IOP Sound Workshop · 6 Matches and match box 16 sheets of A4 paper Assistants ... 6 6 2 2 1 1 2, 3 3 4 4 5 5 6, 2 2 3 3 4 4 5, 2 2 3 3 4 4 5, 6 6 2 2 1 1 2, 3 3 4 4 5 5 6

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Notes by Sue McGrath www.science2life.com 2012 Page 1

Demonstration

Apparatus Packed

Straw oboe

Straws Scissors Paper Cellotape Hole punchers

Sound wagon Sound wagon Vinyl LP

Thunder tube

Thunder tube Cups Cable ties Talking ribbons

Talking Cups Polystyrene, paper or plastic cups String

How sound travels

Air zooka Smoke machine Smoke fluid Tray 6 polystyrene cups

Rijke (singing) Tube

Rijke tube Bunsen flame Retort stand Safety glove

Gramophone Gramophone kit 78 vinyl

Good Vibrations Metal hangers string

Moaning Myrtle demo

Balloons Hex nuts Marker pen Balloon pump

Tuning fork splash Large tuning fork Bowl of water.

Force on a current carrying wire

Tin foil strip Croc clips Connecting wires Wooden DVD frame Tape Large permanent magnet Frequency generator

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Brring, Brring, Brring – it’s a telephone made from string! Nuts & Bolts

• 2 plastic cups or empty yoghurt pots • some string • 2 paper clips • sharpened pencil or object for poking holes. • Thunder tube • Plastic cups • Plastic cable ties

Secrets for Success

Hold the string and gently move your thumb along its length. What do you hear?

With the pencil, carefully poke a small hole in the bottom of each container. Tie the paper clip to one end of the string. Thread the other end through the hole moving from the inside of the cup outwards. The paperclip will keep the string from going all the way through the hole. Hold the container in one hand and gently drag the thumb nail of your other hand along the string moving from the top downwards. What do you hear this time? A much louder sound.

Thread the string through the hole in the second container, this time from the outside to the inside. Tie the second paper clip to the end of the string. The paperclips should both be on the inside of the containers.

Hand one of the containers to your assistant. Move apart so that the string between you is taut. Ask your assistant to put their container to one of their ears while you talk into your container. Make sure that the distance between you is sufficient for you assistant not to be able to hear you without the aid of the ‘telephone’.

• What is the farthest distance you can get the telephones to work using string? • Does using thin metal wire affect this distance? • Does increasing the size of the cup affect the volume of the sounds you hear?

When you investigate make sure that you only change one thing (that’s the variable) at a time. Make a prediction – what do you think will happen as a result of your change? Test it out. Was your prediction correct?

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Science in a Nutshell

When you drag your nail across the string, the rough surface causes your nail and string to vibrate. The container, with its large surface area, amplifies the sound. Large containers will give you louder sounds.

Thunder tubes are readily available in market places or you can get them from science providers such as SciChem or Tim Starr

Just like the string, when the metal spring is plucked separately the sound emitted is very quiet. By attaching it to a resonating cylinder with a drum head the sound is very much amplified. When the tube is shaken with the spring hanging down - whoa! - a surprisingly realistic (and incredibly loud) sound of thunder results. Use your free hand to cover the tube's open end and you can produce a scary "wah-wah" sound. Lightly tapping the spring on a hard surface produces a sound similar to that of a jaw harp. Experiment and come up with your own sounds.

A cheaper version can be made using plastic cable ties and recycled plastic cups or pots.

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Straw Oboe Nuts & Bolts

Straws Scissors 6 Matches and match box 16 sheets of A4 paper Assistants Secrets for Success

With your fingers flatten one end of the straw. Use the scissors to cut the flattened end so it looks like this shape \_/ and the angled lengths should be about 1½ cm in length. Ask your assistants to place the cut ends into their mouths and blow. The whole of the cut section must be fully inside the mouth. If you are lucky enough one or all of them will get a sound. However, if now sounds are forth coming check that the straw is far enough in the mouth, and or ask them to tighten their lips and gently squeeze the straw as they blow – not too much though as air is required to move through the straw!

