake produces a single vibration (a wave pulse). If the end of the spring is shaken back and forth, a continuous series of pulses is produced.

This is a standing wave pattern.

Longitudinal pulses can be produced by holding spring coils together at one end and then releasing.

A continuous wave can be produced by sliding the end of the spring back and forth towards your partner (holding the fixed end)

Longitudinal pulses can be produced by holding spring coils together at one end and then releasing.

A continuous wave can be produced by sliding the end of the spring back and forth towards your partner (holding the fixed end)

Using Graphs to Describe Waves

Displacement – Position Graphs

A displacement – position plot shows the different position of a particles in the medium at a given time for a section of the wave.

Consider a rope that is being vibrated at one end. If we took a snapshot with a camera it might look like this.

-1.5

-1

-0.5

0

0.5

1

1.5

position

dis

pla

cem

en

t

The graph shows the displacement of the rope at an instant in time.

-1.5

-1

-0.5

0

0.5

1

1.5

position

dis

pla

cem

en

t

In transverse waves, the maximum and minimum displacements are called crests and troughs respectively.

The distance between two successive crests OR troughs would be the wavelength

crest

trough

l

Longitudinal waves are a bit tricky to understand because the direction of vibration is not perpendicular to the direction of wave travel. Consider the following images

compressionrarefaction

l

-1.5

-1

-0.5

0

0.5

1

1.5

position

dis

pla

cem

en

t

In longitudinal waves, the maximum and minimum displacements are called compressions and rarefactions respectively.

The distance between centers of two successive compressions OR rarefactions would be the wavelength

center ofcompression

center of rarefaction

l

-1.5

-1

-0.5

0

0.5

1

1.5

Displacement-Time GraphsA displacement – time plot shows how the position of a particle in the medium changes as it vibrates back and forth.

Transverse and longitudinal waves look the same on this type of plot. The only difference is the direction of the particle displacement.

time

dis

pla

cem

en

t

-1.5

-1

-0.5

0

0.5

1

1.5

Amplitude can be measured on the y-axis.

time

dis

pla

cem

en

t

Period can be measured on the x-axis. The period would be the time for one complete wave.

Wave Speed

The speed of a wave is determined by the properties of the medium it travels through. Properties include:

Wave Speed

The speed of a wave is determined by the properties of the medium it travels through. Properties include:

• type of material• elasticity• tension• density (solid, liquid or gas)• temperature

distancespeed=

time

Speed is calculated using the following equation.

Recall:The definition of period

The definition of wavelength

Recall:The definition of period

The definition of wavelength

the time it takes to complete one vibration/oscillation/cycle

the distance a wave travels in a complete vibration/oscillation/cycle

So:

wavelengthwave speed=

period

So:

wavelengthwave speed=

period?

wavelengthwave speed=

period

Try this:

Use frequency instead of period and derive the wave speed equation. Write the equation using appropriate symbols.

Practice:

1. Find the speed of a wave in a metal spring if a pulse travels 7.5 m in 3.0 s.

2. What is the frequency of this wave if the distance between two successive wave crests is 0.25 m?

3. Challenge: A fisherman passes the time by counting waves passing under his boat. He notices that his boat rises and falls 8 times in 65 seconds. He counts 6 wave crests between his boat and a buoy located 54 m away. What is the speed of the water waves?