Waves 2016 EdExcel GCSE Physics Topic 4 W Richards The Weald School 27/07/2019 Waves 2016 EdExcel GCSE Physics Topic 4 W Richards The Weald School

What is a wave? 27/07/2019 A wave is a “movement of energy”. Notice that the “stuff” the wave moves through doesn’t actually move from left to right, but energy does!

Some definitions… 1) Amplitude – this is “how high” the wave is: 27/07/2019 1) Amplitude – this is “how high” the wave is: 2) Wavelength () – this is the distance between two corresponding points on the wave and is measured in metres: 3) Frequency – this is how many waves pass by every second and is measured in Hertz (Hz). By definition, frequency = 1/period (in s).

Transverse vs. longitudinal waves 27/07/2019 Displacement Transverse waves are when the displacement is at right angles to the direction of the wave… Direction Examples – light, water waves Example – sound Displacement Longitudinal waves are when the displacement is parallel to the direction of the wave… Direction

The Wave Equation 27/07/2019 The wave equation relates the speed of the wave to its frequency and wavelength: Wave speed (v) = frequency (f) x wavelength () in m/s in Hz in m V  f You need to learn this equation!

Some example wave equation questions 27/07/2019 A water wave has a frequency of 2Hz and a wavelength of 0.3m. How fast is it moving? A water wave travels through a pond with a speed of 1ms-1 and a frequency of 5Hz. What is the wavelength of the waves? The speed of sound is 330ms-1 (in air). When Dave hears this sound his ear vibrates 660 times a second. What was the wavelength of the sound? Purple light has a wavelength of around 6x10-7m and a frequency of 5x1014Hz. What is the speed of purple light? 0.6ms-1 0.2m 0.5m 3x108ms-1

Another wave equation x v t 27/07/2019 Here’s another equation we can use for waves: Wave speed (v) = distance (x) / time (t) in m/s in m in s x t v You need to learn this equation!

Some example wave equation questions 27/07/2019 A sound wave covers 600m in 2 seconds. What is its speed? A wave travels at a speed of 2m/s. How far will it travel in 20s? How long does light take to reach us from the sun if the sun is 150 million km away and light travels at 3x108m/s? 300m/s 40m 500s

Measuring the speed of waves 27/07/2019 Task: Design and carry out two experiments that will help you to determine: The speed of a ripple in water The speed of sound in air

Q. How does wave speed depend on depth? Water Waves (HT only) 27/07/2019 Q. How does wave speed depend on depth? Design an experiment that explores the relationship between the depth of water and the speed of a wave in that water.

Wavefronts Wavefronts 27/07/2019 A “wavefront” is basically the “crest” of a wave when viewed from above: Wavefronts

What happens when a wave hits a boundary? (HT only) 27/07/2019 What happens when a wave hits a boundary? (HT only) When a wave hits a boundary a number of things could happen: 1) It could be reflected 2) It could be transmitted or refracted through the material 3) It could be absorbed Note that the frequency of the waves doesn’t change. However, the velocity and wavelength might!

Using Wavefronts to show Refraction (HT only) 27/07/2019 Using Wavefronts to show Refraction (HT only) Let’s use wavefronts and the wave equation to show what happens when an electromagnetic wave is refracted: V  f When this wave if refracted it changes direction and slows down. However, the frequency does not change to what happens to the wavelength?

Sound Waves (Physics HT only) 27/07/2019 As we know, sound waves are formed when something vibrates. But how does the sound reach our ears? Air molecules 2) The vibrations pass through air by making air molecules vibrate 3) These vibrations are picked up by the ear drum and other parts of the ear, which also vibrate 1) An object makes a sound by vibrating

Converting Sound to Vibrations 27/07/2019 The ear drum The ear bones In a microphone the sound waves cause a diaphragm to move Note that our ear drum and bones can only vibrate at a certain rate. Therefore our hearing is limited to between 20Hz and 20,000Hz. This is called our “range of hearing”.

Infrasound 27/07/2019 Infrasound is the region of sound BELOW 20Hz (as opposed to ULTRASOUND which is above 20KHz) – it can’t be heard by humans. Some uses: Communication between animals African elephants use infrasound to communicate with other elephants many kilometres away. 2) Detecting movement in remote areas 3) Detecting volcanic eruptions and meteors

Using Waves for Exploration 27/07/2019 Example 1 – Echo sounding In echo-sounding, high frequency sound waves are reflected from an object to measure the distance to that object:

Words – depth, reflected, picture, medium Example 2 - Ultrasound 27/07/2019 Ultrasound is the region of sound above 20,000Hz – it can’t be heard by humans. It can be used in pre-natal scanning: How does it work? Ultrasonic waves are partly _________ at the boundary as they pass from one _______ to another. The time taken for these reflections can be used to measure the _______ of the reflecting surface and this information is used to build up a __________ of the object. Words – depth, reflected, picture, medium

Example 3 - Earthquakes Here’s the structure of the Earth: 27/07/2019 Here’s the structure of the Earth: A thin crust - 10-100km thick A mantle – has the properties of a solid but it can also flow A core – made of molten nickel and iron. Outer part is liquid and inner part is solid How do we know this? These facts have all been discovered by examining seismic waves (earthquakes)