This demo will produce lots of laughter from everyone – the flattened \_/ shaped tip acts like the reed found in most wind instruments. Blowing on the reed causes the straw to vibrate. The vibrations are felt by the lips of the blower and they usually stop blowing first off because the experience is very weird. The buzzing sound is a bit like a musical duck call!

Once you have an assistant that can give you a long note, get them to hold the straw close to their mouth. With the scissors try cutting the end off of the straw, notice what happens to the change in the sound of the note. Keep cutting until the straw is a third of its original length – let your assistant know what you are going to do as they will get fearful of their fingers!!

Repeat the process again but this time using the 6 matches melt 6 holes in the straw about 1 ½ cm apart, starting 1 ½ up from the un-cut end. You can now play your straw oboe!

Let’s try Twinkle Twinkle Little Star: the number 1 represents the top hole covered, 2 the top two, etc until 6 which means all holes are covered.

6 6 2 2 1 1 2, 3 3 4 4 5 5 6, 2 2 3 3 4 4 5, 2 2 3 3 4 4 5, 6 6 2 2 1 1 2, 3 3 4 4 5 5 6

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This is a lovely little demonstration which can be used to help children understand topics such as materials, sound, waves, energy transfer and the relationship between wavelength, frequency and pitch.

When the flattened end of the straw is cut into a V shape and placed into the mouth, the ‘\_/’ shaped reed vibrates when you blow gently this is because when you blow, a pulse of compressed air flows down the straw. The pulse travels down the straw at mach 1, the speed of sound, and bounces off the distant open end. At this point the compressed air changes into a low pressure expansion. When the expanded air reaches the two flattened edges of the straw they are forced closed then bounce open to admit more air. Thus the sound bounces back and forth inside the straw and the flattened edges (reed) open and close (vibrate) creating a sound.

Once you are able to produce a clear sound, cut off successive pieces of the open end of the straw. You will hear the pitch of the note increase. This increase in pitch is due to the wavelength (the symbol used by scientists for wavelength is �) of the standing wave set up in the straw changing – since the wave velocity, v, remains the same the frequency, f, has to change.

v = f� decreasing � (i.e. making straw shorter) results in f increasing (we hear the pitch of the note increase).

The loudness of the note is increased if you blow harder. In school both the loudness and pitch can be displayed on an oscilloscope with the help of a microphone. The loudness of the note can also be increased by attaching a paper cone (use poser paper if possible) to the end of the straw. Try a cone firstly made from one sheet of A4 paper then 4 and lastly 16 sheets.

Students can see the amplitude of the sound wave on the screen of the oscilloscope increase in height as the loudness of the produced note is increased. When the length of the straw is changed the change in pitch is registered on the oscilloscope by the distance between successive wave crests moving closer together for increasing pitch or further apart for lower notes.

By adjusting the tension in your lips and the pressure with which you blow different harmonics can also be played. The straw is an open ended tube – so when you blow and hear a note a standing wave has been set up in the straw. The fundamental wave will have a wavelength = 2L where L is the length of the straw. The wavelength of the second harmonic is equal to L and so on. Illustrations of displacement and pressure patterns in open pipes can be found in many A Level text books.

Give your many assistants a set of straws and ask them to answer the following questions:

How can you change the pitch of the oboe? (change the length)

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Why do you have a different pitch when the length varies? (link to standing wave diagrams in text book and they should see that the wavelength is different for different straw lengths – since v remains constant, f must change)

Where is the least air pressure in the straw oboe? (At the wave antinodes where the air molecules are spread far apart the pressure is least – conversely, the air molecules are all bundled together where the wave nodes are.)

Going Further

Cut a 50 cm length of sticky tape and place the centre of 25 straws onto the tape at 2 cm intervals. Stick a second 50 cm length of tape over the top of the straws to cover the sticky part of the lower tape. Attach a paper clip to each end of every straw. Hang the straw strip from a desk, pull it taunt, and give one of the straws at the top or bottom a tap to start a transverse wave. Increasing tension will increase the speed of the wave, and increasing the density (by adding more paperclips) will decrease the speed of the wave. Although sound is a longitudinal wave, best represented by compression waves in slinky springs, the straw waves can be used to illustrate many of the properties of sound waves.

