P5 - Waves in matter

Question 1

What are longitudinal and transverse waves?

Answer 1

Longitudinal:

• oscillations are parallel to energy transfer
• consists of rarefactions and compressions
• examples include sound waves

Transverse

• oscillations are perpendicular to energy transfer
• has a peak/crest( top of the wave) and a trough(bottom of wave)

Question 2

What is the formula to calculate wave velocity?

Answer 2

wave velocity(m/s) = frequency(Hz) x wavelength(m)

Question 3

How can you measure the velocity of ripples and sound waves?

Answer 3

Ripples:

• use a ripple tank
• find wavelength using a strobe and and a ruler
• find frequency by finding the number of rotations of the motor per second then use the formula

Sound:
- connect a pair of microphones a certain distance apart to an oscilloscope

Question 4

What happens when sound travels across a boundary?

Answer 4

When a wave travels from one medium to another it’s wave velocity can change and so can it’s direction which is called refraction. Frequency doesn’t change
(look at diagrams in physics book)

Question 5

What 3 things can happen to a sound wave at a boundary?

Answer 5

The wave can be:
- reflected (an echo)
- transmitted (possibly refracted)
- absorbed 
What happens depends on the densities of the regions either side of the boundary. If the density differs a lot then a lot of the sound will be reflected.

Question 6

What is ultrasound and how can we use it?

Answer 6

Ultrasound is sound with frequencies above 20kHz and is useful as it has a very small wavelength so it can be focused into a beam

Using ultrasound on a pregnant woman:
- The transmitter beams ultrasound waves into the mother
- The waves reflect from different boundaries
- The machine calculates distances using time and velocity to then produce an image
a gel is placed on the mother to reduce the density difference

Question 7

What happens when a sound wave hits a solid?

Answer 7

When the sound is absorbed it makes the particles in the wall vibrate and energy is transferred.

Question 8

How does the ear detect sound?

Answer 8

The outer ear(pinna) gathers the sound wave and directs it down the auditory canal towards the eardrum which vibrates. As the eardrum vibrates it makes the ossicles(three small bones in the ear; hammer, anvil, stirrup) vibrate. They are like small levers that amplify the vibration and pass it on through the oval window. Then in the cochlea, the fluid transmits the vibrations to small hairs in the cochlea which release chemical substances that makes nerves send a signal down the auditory nerve. The brain then process the signal and you hear a sound.

Question 9

Why can we only hear a range of frequencies?

Answer 9

The hair inside the cochlea are different lengths and resonate at different frequencies. As we get older we lose the shorter hairs and are unable to hear higher frequencies.