1 General Properties of WAVE Sec 3 Exp Science PHYSICS

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General Properties of

WAVE

Sec 3 Exp

Science PHYSICS

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What is a Wave?

Wave is a mechanism that carries energy from one point to another.

A wave is created by vibrations or oscillations of objects or substances.

Waves carry energy from one point of vibrations or oscillations to another.

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2 Types of Waves

• TRANSVERSE

• LONGITUDINAL

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Transverse waves

• Waves that travel in a direction perpendicular to the direction of vibration of particles.

• This means that particles of transverse wave only moves up and down, while wave travel sideways.

• Some examples are rope waves, water waves, light waves and radio waves.

Direction of wave

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Transverse Wave

Transverse wave in air. Direction of wave is from left to right.Notice how the air particles are vibrating with respect to thedirection of wave motion. The air particles are just vibratingup and down about their fixed positions as the wave moves

from left to right.

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Longitudinal Waves

• Waves that travel in the same direction as the direction of vibration of particles.

• Particles of longitundinal waves move side-to-side in a direction parallel to direction of wave.

• Some examples are sound waves and waves produced by springs.

Direction of wave

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Longitudinal Wave

Longitudinal wave in air. Direction of wave is from left to right.Notice how the air particles are vibrating with respect to the

direction of wave motion. Although it appears at first glance thatgroups of air particles are moving from left to right, if you lookclosely, the air particles are in fact just vibrating about a fixed

position.

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Key point 1! for ALL wave motion

During wave motion, the particles are NOT transferred from one point to another.

They just vibrate about their fixed or neutral position.

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Terms used in waves

• Crests

• Troughs

• Displacement

• Amplitude

• Wavelength

• Frequency

• Period

• Wavefront

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Troughs

Crests

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wavelength

wavelength

Displacement /m

Distance/m

amp

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Properties of Wave Motion

• Crest: The highest point of a transverse wave.

Trough: The lowest point of a transverse wave.

Amplitude: Maximum displacement from rest or central position. Unit: meter (m).

Displacement has positive and negative values. Normally when particles moves upwards it’s positive. When downwards its negative.

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Describing a wave:

Wavelength: Shortest distance between two ( λ ) points on the wave that are in

phase. Unit: meter (m)Distance between two troughs.Distance between two crests.

(In phase = both particles move in the same

direction, at the same speed and have the same displacement from rest position.)

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Describing a wave:

Frequency: Number of complete waves ( f ) produced in one second.

Unit: Hertz (Hz) Period: Time taken to produce one ( T

) complete wave. Unit: seconds (s)

Inverse of frequency

(i.e. T = 1 / f )

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Describing a wave:

Wave speed: The distance travelled by a wave (v) in one second.

SI unit: m /s

Wavefront: An imaginary line on a wave that joins all points which have the same

phase of vibration.

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Quick Check1. A boy sings as he plays with a rope. He vibrates

one end of the rope up and down repeatedly, while the other end is fixed to a wall.

(a) What type of wave is obtained in the rope? Transverse(b) Define transverse wave. A wave where its particles vibrate in a

perpendicular direction to that of the direction of wave.

(c) The sound that the boy made while singing is an example of (transverse, longitudinal) wave.

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Quick Check

2. Give one example of a longitudinal wave.

Sound wave

3. Label the wave terms – crest, trough, wavelength and amplitude in the displacement-distance graph below of a transverse wave.

Displacement /m

Distance/m0

Displacement /m

Distance/m0

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Relationship between velocity, frequency and wavelength

• Speed of a wave = frequency of wave X wavelength of the wave

• In symbols,

v = f v = wave speed/velocity (m/s)f = frequency (Hz) = wavelength (m)

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The frequency – period equation

• Knowing frequency, we can find the period of a wave.

• Similarly, if we know the period, we can find the frequency of the wave.

f = 1/T

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The Ripple Tank

• The ripple tank allows us to observe the behaviour of waves easily.

• Basically waves behaves similar to that of light:

• Waves reflect• Waves refract

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Measuring wavelength

1 wavelength

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Displacement vs distance Graph

• Waves can be plotted on a displacement vs distance graph.

• The displacement refers to displacement of particles, while distance refers to the distance of particles from a starting point.

• From this graph, we can find:1. Wavelength2. Amplitude3. Distance of particles from a position

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wavelength

Displacement /m

Distance/m

amp

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wavelength

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Displacement vs time Graph

• Waves can be plotted on a displacement vs time graph.

• The displacement refers to displacement of particle just one particle, while time refers to the time of that same particle moves a particular displacement

• From this graph, we can find:1. Period of wave2. Amplitude

• From period, we can calculate the frequency of the wave

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Displacement /m

Time/s

amp

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period

period

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A displacement – time graph of a wave with wavelength 0.4m is shown.

(a) Calculate the frequency of the wave. Period of wave is 0.5 s So f = 1/T = 1/0.5 = 2 Hz(b) Calculate the speed of the wave.

Speed = frequency x wavelength = 2 x 0.4 = 0.8m/s

Eg. Displacement vs time Graph

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Displacement /m

Time/s0

0.25 0.5 0.75 1.0

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Quick Check

1. Calculate the wave speed of a wave of frequency 500 Hz and a wavelength of 0.02 m.

v = 500 x 0.02 = 10 m/s

2. Calculate the wavelength of a wave that has a frequency of 2kHz and speed of 5m/s.

wavelength = v/f = 5/2000 = 0.0025 m

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Quick Check3. A wave of speed 330 m/s has a wavelength of 2m.

(a) Calculate the frequency of the wave.

frequency = v/wavelength = 330/2 = 165 Hz

(b) Calculate the period of the wave.

period = 1/f = 1/165 = 0.006 s