Dominoes (or Jenga pieces) can be used to illustrate how sound is transmitted to our ears without the transfer of mass. Arrange as many dominoes as possible in a row – ask the class to describe the disturbance as it passes through the dominoes. Does it travel at a constant speed? If you have enough pieces organise your assistants to work in teams. Get them to investigate what happens to the speed if the dominoes are closer together or further apart. Then link this to the increase in the speed of sound as the density of the material through which it travels increases.

How does sound bounce off an open end of a tube? This can be demonstrated firstly with the aid of a bottle brush and a tube. Push the brush inside the tube (could be made from a piece of rolled up card) – when the brush exits the tube its’ bristles spring outward this movement demonstrates what the compressed air in a sound wave does. But air is more elastic than bristles. A large tube (PVC) and a lead weight which is one end tied to a long spring of rubber bands can be used to illustrate the compression overshoot. Without letting go of the rubber band (!) drop the weight down the tube. When the lead weight comes out of the end of the tube, it stretches the rubber bands and hence overshoots its equilibrium position, just as air does. The air compression overshoots just like the lead weight and pulls a partial vacuum at the end of the tube which goes back down the tube as an expansion, and that is how sound bounces back and forth inside a tube.

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Good Vibrations Nuts & Bolts

• Metal spoon • Metal coat hanger • Metal slinky spring • Wooden spoon • 1 ½ metres of string

Secrets for Success

Tie the metal spoon to the centre of the string. Wrap one end of the string, about 5 times, around the index finger of your left hand and the other around the index finger of your right hand. Put the tips of both fingers into your ears. At this stage you may feel a bit silly if others are in the room watching you but the experience will be worth it and all will want to try!

Now bend forward allowing the spoon to strike against different surfaces – you will hear loud, alarming, chiming sounds! Repeat using other objects such as a coat hanger, slinky spring or wooden spoon.

Do different objects produce different sounds?


http://video.about.com/deafness/Cochlear-Implants.htm here is a nice video on how the ear works

The object attached to the string will vibrate when it is hit. The sound waves are transmitted to the ear drums via the string and your fingers. Solid materials are much more effective in transmitting sound energy than the air which is a gas.

When objects vibrate sound is produced. Sound vibrations can travel through solids, liquids, or gases: this vibration is called a sound wave. On a molecular level, as molecules bump against each other, they transfer the sound wave from one molecule to the next. Thus sound travels from the source to the receiver.

The speeds that sound travels depend on the material in which it is travelling and the molecular packing of that material. Molecules are packed most densely in solids and least densely in gases. The closer the molecules are together, the better sound travels.

In this activity, when the string and fingers were plugged into the ears, the sound travelled better than just through the string and air. In addition, the bones in your head act as resonators and also vibrate to make the sound fuller. Tightly woven string works better than loosely woven yarn, especially if the length is over 45 cm.

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Singing Rod Nuts and Bolts

• Singing rod – aluminium rod • Roisin • Polystyrene cup

Secrets for Success

Go to this link to see the rod and other musical objects being played by Steve Spangler

Crush a pea sized amount of rosin on a piece of paper until it is ground into power.

Cover the tips of your thumb, index and middle finger of your non-dominate hand with rosin as if you were going to use the rosin to make a finger print.

With your other hand grasp the metal rod with your thumb and index finger exactly in the middle of the rod.

Stroke one half of the rod with your rosin covered fingers. After four or five strokes a standing wave will build up as the other end of the rod begins to resonate.

Place the polystyrene cup at the end of the rod, the sound is not only amplified but this demonstration also shows that sound is a longitudinal wave. If the cup is placed along the length of the rod there is no amplification – the wave is not transverse.


