The Energy of Waves Physical Science

Section 1: The Nature of Waves A wave is any disturbance that transmits energy through matter or space. Examples: ocean waves, microwaves, sun waves, and sound waves (radio, telephone and voice)

Waves and Energy Waves move through a medium, which is the substance through which a wave can travel. Example: sound travels through air (air is the medium). The waves travel through the medium, but the medium does not travel with the waves. For example: water waves. It looks like the water is moving forward, but it is only moving up and down with the waves.

Waves Transfer Through a Medium When the energy of a wave transfers through a medium, the particles in the medium vibrate, and it passes the energy to the next particle, which also vibrates. Example: sound through the air. When you release the energy into the air, it causes the air particles to vibrate. That is how sound travels. Question: When would you scream but not be heard?

Energy Transfer Without a Medium Electromagnetic waves are waves that don’t require a medium, so they can travel through a vacuum. These waves include visible light, microwaves, TV/radio signals, and Xrays. They can travel through a medium, but they don’t need one.

Light Years Because planets and stars can be very, very far away, they are measured in light years, the amount of time it takes light to travel. Since light travels at 299,792,458 meter per second, how could you find out how many meters it travels in a year? This means, when you see stars at night, the light you are seeing could have been emitted by the star before the Earth was even formed!

Types of Waves 3 types: transverse, longitudinal and surface waves Transverse Waves: Waves in which the particles vibrate with an up-and-down motion. The particles in a transverse wave move across (perpendicular) the direction the wave is traveling. The top of each wave is called a crest, and the bottom of each wave is called the trough. Example: all electromagnetic waves.

Transverse Waves

Longitudinal Waves In a longitudinal wave, the particles of the medium vibrate back and forth along the path that the wave travels. Think of the coils on a spring. You can stretch out the coils (rarefaction) and then let them bounce back (compression). In a longitudinal wave, the compressions are the crests of the wave, and the rarefactions are the troughs. Example: sound waves

Longitudinal Waves

Surface Waves A combination of longitudinal and transverse waves is called surface waves. They look like transverse waves, but the medium moves in circles instead of up and down. Ocean waves are surface waves because they travel at the water’s surface, where the water meets the air.

Surface Waves

Section 2: Properties of Waves Have you ever been on a boat on the water that wasn’t moving? Do you remember the gentle rolling of the boat on the waves? Did you ever have a speeding boat pass by? If so, what did your boat do? There are 4 properties of waves: amplitude, wavelength, frequency, and wave speed.

Amplitude The property of waves that is related to the height of the wave is called amplitude. The amplitude of a wave is the maximum distance the wave vibrates from its rest position. The larger the amplitude, the taller the wave. It takes more energy to move a larger wave, so a wave with a large amplitude carries more energy than a wave with a small amplitude.

Ocean Waves vs. Tsunami

Wavelength A wavelength is the distance between any two adjacent crests or compressions, or the distance between any two adjacent troughs or rarefactions.

The Power of Waves http://www.abc.net.au/news/specials/japan- quake-2011/

Wavelengths

Frequency Frequency is the number of waves (either the crest or the trough) that pass a given point in a given amount of time. You can measure frequency by counting the number of troughs or crests (one or the other, not both) during that time. It is measured in hertz (Hz). A hertz is equal to one wave per second. The higher the frequency, the more energy the wave is carrying.

Wavelength and Frequency Because wavelength and frequency are closely related, you can see how the amount of energy the wave is carrying affects both. The more energy a wave has, the shorter the wavelength, the greater the frequency. The less energy a wave has, the longer the wavelength, the less frequency.

Don’t Touch that Dial! Think about your favorite radio station – what is it? The number is the frequency of the waves that station is emitting. Which have higher frequencies, AM or FM stations?

Wave Speed Wave speed is the velocity at which a wave travels. You can calculate it as Velocity = frequency x wavelength or V = f x λ To calculate wave speed, you would take the distance (between crests or troughs), and divide it by the time it takes to get a certain point.

The water waves below are traveling along the surface of the ocean at a speed of 2.5 m/s and splashing periodically against Wilbert's perch. Each adjacent crest is 5 meters apart. The crests splash Wilbert's feet upon reaching his perch. How much time passes between each successive drenching? Answer and explain using complete sentences. Wilbert’s Dilemma The water waves below are traveling along the surface of the ocean at a speed of 2.5 m/s and splashing periodically against Wilbert's perch. Each adjacent crest is 5 meters apart. The crests splash Wilbert's feet upon reaching his perch. How much time passes between each successive drenching?

Wilbert’s Dilemma How to solve Wilbert’s problem: If Velocity = frequency x wavelength then 2.5m/s ? X 5 m frequency = .5

Section 3: Wave Interactions Planets do not emit light. So why does the moon shine so brightly, particularly a full moon? Reflection occurs when a wave bounces back after striking a barrier. All waves can be reflected. What do you call reflected sound waves?

Wave Interactions Refraction is the bending of a wave as it passes at an angle from one medium to another. Have you ever looked down at your feet when you are standing in a pool? What have you noticed about your feet and legs?

Mutant Student? When waves move to a different medium, they begin traveling at a different speed. The reason why your legs look like the are in the “wrong” place is because the light waves that traveled through the air go through the water at a different speed. As soon as the wave hits the water, it bends as it slows down, so your eyes see your legs further out than they really are!

Diffraction Most waves travel in a straight line (such as light waves). However, some waves can curve or bend when they reach the edge of an object. This bending is called diffraction. Sound waves can diffract because they have longer wavelengths than light and can bend around the corners.

Reflection, Refraction and Diffraction   http://www.youtube.com/watch?v=Smoo3jKt sDI http://www.acoustics.salford.ac.uk/schools/t eacher/lesson3/flash/whiteboardcomplete.sw f

Interference Have you ever walked into a large party or crowd, and you hear a lot of people talking. You can’t understand anyone because too many people are talking. This is an example of interference. Interference is when 2 or more waves share the same space, so they overlap.

2 Types of Interference Constructive Interference – When the crest of one wave overlaps the crest of another wave. This increases the energy of the wave, so it has a higher amplitude. Example: a band playing instead of just 1 person. Destructive Interference – when the crest of one wave matches the trough of another wave. They cancel each other out, so all you get is noise. This is what happens in a crowd.

Constructive and Destructive Waves

Standing Waves A standing wave is a wave that forms a stationary pattern in which portions of the wave are at the rest position (destructive), while other portions have a large amplitude (constructive). It happens as the wave is traveling back and forth – it looks like it is standing still, but energy is traveling through the medium.

Resonance Standing waves cause resonance frequencies. In other words, the medium vibrates with the same frequencies as the wave. For example, when you strike an object, it begins to resonant with the same frequency of the energy that you struck it. Example: a drum. The sound you hear is the vibration of the wave in the air that matches the vibration of the energy that you used to hit the drum.

Resonance Videos https://www.youtube.com/watch?v=JDnNmLk Q3Bc www.youtube.com/watch?v=ofESdVdX-fY

Tacoma Narrows Bridge In 1940 a suspension bridge was built in Tacoma, Washington. Suspension bridges are designed to sway in the wind to a certain degree, which gives them their own energy waves. The high winds on Nov. 7th, 1940 matched those waves of the bridge (constructive waves), which amplified the bridges movement, causing its collapse.

Tocoma Narrows Bridge Collapse https://www.youtube.com/watch?v=nFzu6CNt qec

The New Tacoma Narrows Bridge