Force, Energy & Communication

Lesson 13 & 14

WAVES

Lesson objectives

Re-establish that all sound is caused by vibrations (waves)

Revise the links between pitch and wavelength; volume and amplitude.

Know a method for calculating the speed of sound

Learn the basic wave equation Measure the critical angle for a glass block

WAVES – the basics

Turn to p 158 of PHYSICS text book

WAVES – the basics

Waves vibrate in a regular fashion.

WAVES – the basics

All sound waves are caused by vibrations.

WAVES – the basics

Waves can transfer energy or information from one place to another.

WAVES – the basics

While the wave may transfer information or energy, it does not transfer matter.

WAVES – the basics

Nearly all the information we transfer is done by waves e.g. sound, radio, mobile phones, tv, etc

Measuring the speed of sound using the echo method

Measuring the speed of sound using the echo method

Speed = distance / time

Measuring the speed of sound using the echo method

Expt. no. Distance from wall (m)

No. of claps

Total distance travelled

Time (s) Speed of sound (m/s)

1 49 10

2 49 10

3 49 10

4 49 10

Measuring the speed of sound using the echo method

Expt. no. Distance from wall (m)

No. of claps

Total distance travelled (m)

Time (s) Speed of sound (m/s)

1 49 10 980 3.62

2 49 10 980 3.31

3 49 10 980 4.01

4 49 10 980 3.54

Measuring the speed of sound using the echo method Text book value for the speed of sound

through air is…..

Measuring the speed of sound using the echo method Text book value for the speed of sound

through air is…..330 m / s

The Wave Equation

A formula you are expected to KNOW!!

The Wave Equation

Turn to page 171 in the text book.

The Wave Equation

The wavelength ( )of a wave depends on two things:

1 the frequency of the wave

2 the speed of the wave

The Wave Equation

This can be expressed by the wave equation

The Wave Equation

This can be expressed by the wave equation

speed (m/s) = frequency (Hz) X wavelength (m)

The Wave Equation

This can be expressed by the wave equation

speed (m/s) = frequency (Hz) X wavelength (m)

N.B. frequency is measured in Hertz (or kilohertz). Hertz is another way of saying the number of waves in a second. E.g. 30 Hz = 30 waves every second

The Wave Equation

Copy out the example calculation off p171

Total Internal Reflection

Refraction happens when light is coming out of something dense like glass or water.

If the angle of the light is shallow enough, the light ray won’t come out at all but reflects back into the dense substance.

This is called total internal reflection because all the light reflects back in.

The angle over which total internal reflection occurs is called the critical angle.

The Wave Equation

Measuring the critical angle

Follow the instructions on your experiment sheet.

HOMEWORK

Complete all questions on the sheet on “Sound waves”.

Due in next Thursday