RMS Speed
Properties of Matter

Molecular speeds

for 14-16

Students can get a feeling for the way gas molecules move and collide by looking at the speed at which a coloured gas diffuses into air and then how quickly it moves into empty space. This can then be linked to the speed of sound and to the effect of different gas densities.

Up next

Measuring the speed of sound using echoes

RMS Speed
Properties of Matter

Measuring the speed of sound using echoes

Practical Activity for 14-16

Class practical

Echoes are used outdoors to estimate the speed of sound. Good weather has to be ordered at the same time as the equipment!

Apparatus and Materials

  • Stopwatch
  • Large reflecting surface, preferably outdoors

Health & Safety and Technical Notes

Read our standard health & safety guidance


Obviously, you want strong echoes from one reflecting surface and not several!

Procedure

  1. The experimenter stands as far away as possible from a large reflecting wall and claps their hands rapidly at a regular rate.
  2. This rate is adjusted until each clap just coincides with the return of an echo of its predecessor, or until clap and echo are heard as equally spaced.
  3. Use a stopwatch to find the time between claps, t. Make a rough measurement of distance to the wall, s. Thus the speed of sound, v = 2 s/t in the first case.

Teaching Notes

  • Students are far more likely to grasp and to remember how to get the estimated speed of sound if you can arrange for them to undertake this experiment in pairs.
  • Newton used echoes to estimate the speed of sound, in an outdoor corridor at Trinity College, Cambridge. It is alleged that the sound he produced was able to lift a door knocker at the far end of the corridor.
  • Discuss why a rough measurement of the distance is adequate.

This experiment was safety-tested in July 2006

Up next

Factors affecting the speed of sound

RMS Speed
Properties of Matter

Factors affecting the speed of sound

Practical Activity for 14-16

Demonstration

This compares the speed of sound at different pressures and in different gases.

Apparatus and Materials

  • Tube at least 1 m long and about 8 cm diameter
  • Demonstration oscilloscope
  • Miniature loudspeaker or earpiece
  • Microphone
  • Amplifier
  • Vacuum pump
  • Supplies of carbon dioxide and natural gas
  • Cotton wool
  • Balloon

Health & Safety and Technical Notes

Vacuum pumps and gas cylinders are heavy: the Manual Handling Regulations must be complied with.

A methane-air mixture, 5 litres in volume, presents a possible fire hazard. Reduce the risk by ensuring that there are no possible sources of ignition in the vicinity.

Read our standard health & safety guidance


This experiment uses the same method as:

Measuring the speed of sound 3


The output from the calibrated time-base of the oscilloscope is fed via the amplifier through the bung in one end of the tube to the earphone or loudspeaker. At the other end of the tube there should be leads running through the bung to the microphone, so it can be connected to the oscilloscope. One of the bungs should also be pierced by a glass tube, which is connected by pressure tubing to the vacuum pump.

Place a little cotton wool in the tube to dampen out any standing waves and make the display clearer.

Do not admit carbon dioxide directly from a cylinder to the tube: this may blow out one of the bungs. Either fill a balloon first, or blow the gas through the open tube.

Procedure

  1. Set the Y-gain of the oscilloscope to about 0.1 V/cm and the time-base to about 1 ms/cm. The pulse received by the microphone should be displayed on the screen.
  2. Pump some air out of the tube. The signal should not have moved across the screen, but it should have a lower amplitude. This shows that the speed of sound is independent of pressure.
  3. After pumping out as much air as possible, admit natural gas and it will be found that the speed of sound is greater.
  4. After re-evacuating, admit some carbon dioxide and the speed will show a marked decrease.

Teaching Notes

  • You can ask students whether the speed of sound would change at different altitudes in the atmosphere. This experiment shows that it would not as long as it is only the pressure which changes.
  • In step 2, there will be a drop in temperature if the vacuum pump rapidly reduces the amount of air in the tube. This will creep back up to room temperature with a corresponding movement of the signal on the screen.
  • Up a mountain at 1500 m, the pressure will be about 25% less than at sea level. Mountaineers who estimate the speed of sound by hand clapping, find the speed of sound to be nearly the same as at sea level. There will be a small reduction due to the lower temperature.
  • The reason for this is that as long as the temperature does not change, molecules will have the same average speeds regardless of their density. At lower pressures they are merely further apart.

This experiment was safety-tested in July 2006

Up next

Measuring the speed of sound 2

RMS Speed
Properties of Matter

Measuring the speed of sound 2

Practical Activity for 14-16

Demonstration

An oscilloscope is used to time a pulse of sound travelling from a speaker to a microphone.