Every object has a natural frequency. If you drop a set of car keys it makes a recognizable sound, as does dropping a pin. The sound that it makes is the natural frequency of the keys and the pin. You can get the rim of the glass to vibrate at the natural frequency of the glass by rubbing the rim with your finger. Your fingertips cause the glass to vibrate at its natural frequency. If this happens over and over again a standing wave builds up and you can get a sound with a lot of volume.

If you cause an object to vibrate at its natural frequency by having another object near vibrating at the same frequency you can set up a standing wave in the first object. This process is called resonance. This is what is happening with the singing rod. Since you are holding the rod at its middle point, the natural frequency of both ends of the rod is the same. As you set up a standing wave with your hand on one

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end of the rod, there is a standing wave set up due to resonance on the other end of the rod. Be careful, you can get the rod to sing loud enough to be painful to some people’s ears.

Reinforcement Activities Search the internet for small video clips of the Tacoma Narrows Bridge Collapse (here is one) or Galloping Gurdie. Both of these names reference a bridge that collapsed due to resonance. The wind blew across the bridge at the same frequency as the natural frequency of the bridge. The standing wave produced was a couple of meters high and eventually the bridge shook so violently it collapsed. It is a great video for getting your students thinking and talking about standing waves and resonance.

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Water Music Nuts & Bolts

• Six concave wine glasses, same size • Measuring jug - small • Water • Food colouring or paint • Metal spoon • Egg cup filled with vinegar

Secrets for Success

Pour water into the glasses so that you have six different levels. Add food colouring or paint to each glass so that you can easily see the water levels – try to get red, orange, yellow, green, blue and purple.

Tap each glass with the spoon to create different sounds. The glass with the most water will produce a note much lower than the one with the least water in it; the pitch of the note increases as the water volume decreases.

Does it make any difference if you hit the glass with a metal, plastic or wooden spoon?

Does the surface the glasses stand on affect the sounds heard?

Can you adjust the water levels in the glasses so that you can play a tune you know?

Once you have mastered this make a note of the volumes of water used to make the set up quicker in your next session.

Colour Volume of water (ml) R Red O Orange Y Yellow G Green B Blue P Purple

Using your spoon hit the glasses in the following sequence; can you guess the tune?

R R B B P P B

G G Y Y O O R

B B G G Y Y O

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B B G G Y Y O

R R B B P P B

G G Y Y O O R

Going Further

Now wash your hands with soap to clean away any dirt or oils. Choose one of the glasses, hold it steady with one hand, dip a finger in some vinegar, keeping contact with the glass at all times, move your finger at a constant speed around the rim of the glass, vary the speed until you hear a loud, clear note.

Repeat with the other glasses. What happens to the pitch of the note as the volumes of water in the glass increases or decreases?

Find a way to secure the glasses to the table; blu tac, sellotape or build a holder, can you play the above tune or the tune of your choice?

See my attempts here


The vinegar (or methylated spirits) is used to ensure there is no grease on your finger tips. Best effects are achieved if a steady pressure is applied between a clean finger and glass, this is because it allows you to get the stick-slip motion required to produce sound.

Sound is usually considered to be a vibration having a range of frequencies that can be detected by the human ear. This is between 50 Hz (50 vibrations a second) and 20 000 Hz (a staggering 20 000 vibrations a second).

Different materials vibrate differently. For us to hear a sound produced by vibrations then sound waves need to travel to our ear drums. The speed at which sound travels is also dependant on the material it is travelling through. The speed depends on how closely packed the particles in the medium, the sound is travelling through, are.

In gases the particles are far apart, the speed of sound in air is between 300 and 350 m/s. It travels fastest in damp or polluted air. Why do you think this is the case? The particles are closer together – see domino effect in straw demonstration.

In water sound travels between 1200 and 1500 m/s. It travels faster in sea water than in fresh water. Why is this?

The speed of sound in steel is around 5000 m/s, approximately 15 times faster that the speed of sound in air. In old cowboy and Indian movies the actors are seen putting their heads on the railway tracks or the ground. What are they listening for? And why do they use a solid to hear for it?

