Lesson 3&4 - Waves Required Practical

Amplitude, frequency, wavelength, wave speed

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Amplitude, frequency, wavelength, wave speed

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• Put this video on Google classroom as pre-learning or for students to watch after the lesson.
• Start
• Recall questions
• Main
• How loudspeakers produce sound.
• Demo buzzer in a box under vacuum - instructions.
• Ripple tank required practical - Instructions
• Use slowed down video to find frequency and wavelength, note on the practical sheet and calculate speed.
• Waves on a string required practical - instructions
• 6 mark question - self mark (full 13 mark version here)
• Plenary
• Evaluation of practicals

Amplitude, frequency, wavelength, wave speed

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Demo 1 - Buzzer in a vacuum

Safety

• equipment required ; Vacuum pump (very heavy!) and buzzer in box

Aim

• To show that sound cannot travel through a vacuum

Top tips

• Make sure the lid of the buzzer box is properly seated so it makes an airtight seal.
• Clamp the rubber tube between the vacuum pump and the box before the vacuum pump is switched off.
• Ensure the buzzer is not touching the case of the box as vibrations can pass through it and the buzzer will still be heard under vacuum.

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Method

• Switch on buzzer - annoying, loud, high-pitched sound will be heard.
• Put the lid on the buzzer box and connect the hose to the air intake of the vacuum pump. Buzzer should still be heard at this point.
• Switch on vacuum pump and remove all the air from the buzzer box.
• Clamp the rubber hose between the box and the vacuum pump.
• Switch off the vacuum pump -the box should be silent. Point out that the red LED is still on, so we know the buzzer is still on.
• Discuss why the buzzer can’t be heard.
• Slowly let the air back into the box by loosening the clamp - buzzer should get gradually louder.

Amplitude, frequency, wavelength, wave speed

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Demo 2 - Waves Required Practical 1 - Ripple tank

Safety

• Equipment required: Ripple tank and accessories. Lamp, power supply for vibration generator., 30cm ruler, 1 sheet A3 white paper.
• Take care with water and electrical devices!

Aim

• Required practical. To measure wavelength and frequency of waves in ripple tank and use this to calculate wave speed.
• Students must be able to explain how to calculate the speed of a wave using this method.

Top tips

• Set this up and test it in advance of the lesson - It is quite fiddly to get the dipper at the correct speed.
• Turn off the lights in the room so make the waves easier to see.
• The slower you have the dipper speed, the easier it is to count the waves.
• You can use the video on slide 17 to collect data for the calculations.

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Method

• Adjust the dipper so it just touches the water surface and produces clear waves.
• Waves should be viewed on the white paper below the tank, not by looking through the water.
• Use meter ruler and count how many waves there are in a given distance (50cm?) then divide by the number of waves to find wavelength. Best done from a photo or freeze the Video.
• To find the frequency count how many waves pass a point in 10 seconds. Divide the number of counted waves by 10 to find the frequency of the waves. This impossible in real time so watch the video for 10 seconds on 0.25 speed.
• Use your results and the wave equation to find the speed.

Amplitude, frequency, wavelength, wave speed

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Demo 3 - Waves Required Practical 2, waves on a string

Equipment

• Vibration generator, power supply, string, 100g mass on hook, wooden bridge, pulley, red string (at least 1.2 metres)

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Aim

• Required practical. Students must be able to explain how to calculate the speed of a wave using this method.

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Top tips

• Set this up and test it in advance of the lesson.
• If you’ve never done this practical before watch this video (start at 7 min 17 sec) or speak to Phil.
• Details of the practical are also on page 181 of the physics textbook.
• Set up the equipment so the point where the string is attached to the vibration generator and the wooden bridge are 1 metre apart.

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Method

• Set up equipment as per the diagram on slide 20
• Hang a 100g mass from the string hanging over the pulley.
• Switch on the vibration generator and adjust the frequency until a single node (bit not moving) appears steady. Should be about 32Hz. This is a single wave with a wavelength of 1m.
• Use the wave equation and the frequency from the vibration generator to calculate the speed of the wave.
• Repeat this for 2,3,4,and 0 nodes.
• Photos and data are on the next slide.

