Topic 4 - Waves Flashcards by Ron Feder

Waves basics

What do waves do?

Waves transfer energy from one place to another without transferring any matter.

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Waves basics

What happens when a wave travels through a medium?

The particles of the medium vibrate and transfer energy between each other e.g. if you drop a twig in a stagnant pool the ripples don’t carry the twig away as the water only moves up and down.

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Waves basics

Define amplitude

Maximum displacement from rest position of a wave that measures the amount of energy the wave transfers

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Waves basics

Define trough

The lowest point of the wave

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Waves basics

Define crest

The highest point of the wave

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Waves basics

What is the rest position of a wave

The point of zero displacement on a wave (undisturbed)

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Waves basics

What is the wavelength of a wave

The distance from trough to trough or crest to crest measured in metres

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Waves basics

Define frequency

Number of oscillations a wave completes every second (Hz). 1 Hz is 1 wave per second

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Waves basics

Define period

The time it takes for one oscillation to occur. (seconds)
Period = 1 / frequency

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Waves basics

Define wave speed

Wave speed (m/s) = distance (m) / time (seconds)

How far the wave travels every second

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Transverse and longitudinal

What is a transverse wave?
Give examples

A wave where the oscillations are perpendicular to the direction the wave travels/perpendicular to the direction of energy transfer.

Examples in include:
* All electromagnetic waves
* S-waves
* Water waves

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Transverse and longitudinal

What is a longitudinal wave?
Give examples

A wave where the oscillations are parallel to the direction the wave travels

Examples include:
* Sound waves
* P-waves

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Measuring wave speed

How do you measure the speed of sound using on oscilloscope?

Set up the oscilloscope so the detected waves at each microphone are shown as separate waves.
Start with both microphones next to the speaker then move one away until the 2 waves are alinged again but 1 wavelength apart.
Measure distance between 2 microphones which = wavelength.
Use formula velocity = frequency * wavelength (the frequency = frequency set in the signal generator)

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Measuring speed of water waves

How do you measure the speed of water ripples?

Using a signal generator attached to the dipper of a ripple tank you can create water waves at a set frequency.
Dim the lights in the room and turn on a strobe light above the ripple tank which will create a wave pattern shadow below.
Alter the frequency of the strobe light so the shadows appear to be still
Distance between 2 shadows = wavelength (λ)
Measure to find average
Use velocity (m/s) = frequency (Hz) * wavelength (λ) to calculate the speed

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Waves at boundaries

What are the 3 possible outcomes when a wave reaches a boundary between 2 materials?

Absorbed - The wave transfers energy into the material’s energy stores (often thermal)
Transmitted - The wave carries on travelling through the material (leads to refraction)
Reflection - The wave is ‘sent back’

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Why would different substances and waves do one of the three?

(Absorption, transmission, reflection)

Depends on the wavelength and the properties of the material

Refraction

What affects a waves speed?

Density of the substance it is travelling through

Refraction

What happens when a wave hits a boundary at an angle?

Its change in speed causes a change in direction

Refraction

As the change in speed increases, how does this impact the change in direction?

Increases change in direction

Refraction

What is a normal?

An imaginary perpendicular line to the boundary

Refraction

Fill in the gaps:

The wave bends ??? the normal if it slows down and ??? from the normal if it speed up

The wave bends towards the normal if it slows down and away from the normal if it speed up

Refraction

What type of wave usually goes slower in denser materials?

EM (electromagnetic) waves such as light

Refraction

Pick the correct answer:
Short/long wavelengths bend more when crossing a boundary

Short wavelengths bend more when crossing a boundary

Refraction

What happens to the wavelength of a wave as it is slowed down?

The wavelength decreases

# **Critical angles** What is total internal reflection?

When a wave crosses a boundary it is **reflected back** into a material

# **Critical angles** When does total internal reflection occur?

When the **angle of incidence > the critical angle**

# **Critical angles** What is the critical angle

The angles where a ray travelling in a tranparent medium can hit the boundary **without being reflected in the medium**

# **Sound waves** What are sound waves caused by?

**Vibrating** objects

# **Sound waves** Are sound waves transverse or longitudinal

**Longitudinal** waves

# **Sound waves** What happens when a sound wave passes through a solid?

**Particles** in the solid **vibrate**

# **Sound waves** From slowest to fastest, state the speed of sound waves in the three states of matter (solid, liquid,gas)

1. Gas 2. Liquid 3. Solid This is because **longitudinal waves travel through particle collision**, and because **solids have the highest density**, sound travels **faster in solids**, making gas slowest due to lowest density of particles.

# **Sound waves** What happens to the frequency when it travels through a different medium?

The frequency of the sound wave **doesn't** change

# **Sound waves** What is the range of frequencies that the human ear can detect (hear)?

20 - 20,000Hz Anything **below** 20 Hz = **Infrasound** Anyhting **above** 20,000 Hz = **Ultrasound**

# **The human ear** How does the ear work?

Sound waves that reach the **eardrum cause it to vibrate**. The vibrations are **passed through your earbone** to the **semicircular canals** and into the **cochlea**. The **cochlea converts these vibrations into electrical signals** that are **sent to the brain** and are **interpreted by it as sound**. Below is a link that contains an annotated diagram of the ear. Copy it into google to view: [https://www.savemyexams.com/a-level/physics/aqa/17/revision-notes/10-medical-physics/10-2-physics-of-the-ear/10-2-1-structure-of-the-ear/]

# **Using Infrasound and Ultrasound** How can ultrasound be used to look inside things?

**Ultrasound** waves can **pass through the body** but wehn they **reach a boundary** between 2 medias some of the wave is **refected** back which is **processed by a computer** to **form an image**.

# **Using Infrasound and Ultrasound** How can ultrasound be used for sonar to locate objects in water

A **pulse of ultrasound is sent downwards** by the submarine (travelling at **1500m/s**) and it takes **4 seconds** to get to the sea floor and **back**. 4 / 2 = 2 seconds. 2 * 1500 =3,000 3000m - So the waiter is **3000m** deep.

# **Core Practical** Describe a practical to measure wave speed on water

1. Set up a **ripple tank** with a **ruler** above the water 2. Change the voltage at the motor until **2 waves can be seen at once** (around half the length of the tank 3. **Count** how many **waves** are **formed** in **10 seconds** and record it 4. **Estimate wavelength** using ruler. Use wavelength and frequency to **find out the wave speed**

# **Core Practical** Describe an experiment to measure wave speed in solids?

1. **Suspend** a **metal rod** hoprizontally using **clamp stands** and **rubber bands** 2. Hit one end of the rod using a hammer and hold a **frequency meter** and **note down the frequency. 3. **Measure** the **length** of the **rod** - The **wavelength** will be **2 * the rod** 4. Use the **frequency** and **wavelength** to **find the speed of sound** in a rod**