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Rijke tube A Rijke's tube turns heat into sound, by creating a self-amplifying standing wave. It is an entertaining phenomenon in acoustics and is an excellent example of resonance.

The Rijke tube is a metal pipe, with a wire mesh in the lower half of a vertical metal pipe, note friction with the walls is sufficient to keep the gauze in place. To achieve the sound the tube is suspended over a Bunsen burner.

Secrets for Success

Place the disc of gauze about a quarter of the way up the metal tube. My tube is 80 cm in length and 3.5 cm in diameter.

With the tube vertical and the gauze in the lower half, heat the gauze with a flame until it was glowing red hot.

When you remove the flame, you will hear a loud sound from the tube which lasted until the gauze cooled down (about 10 s).

Lord Rayleigh, who wrote the definitive textbook on sound in 1877, recommends this as a very effective lecture demonstration. He used a cast iron pipe 1.5 m long and 12 cm diameter with two layers of gauze made from iron wire inserted about quarter of the way up the tube. The extra gauze is to retain more heat, which makes the sound longer lasting. He reports in his book that the sound rises to such intensity as to shake the room!


The sound comes from a standing wave whose wavelength is about twice the length of the tube, giving the fundamental frequency. The flow of air past the gauze is a combination of two motions. There is a uniform upwards motion of the air due to a convection current resulting from the gauze heating up the air. Superimposed on this is the motion due to the sound wave. For half the vibration cycle, the air flows into the tube from both ends until the pressure reaches a maximum. During the other half cycle, the flow of air is outwards until the minimum pressure is reached. All air

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flowing past the gauze is heated to the temperature of the gauze and any transfer of heat to the air will increase its pressure according to the gas law.

As the air flows upwards past the gauze most of it will already be hot because it has just come downwards past the gauze during the previous half cycle. However, just before the pressure maximum, a small quantity of cool air comes into contact with the gauze and its pressure is suddenly increased. This increases the pressure maximum, so reinforcing the vibration. During the other half cycle, when the pressure is decreasing, the air above the gauze is forced downwards past the gauze again. Since it is already hot, no pressure change due to the gauze takes place, since there is no transfer of heat. The sound wave is therefore reinforced once every vibration cycle and it quickly builds up to give large amplitudes of vibration, resulting in a loud sound.

This explains why there is no sound when the flame is heating the gauze. All air flowing through the tube is heated by the flame, so when it reaches the gauze, it is already hot and no pressure increase takes place.

When the gauze is in the upper half of the tube, there is no sound. In this case, the cool air brought in from the bottom by the convection current reaches the gauze towards the end of the outward vibration movement. This is immediately before the pressure minimum, so a sudden increase in pressure due to the heat transfer tends to cancel out the sound wave instead of reinforcing it.

The position of the gauze in the tube is not critical as long as it is in the lower half. To work out its best position, there are two things to consider. Most heat will be transferred to the air where the displacement of the wave is a maximum, i.e. at the end of the tube. However, the effect of increasing the pressure is greatest where there is the greatest pressure variation, i.e. in the middle of the tube. Placing the gauze midway between these two positions (one quarter of the way in from the bottom end) is a simple way to come close to the optimal placement.

The Rijke tube is considered to be a standing wave form of thermo-acoustic devices known as "heat engines" or "prime movers".

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How do gramophones work? Gramophones play records by means of a needle, which traces around grooves in a record, and a cone that amplifies the sound of the needle's vibrations. Although the look and sound quality of professionally built gramophones is difficult to replicate at home, you can build your own functional version with materials you have lying around the house. You can finish this project in a few minutes and use it to impress your friends and family. Read more: How to Make Your Own Gramophone | eHow.com http://www.ehow.com/how_8573193_make-gramophone.html#ixzz1vqtGTN9f

Definitions

"Gramophone" is a term that can be used to refer to any record player (as can the term "phonograph"). In general, however, the term is used to refer to early record players that were driven by mechanical means and reproduced sound without electrical amplification. These record players are quite different from modern turntables, but the records that store the sound and the needles that turn it into audible music work in essentially the same way.

