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Chapter 20. Waves. Section 1: Objectives. Describe how waves transfer energy without transferring matter. Distinguish between waves that require a medium and waves that do not. Explain the difference between transverse and longitudinal waves. Wave Energy.
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Chapter 20 Waves
Section 1: Objectives • Describe how waves transfer energy without transferring matter. • Distinguish between waves that require a medium and waves that do not. • Explain the difference between transverse and longitudinal waves.
Wave Energy • A wave is any disturbance that transmits energy through matter or empty space. • Energy can be carried away from its source by a wave. • The material through which the wave travels does not move with the energy.
Wave Energy • As a wave travels, it does work on everything in its path. • The waves in a pond do work on the water to make it move up and down. • The waves also do work on anything floating on the water’s surface. • The fact that the water and floating objects move tells you that the waves are transferring energy.
Wave Energy • Most waves transfer energy by the vibration of particles in a medium. • A medium is a substance through which a wave can travel. • Sound waves, water waves, and seismic waves all need a medium through which to travel.
Wave Energy • Visible light waves, microwaves, radio waves, and X rays are examples of waves that can transfer energy without going through a medium. • These waves are electromagnetic waves. • Electromagnetic waves do not require a medium like other waves.
Wave Energy • Transverse Waves are waves in which the particles vibrate perpendicularly to the direction the wave is traveling. • Transverse waves are made up of crests and troughs. • Water waves, waves on a rope, and electromagnetic waves are examples of transverse waves.
Wave Energy • Longitudinal Waves are waves in which the particles vibrate back and forth along the path that the waves moves. • Longitudinal waves are made up of compressions and rarefactions. • Waves on a spring are longitudinal waves.
Wave Energy • Sound Waves are longitudinal waves. Sound waves travel by compressions and rarefactions of air particles, as shown below.
Wave Energy • Combinations of Waves A transverse waves and a longitudinal wave can combine to form a surface wave. • Surface waves look like transverse waves, but the particles of the medium move in circles rather than up and down.
Chapter 20 Sec. 1 Pop Quiz • 1) T/F Energy can’t be carried away from its source by a wave. • 2) T/F As a wave travels it does work on the path. • 3) How do most waves transfer energy? • 4) List 1 type of wave that does not require a medium to pass through. • 5) What is the difference between transverse and longitudinal waves? • 6) What is a surface wave?
Section 2: Objectives • Identify and describe four wave properties. • Explain how frequency and wavelength are related to the speed of a wave.
Parts Of A Wave • The amplitude of a wave is the maximum distance that the particles of a medium vibrate from their rest position. • A wave with a large amplitude carries more energy than a wave with a small amplitude.
Parts Of A Wave • A wavelength is the distance between any point on a wave to an identical point on the next wave. • A wave with a shorter wavelength carries more energy than a wave with a longer wavelength does.
Parts Of A Wave • Frequency is the number of waves produced in a given amount of time. • Frequency is usually expressed in hertz (Hz). • One hertz equals one wave per second. • If the amplitudes are equal, high-frequency waves carry more energy than low-frequency waves.
Parts Of A Wave • Wave Speed is the speed at which a wave travels. • Wave speed (v) can be calculated using wavelength () and frequency (f), by using the wave equation, which is shown below: • v f
Example # 1 • What is the frequency of a wave if the wave has a speed of 12 cm/s and a wavelength of 3 cm? • Remember: v f • = v / f (Rearranged by dividing by f) • F = V / (Rearranged by dividing by )
Example # 2 • A wave has a frequency of 5 Hz and a wave speed of 18 m/s. What is the wavelength? • Remember: v f • = v / f (Rearranged by dividing by f) • F = V / (Rearranged by dividing by )
Parts of A Wave • Frequency and wavelength are inversely related. • So, if one value is doubled, the other value will be cut in half. • The wave speed of a wave in a certain medium is the same no matter what the wavelength is. • So, the wavelength and frequency depend on the wave speed, not the other way around.
Chapter 20 Sec. 2 Pop Quiz • 1) What is the unit used to measure frequency? • 2) What is 1 Hz equal to? • 3) What is the formula used to calculate wave speed? • 4) How are frequency and wavelength related? • 5) What will happen to the speed of a wave in a certain medium if the wavelength changes?
Section 3: Objectives • Describe reflection, refraction, diffraction, and interference. • Compare destructive interference with constructive interference. • Describe resonance, and give examples.
Wave Angles • Reflection happens when a wave bounces back after hitting a barrier. • Light waves reflecting off an object allow you to see that object. • A reflected sound wave is called an echo. • Waves are not always reflected when they hit a barrier. • A wave is transmitted through a substance when it passes through the substance.
Wave Angles • Refraction is the bending of a wave as the wave pass from one medium to another at an angle. • When a wave moves from one medium to another, the wave’s speed and wavelength changes. • As a result, the wave bends and travels in a new direction.
Wave Angles • Diffraction is the bending of waves around a barrier or through an opening. • The amount of diffraction depends on its wavelength and the size of the barrier or opening the wave encounters.
Wave Angles • Interference is the result of two or more waves overlapping. • Constructive Interference happens with the crests of one wave overlap with the crests of another wave or waves. The troughs of the waves also overlap. • Constructive Interference results in a new wave that has a larger amplitude than the original waves had.
Wave Angles • Destructive Interference happens with the crests of one wave and the troughs of another wave overlap. • Destructive interference results in a new wave that has a smaller amplitude than the original waves had. • When the waves involved in destructive interference have the same amplitude and meet each other at just the right time, the result is no wave at all.
Wave Angles • Standing Waves are waves that appear to be standing still. • A standing wave only looks as if it is standing still. • Waves are actually going in both directions. • In a standing wave, certain parts of the wave are always at the rest position because of total destructive interference. • Other parts have a large amplitude because of constructive interference.
Wave Angles • The frequencies at which standing waves form are called resonant frequencies. • Resonance happens when an object vibrating at or near the resonant frequency of a second object causes the second object to vibrate.
