WAVES Chapter 20 p.576-601

Section 1

Objectives 1. Explain the relationship among waves, energy, and matter 2. Describe the difference between transverse waves and compressional waves

A. Waves Carry Energy Waves (rhythmic disturbances that carry energy without carrying matter) can carry energy by itself or with matter Example – if a boat makes waves, then just energy is moving through the wave in an outward motion, but if you throw a ball, the energy is traveling in the direction of the ball along with the matter of the ball

But how does a wave carry energy without transporting matter? If a line of students were standing shoulder to shoulder and passing a ball to each other without moving their feet and only their hands (ball is being passed side to side) then the ball is not actually moving, but the people are. The energy is traveling through the students, not the ball. Would the ball be able to move without the students? But the ball can represent the motion of the energy that is being moved. So… what do the people represent if the ball represents energy? Answer: Waves!

B. Wave Motion! Ever wondered how the waves move the way they do? Ever been in a pool or the lake and you bobbed up and down instead of moving forward or backward? Water molecules actually move in a circular motion which causes that bobbing motion rather than a distinct forward or backward motion. The molecules will bump into each other and transfer the energy from one to another. It depends on the amount of energy exerted that determines on the strength and size of the wave Any type of energy will cause a wave to move outward, but if there isn’t enough energy it won’t be strong enough to actually move or carry matter.

C. What do you think? How can being near a source of deep, loud sounds, such as heavy machinery or the subwoofer of a stereo, demonstrate that waves carry energy??? Answer: Try this: With a partner, turn to each other and take turns saying hello in a deep voice. You can actually feel and hear the vibration coming from their voice. You can feel the vibrations as well as hear the sound!

D. Mechanic Waves Mechanical waves (use matter to transfer energy) travel through matter called mediums In an example of ripples on a pond, what would be the medium? Answer: The mechanical wave travels as energy is transferred from particle to particle in the medium which can be liquid, solid or gas. You would not be able to hear a sound wave if it weren’t for air in which the air molecules transfer the energy to your ear. Water!

E. What do you think? Why do you think the word mechanical is appropriate for waves that need matter to move? Hint: think about what a machine does…. Think about why your mechanical pencil is called “mechanical” instead of just “pencil”. Answer: One definition of a machine is: a device that transmits energy. In a mechanical wave, matter transmits energy.

F. Transverse Waves Transverse wave (a type of mechanical wave that the wave energy causes the matter in the medium to move up and down or back and forth at right angles to the direction the wave travels) Example: a rope tied to the wall and you moving it up and down. Watch as the rope appears to move away from you but it is not. The wave is what is moving away. The high points of the waves are called crests The low points of the waves are called troughs

Diagram of a wave

G. Compressional Waves Compressional wave (a type of mechanical wave in which matter in the medium moves forward and backward in the same direction that the wave travels) Example: Imagine holding a slinky in both hands with them stretched apart. You move one hand and a wave is sent to the other end. The slinky did not leave your hand and travel with the wave. Instead the part of the slinky that moved like a wave bounced back and forth until it was at its origin in a resting position

Slinky Waves

H. Sound Waves A type of compressional wave Example: your voice is created when your vocal cords move in which you feel a vibration same as if you were to touch the speaker on a stereo when music is playing All waves are produced by something that vibrates

I. Making Sound Waves When something is producing sound it actually touches some of the air molecules around it in which those air molecules carry the energy that travels to your ear But from the beginning source of the sound, the medium pushes the air molecules making them squeeze together Compression (the group of molecules that are squeezed together)

When the medium moves downward during the vibration, the molecules have space to move away from each other Rarefraction (a place where molecules are far apart)

J. Electromagnetic Wave Electromagnetic wave (a wave that does not require matter to carry energy) These are transverse waves that are produced by the motion of electrically charged particles Can travel through mediums that are solid, liquid or gas just like what? Answer: Example: Radio waves. They don’t stop when they reach your house. They travel right through it. Mechanical Waces!

Unlike mechanical waves, electromagnetic waves can travel through space or a vacuum where no matter exists What is another type of wave, other than a radio wave, that can travel through space? Answer: Light wave!

But wait! There’s more! Other waves that travel through space include: INFRARED

VISIBLE

ULTRAVIOLET

Another type of electromagnetic wave: X-RAY This is actually Mr. DiFabio’s elbow!

X-rays don’t just help medically, but also security!

Review! 1. What is a wave? Rhythmic disturbances that carry energy without carrying matter 2. What type of mediums can both mechanical and electromagnetic wave travel through? Solid, liquid, or gas 3. What type of waves causes the matter to move up and down or back and forth at ninety degree angles? Transverse wave

5. What are all waves produced from? 4. What type of wave only carries the energy forward and backward in the same direction the wave travels? Compressional wave 5. What are all waves produced from? vibrations 6. What is it called when the molecules are squeezed together? Compression 7. What is it called when the molecules are far apart? rarerefraction 8. What type of wave doesn’t need matter as a medium? Electromagnetic wave 9. What are types of waves that can travel through space? Light, infrared, visible, radio, ultraviolet