The Gramophone Record

Except for the early cylinder records invented by Thomas Edison, all records use a spiral groove to store sound. The earliest records had recordings on only one side, but soon companies started printing double-sided records which could store twice as much sound. The groove in the record looks straight and uniform, but it actually has small back-and-forth eddies which store the sound.

The Gramophone Player

Like other record players, gramophones read the sound with a small needle which fits into the groove in the record. That needle is attached to a diaphragm, which in turn is attached to a horn. The record is turned at a fairly constant speed by a spring-driven motor. As the record turns, the grooves make the needle vibrate back and forth. These vibrations are transmitted to the diaphragm, which itself vibrates, creating sound. That sound is then channeled out of the horn into the room.

Early Records

Early records were made by machines that worked like gramophone players, only in reverse. Musicians would play into the horn, which would channel the sound into the diaphragm. The diaphragm would transfer the sound vibrations into a needle. The needle itself rested on a record topped with a layer of wax. As the needle wiggled, it

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would inscribe the sound into the wax. Finally, the record would be treated to fix the recording so it could be played back.

Moaning Myrtle Spooky Balloon Trick Make a seriously spooky sound. Feel the hairs stand up on your neck.

Nuts and Bolts

• Balloon • Hex nut - this is a metal nut with six sides. • penny • Marker pen

Secrets for Success

Before you blow the balloon up, put the hex nut inside the balloon. Then blow up the balloon with a pump and tie off the end.

With your whole hand firmly holding the top of the balloon, swirl it around and around. The nut should start to spin around inside of the balloon.

Stop spinning and listen.

Repeat with a penny… what doe you notice about the sound? How about different sizes of hex nuts?


The spooky sound is made when the sides of the hex nut scrape quickly around the inside of the balloon. This makes the balloon and hex nut vibrate, and this vibration is sound that travels through the air to your ears. The larger the vibrations the louder the sound.

The hex nut keeps spinning inside the balloon even when you have stopped spinning it, as it wants to keep doing what it is doing - spinning around and around. This is called inertia. The hex nut will keep moving at the same speed and in the same direction until the force of friction slows it down and it falls back to the bottom of the balloon.

Note the change in pitch as the speed of the hex nut changes.

Balloons inflated to different sizes will give different notes for a hex nut spinning at the same speed.

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Loudspeakers à la Scientific Sue This is a lovely classroom activity; I first started doing it whilst I was the Education Programme Leader at W5, Belfast’s world renowned interactive Science Centre. I start this workshop with several interactive activities, which explore magnetic fields and the magnetic effect of a current and explain the production of vibrations and hence sound. This investigation helps teach what can be a conceptually difficult topic in an innovative way, using everyday objects to bring the relevance of science and technology to pupils. Nuts and Bolts Each group needs:

• any personal music player with an earpiece socket (playing rock music!); • sandpaper; • selection of containers (tin cans, plastic cups); • access to a sound intensity meter (optional).

Additionally each pupil needs:

• film canister lid or something similar; • neodymium magnet 10 mm × 8 mm; • 4 m enamel coated resistance wire, e.g. 32 swg; • 1 earpiece jack; • small piece of Blu-tack or double sided tape.

Secrets for Success • Use the Blu-tack to attach the magnet to the centre of the film canister lid. Draw

the Blu-tack up the sides of the magnet. • Wrap the wire around the magnet (ensure that a 20 cm length of wire is left

hanging)—the Blu-tack will keep the wire in place—take note of how many turns are in your coil—leave 20 cm of wire at the end.

• Using the sandpaper remove 1 cm of the enamel coating from the end of each wire. Unscrew the jack. Push the wires through the black plastic section.

• Thread each end through one of the two metal connectors. Twist each wire to make a firm connection.

• Re-screw the jack together. • Plug your jack into the music source. Switch volume to maximum. Hold the lid,

magnet and coil in your hand and feel the vibrations. Now place the canister lid to your ear and enjoy the music!

