Waves Part I

1. Waves Part I Phys 4a. Students know waves carry energy from one place to another. Phys 4b. Students know how to identify transverse and longitudinal waves in mechanical media, such as springs and ropes, and on the earth (seismic waves). Phys 4d. Students know sound is a longitudinal wave whose speed depends on the properties of the medium in which it propagates.

2. The Nature of Waves • What is a wave? • A wave is a repeating disturbance or movement that transfers energy through matter or space from one place to another.

3. Waves transfer energy, not matter. The water waves below are carrying energy but are not moving. Waves can only exist as they have energy to carry.

4. Categories of Waves

5. Longitudinal vs. Transverse • One way to categorize waves is on the basis of the direction of movement of the individual particles of the medium relative to the direction in which the waves travel.

6. Mechanical vs. Electromagnetic • Another way to categorize waves is on the basis of their ability or inability to transmit energy through a vacuum (i.e., empty space). • Electromagnetic waves can travel through a vacuum (empty space) • Mechanical waves cannot travel through empty space

7. Mechanical Waves • Mechanical waves are waves which require a medium. • Medium - A form of matter through which the wave travels (such as water, air, glass, etc.). • Mechanical waves are not capable of transmitting energy through a vacuum.

8. Mechanical Waves • Waves such as light, x-rays, and other forms of radiation do not require a medium. • Sound waves are incapable of traveling through a vacuum • Examples of mechanical waves: slinky waves, water waves, and sound waves

9. Electromagnetic Waves • All light waves are examples of electromagnetic waves • An electromagnetic wave is a wave which is capable of transmitting its energy through a vacuum (i.e., empty space). Electromagnetic waves are produced by the vibration of charged particles. • Electromagnetic waves, which are produced from the sun, subsequently travel to Earth through the vacuum of outer space. Were it not for the ability of electromagnetic waves to travel to through a vacuum, there would undoubtedly be no life on Earth.

10. Check your understanding A sound wave is a mechanical wave; not an electromagnetic wave. This means that • particles of the medium move perpendicular to the direction of energy transport. • a sound wave transports its energy through a vacuum. • particles of the medium regularly and repeatedly oscillate about their rest position. • a medium is required in order for sound waves to transport energy

11. Transverse Waves

12. Transverse Waves • Examples: light, ocean, all electromagnetic waves, and seismic waves • In transverse waves, the molecules of the medium oscillate perpendicular to the direction of propagation

13. Electromagnetic Spectrum (Transverse Wave)

14. Electromagnetic Waves • IN A VACUUM - all electromagnetic waves move at a speed of 3.0 x 108 meters/sec • Electromagnetic waves exist with an enormous range of frequencies. This continuous range of frequencies is known as the electromagnetic spectrum

15. Seismic Waves • What are seismic waves? • An energy wave which vibrates through the earth’s crust as the crust bends or breaks. Seismic waves exist as both transverse and longitudinal waves. Some travel through the earth and some travel across the earth’s surface.

16. S-Waves (Secondary) • Are transverse waves, or shear waves. • The ground is displaced perpendicularly to the direction of propagation. • S waves can travel only through solids, as fluids (liquids and gases) do not support shear stresses. • Their speed is about 60% of that of P waves in a given material. S waves are several times larger in amplitude than P waves for earthquake sources.

17. P-Waves (Primary) • Are longitudinal waves, or compressional waves. • The ground is alternately compressed and dilated in the direction of propagation. • In solids, these waves generally travel almost twice as fast as S waves and can travel through any type of material. • In air, these pressure waves take the form of sound waves, hence they travel at the speed of sound. • When generated by an earthquake they are less destructive than the S waves and surface waves that follow them, due to their bigger amplitudes.

19. Anatomy of a Seismic Wave

20. Properties of Waves

21. Wave Rest position

22. Wave • The amplitude is the height of the wave. • The wavelength is the distance from one wave top, or crest, to the next. • The Crest is the highest point of the wave • The through is the lowest point of a wave • The rest position of a wave is called a node or nodal line

23. Frequency of Waves • Frequency is how many waves/second, in units of 1/s or Hertz (Hz)

24. Frequency vs. Wavelength

25. Period vs. Frequency • Frequency is the amount of cycles or waves per second, measured in Hertz • A period is how much time it takes for 1 cycle or wave to pass • Period and frequency have an inverse relationship to one another

26. Check your understanding • Complete the sentence: • The larger the frequency the _______ the wavelength. • Frieda the fly flaps its wings back and forth 121 times each second. The period of the wing flapping is ____ sec. • A period of 5.0 seconds corresponds to a frequency of ________ Hertz. • As the frequency of a wave increases, the period of the wave ___________.

27. Amplitude and Energy of a Wave

28. Amplitude and Energy • a wave is an energy transport phenomenon which transports energy along a medium without transporting matter. • The amount of energy carried by a wave is related to the amplitude of the wave. • The energy transported by a wave is directly proportional to the square of the amplitude of the wave. This energy-amplitude relationship is sometimes expressed in the following manner.

29. Wave Speed • Speed = wavelength x frequency v=fλ • Units are in meters/second • Waves carry energy and NOT matter

30. Check your understanding • What’s the formula for wave speed? • The sound from a 60 Hz electric razor spreads out at 340 meters per second, what is its frequency? Period? Speed? wavelength?

31. Longitudinal Waves

32. Longitudinal Waves • Examples: sound, seismic P-waves • Longitudinal waves are waves that have vibrations along or parallel to their direction of travel; that is, waves in which the motion of the medium is in the same direction as the motion of the wave. • The speed of a longitudinal wave depends upon the medium through which it travels.

33. Compressional Longitudinal Wave • On a compressional wave the area squeezed together is called the compression. • The areas spread out are called the rarefaction. • The wavelength is the distance from the center of one compression to the center of the next compression.

34. Sound • Sound is a longitudinal wave • Sound is a mechanical wave • Sound is a pressure wave

35. Sound Waves • Sound waves in air (and any fluid medium) are longitudinal waves because particles of the medium through which the sound is transported vibrate parallel to the direction in which the sound wave moves • Sound waves are mechanical waves because it needs a medium to travel through, cannot travel through empty space • Since a sound wave consists of a repeating pattern of high pressure and low pressure regions moving through a medium, it is sometimes referred to as a pressure wave

36. Check your understanding • A sound wave is different than a light wave in that a sound wave is a. produced by an oscillating object and a light wave is not. b. not capable of traveling through a vacuum. c. not capable of diffracting and a light wave is. d. capable of existing with a variety of frequencies and a light wave has a single frequency.

37. Review Waves all carry energy Transverse Waves -Light, EM waves, S-waves, ocean waves -Can travel through empty spaces -Matter and energy travel in perpendicular direction Longitudinal Waves -Sound, P-waves -Cannot travel through empty space -Matter and energy travel in parallel direction