Amplitude, frequency, wavelength, wave speed

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Demo 3 -cont.

A Node is the bit of the string not moving.

Top picture has 1 complete wave and 1 node.

The second picture is 1 and a half waves, 2 nodes.

Amplitude, frequency, wavelength, wave speed

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Lessons 3 and 4 - Waves Required Practical

Answer in your book:

• Give the wave speed equation.
• Define wavelength
• Draw a wave and label the following parts: wavelength, amplitude, crest, trough

Amplitude, frequency, wavelength, wave speed

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Check

• Give the wave speed equation. Wave speed = frequency X wavelength
• Define wavelength The distance from one wave peak to the next, measured in metres.
• Draw a wave and label the following parts: wavelength, amplitude, crest, trough

Amplitude, frequency, wavelength, wave speed

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Lessons 3 and 4 - Waves Required Practical

Lesson objectives

• To explain how sound waves are produced.
• To investigate the suitability of apparatus to measure the frequency, wavelength and speed of waves in a ripple tank and waves on a string.

Amplitude, frequency, wavelength, wave speed

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Sound waves

Describe how this wave would look different if the sound was louder.

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Amplitude, frequency, wavelength, wave speed

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How are sound waves produced?

Amplitude, frequency, wavelength, wave speed

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Sound waves

The vibrating cone in a loudspeaker produces a longitudinal pressure wave in the air, this is a sound wave. If the frequency of the vibration is changed, the pitch of the sound will also change.

What will happen to the pitch if the frequency is increased?

It will increase. The higher the frequency, the higher the pitch.

Amplitude, frequency, wavelength, wave speed

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Demo - Bell in a vacuum

Can you explain what is happening?

Amplitude, frequency, wavelength, wave speed

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Bell in a vacuum demo

Sound waves are mechanical so they need a medium (particles) to travel. This means they cannot travel through a vacuum.

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When the air is removed from the jar the sound can’t travel from the buzzer to your ear, so you can’t hear it.

Amplitude, frequency, wavelength, wave speed

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Waves Required Practical

We will use a ripple tank to measure the frequency, wavelength and speed of waves in water.

Read the instructions on the required practical sheet

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Amplitude, frequency, wavelength, wave speed

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Adjustments to the video in the next slide.

Slow the video down to quarter speed.

In the video settings (cog on the right hand side of the screen at the bottom) select “playback speed” and set to 0.25.

You can then allow the class to easily count the number of waves passing a point in 5 seconds (use the video time to measure 5 seconds)

Pause the video to measure the wavelength.

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Amplitude, frequency, wavelength, wave speed

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Let’s make things a bit easier

When I pause the video measure the wavelength of the waves.

When I slow down the video count how many waves pass a point on the ruler in 5 seconds (Use the video timer to measure 5 seconds) and divide this by 5 to get the frequency.

Write your results on the required practical sheet and calculate the speed of the waves.

Amplitude, frequency, wavelength, wave speed

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Waves Required Practical

Waves required practical - waves on a string

We are going to calculate the speed of the waves on a piece of string.

How could we calculate this?

Amplitude, frequency, wavelength, wave speed

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Waves Required Practical

Exam question

Exam technique: List the equipment used and how each of the three quantities can be determined.

Explain how a student could make appropriate measurements and use them to determine the wavelength, frequency and speed the waves.

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(6 marks)

Amplitude, frequency, wavelength, wave speed

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Mark scheme

Amplitude, frequency, wavelength, wave speed

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Mark scheme

Amplitude, frequency, wavelength, wave speed

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Evaluation

Which piece of equipment was most suitable for measuring the wavelength, frequency and speed of the waves?

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Why do you think this?

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Amplitude, frequency, wavelength, wave speed

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Evaluation

• What were the possible sources of errors in our investigations? What type of errors were they?

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• How could we improve the experiments to minimise these errors?

Amplitude, frequency, wavelength, wave speed

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Check your notes

Amplitude, frequency, wavelength, wave speed

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Logos / symbols

Amplitude, frequency, wavelength, wave speed

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