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Any of the empty containers can be used to amplify the sound by attaching the flat surface of the canister lid to its base. Pupils can experiment with different containers, different number of coils and different standard with gauges (swg) of wire. You can use the sound intensity meter to compare the sounds produced from an array of loudspeakers. Science in a Nutshell A couple of hundred years ago Faraday found out that when electrical charges flow through a wire a magnetic field is induced around that wire. By creating a coil (a solenoid) we can increase the magnetic field strength… the strength of the field is proportional to the number of turns. The coil thus becomes a magnet – an electromagnet. Faraday’s right hand rule allows us to find the magnetic north of this electromagnet. It states that when the right hand is wrapped around the coil with the fingers in the direction of the conventional current (ie in the opposite direction to the flow of electrons), the thumb points in the direction of the magnetic north pole. By changing the direction of the current you make the magnetic poles flip over.

We can now get vibrations by placing our electromagnet close to a permanent magnet. By switching the direction of the current flowing in the coil you change magnetic orientation of the coil’s end facing the permanent magnet. When the poles are the same they push apart, when opposite they pull together.

The music source contains electronics which not only vary the rate at which the poles change in the coil but also the flow of electricity thus allowing for small and large vibrations to be formed.

We hear these variations as music.

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Force on a current-carrying wire An example of teacher resource materials on the IOP education site Search Results All 2 results for force on a current carrying wire

1. TAP 412-3: Force on a current-carrying wire (Score: 1.0000)

Vary the current gradually up to about 5 or 6 A. (This will depend on your power supply.) 2. How does the force

on the wire (or the change in weight of the magnets) depend on the current in the wire? 4. To investigate how the

length of wire in the field affects the force, the single wire can be replaced by wires of different length. 1. The two

graphs should show that the force is proportional both to the current in the wire and to the length of wire in the

field.

http://tap.iop.org/fields/electromagnetism/412/file_46928.doc

Tap 412-1: Forces on currents

These activities allow qualitative investigation of the forces on currents.

You will need: �aluminium foil

�scissors

�large horseshoe magnet

�two clip component holders

�four long 4 mm leads

�power supply, 0–12 V plus rheostat (4 A) (or Westminster Very Low Voltage power supply)

Safety If the current is too large then the foil will get hot and the forces will be too large and tear the foil. When using two strips of foil be aware that if they touch sparking may occur. Care is needed, and the pd must remain modest

You will be making some general observations about forces on currents. As force is a vector, you will be interested to see how large the force is and in what direction it acts. Systematic observations will be essential.

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The force on a current in a magnetic field The current passes through a strip of aluminium foil. The movement of this foil shows the size and direction of the force. The foil is placed in the magnetic field of a large permanent magnet and connected to the power supply.

1. Set the power supply to 1 V, switch on, and observe what happens to the foil.

2. Increase the power supply to 2 V. What happens?

3. Now reverse the connection to the power supply. What happens?

4. Finally, reverse the magnet. What happens to the foil?

What you have seen 1. The size of the force on a current in a magnetic field depends on the size of the current.

2. The direction of the force depends on the direction of the current and the direction of the magnetic field.

3. The direction of the force is at right angles to the current and to the magnetic field. (Fleming’s left-hand rule.)

4. You have not seen that the size of the force also depends on the size of the magnetic field. But you can show that the magnetic field is essential to the force by removing the magnet!

Here is the set –up used by me in the sound session.

I connected the aluminium foil to a frequency generator and used this to test the hearing range of the audience!

The structure is a DVD rack from Ikea.

Finally there is a huge source of material on the IOP site…go and explore!!!

http://www.iop.org/education/index.html

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www.science2life.com

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IOP Sound Workshop · 6 Matches and match box 16 sheets of A4 paper Assistants ... 6 6 2 2 1 1 2, 3 3 4 4 5 5 6, 2 2 3 3 4 4 5, 2 2 3 3 4 4 5, 6 6 2 2 1 1 2, 3 3 4 4 5 5 6