3.2 - Stationary Waves : Waves

Question 1

How are stationary waves produced?

Answer 1

By the superposition of 2 waves of the same frequency and amplitude travelling in opposite directions.

Question 2

What is the principle of superposition?

Answer 2

When 2 or more waves with the same frequency arrive at a point, the resultant displacement is the sum of the displacements of each wave.

Question 3

What is the difference between how stationary and progressive waves carry energy?

Answer 3

Stationary waves store energy, progressive waves transfer energy.

Question 4

What is difference between progressive and stationary waves in terms of amplitude?

Answer 4

Progressive - All points have the same amplitude
Stationary - Each point has a different amplitude depending on the amount of superposition

Question 5

What is difference between progressive and stationary waves in terms of phase difference?

Answer 5

Progressive - points exactly a wavelength apart are in phase. The phase of points within one wavelength can be between 0 and 360 degrees
Stationary - Points between nodes are in phase, points on either side of a node are out of phase.

Question 6

What is difference between progressive and stationary waves in terms of wave speed?

Answer 6

Progressive - Wave speed is the speed which the wave travels through a medium.
Stationary - Each point on the wave oscillates at a difference speed. The overall wave does not move.

Question 7

What are nodes?

Answer 7

Regions where there is no vibration. (minimum)

Question 8

What are antinodes?

Answer 8

Regions where the vibrations are at their maximum amplitude. (maximum)

Question 9

What which points on a stationary wave are in phase/ out of phase?

Answer 9

• Points between nodes are in phase
• Points that have an odd number of nodes between them are out of phase
  Points that have an even number of nodes between them are in phase

Question 10

What happens when 2 waves superpose each other in phase?

Answer 10

This causes constructive interference. The peaks and troughs line up on both waves and the resultant wave has double the amplutude.

Question 11

What happens when 2 waves superpose each other in anti-phase?

Answer 11

This causes destructive interference. The peaks on one wave line up with the troughs of the other. The resultant wave has no amplitude.

Question 12

What type of wave does superposition effect?

Answer 12

All types - transverse and longitudinal

Question 13

What must happen for waves to superpose?

Answer 13

The waves must have the same wavelength and similar amplitude.

Question 14

How do stringed instruments (like guitars) produce sound?

Answer 14

Vibrations caused by stationary waves on a stretched string.

Question 15

How do instruments such as the clarinet and organ produce sound?

Answer 15

Sound waves can be produced as a result of the formation of stationary waves inside an air column.

Question 16

What are 3 practical examples of stationary waves?

Answer 16

• a stretched string (string instruments)
• microwaves with a reflector (reflector creates stationary wave)
• sound waves inside an air column (shows how clarinets and organs work)

Question 17

What are harmonics?

Answer 17

Any system in which standing waves can form has numerous natural frequencies. The set of all possible standing waves are known as the harmonics of a system.

Question 18

What is the first harmonic/ fundamental frequency?

Answer 18

A stationary wave with a single loop made up of 2 nodes and an antinode.

Question 19

What is the wavelength of the first 3 harmonics?

Answer 19

First - 1/2 wavelength
Second - A full wavelength
Third - 3/2 of a wavelength

Question 20

What is the equation for finding the frequency of the first harmonic?

Answer 20

f = 1/2L√T/µ
f = frequency
L = length of string
T = tension of string
µ = mass per unit length of the string

Question 21

What are the aims of the required practical: Investigating stationary waves?

Answer 21

• To measure how the frequency of the first harmonic is affected by changing one of the following variables:
• Length of string
• Tension of string (when we did it we changed this)

Question 22

What is the independent variable of the ‘investigating stationary waves’ required practical?

Answer 22

Either length or tension or mass per unit length.

Question 23

What is the dependent variable of the required practical that investigates stationary waves?

Answer 23

Frequency of the first harmonic.

Question 24

What are the controlled variables of the required practical?

Answer 24

Same length of the string and the same string (mass per unit length, µ)

Question 25

What is the equipment needed for the required practicalthat investigates stationary waves?

Answer 25

• signal generator
• vibration generator
• retort stand
• 2kg mass
• 2m of string
• pulley
• wooden bridge
• mass hanger + 100g masses
• metre ruler
• top-pan balance

Question 26

What is the resolution of measuring equipment for the required practical that investigates stationary waves?

Answer 26

metre ruler - 1mm
signal generator - 10nHz
Top-pan balance = 0.05g

Question 27

What do you do with the required practical ‘investigating stationary waves’
results?

Answer 27

Use the tension to plot a graph and use the gradient (1/2Lõ). Compare graph gradient with calculated gradient and work out percentage error.
- evaluate the experiment and look for systematic/ random errors that may have occurred.