Apparatus and Materials

  • Oscilloscope


  • Microphone
  • Ticker-timer
  • L.T. supply for the timer
  • Cell, 1.5 V
  • Loudspeaker, small (about 4 ohms)
  • Resistor (about 2 ohms)
  • Drawing pin
  • Scrap of kitchen foil

Health & Safety and Technical Notes

Read our standard health & safety guidance


The ticker-timer acts as a pulse generator: making 50 pulses per second if the timer is polarized.

The spike on its blade hits the drawing pin with a scrap of kitchen foil underneath, making brief contact that completes a circuit to give a pulse through the loudspeaker.

The resistor inserted in the loudspeaker circuit provides a small potential difference to be taken to the oscilloscope. The potential difference triggers the time-base as the speaker emits a pulse of sound.

If that pulse fails to appear on the screen, try reducing the value of the resistor.

The microphone receives pulses of sound from the loudspeaker and shows them on the oscilloscope trace.

The CLEAPSS Lab Handbook (Section 12.14} has several pages of guidance for teachers and technicians about using oscilloscopes.

Procedure

  1. Set the gain of the Y-amplifier of the oscilloscope to 0.1 volts/cm and the time-base to about 0.5 milliseconds/cm.
  2. Start with the microphone about 0.25 metres from the loudspeaker and move it steadily away. Students should see the microphones pulse moving away from the loudspeakers pulse and growing smaller.
  3. Measure the distance that the pulse moves when the microphone is moved, say, 0.5 metre. Find the time corresponding to that from the time-base calibration. From this, calculate the speed of sound.

Teaching Notes

  • Concentrate on the pulse picked up by the microphone, rather than looking for the time between the emitted pulse and that received. This avoids any uncertainty in the position of the emitted pulse.
  • The accuracy of the result obtained for the speed of sound will depend on the accuracy of the calibration of the oscilloscope. If the calibration is doubted, check by connecting the output of the 1000 Hz oscillator to the Y-input.

Up next

Measuring the speed of sound 3

RMS Speed
Properties of Matter

Measuring the speed of sound 3

Practical Activity for 14-16

Demonstration

An oscilloscope is used to find the time of travel of a pulse of sound generated by its time-base.

Apparatus and Materials

  • Demonstration oscilloscope
  • Microphone
  • Amplifier and loudspeaker

Health & Safety and Technical Notes

Read our standard health & safety guidance


This demonstration is only possible if the oscilloscope has an output terminal from its calibrated time-base. This is referred to as the sweep output, and may be on the back.

As the time-base sends the spot on the screen back to the left at the start of its trace, a pulse comes from the sweep output and the amplifier generates an audio signal which is sent to the speaker.

Procedure

  1. Connect the sweep output and the earth terminals of the oscilloscope to the input of the amplifier. Set the volume control of the amplifier very low, and connect its output to the loudspeaker.
  2. Start with the time-base set to 100 ms/cm.
  3. Connect the microphone to the input and earth connections of the oscilloscope and set the Y-gain to 0.1 volts/cm.
  4. Position the microphone and loudspeaker so they face one another with an initial separation of about 0.1 metres.
  5. At 100 ms/cm the signal received by the microphone should just be seen. At 10 ms/cm it should be clearer, and better still at 1 ms/cm.
  6. As you move the microphone away from the loudspeaker, its signal will be seen to move across the screen. This is because of the time taken for the signal from the loudspeaker to travel through the air to the microphone.
  7. Record the distance (d) moved by the signal on the screen when the microphone is moved 1 metre. Using the corresponding time, the speed of sound can be found.

Teaching Notes

  • The value of starting at 100 ms/cm is that students can see that the click from the loudspeaker corresponds to the start of the sweep.
  • Since the sweep output is feeding the Y-input, the picture is automatically synchronized. However, the input signals can influence the starting of the trace and could produce erratic pictures. Set the time-base to run independently of any input signal.
  • The accuracy of the result obtained for the speed of sound will depend on the accuracy of the calibration of the oscilloscope.

This experiment was safety-tested in July 2006

Up next

Different densities of gases

RMS Speed
Properties of Matter

Different densities of gases

Practical Activity for 14-16

Demonstration

This shows that carbon dioxide and hydrogen have very different densities to air.

Apparatus and Materials

  • Carbon dioxide
  • Hydrogen
  • Light source, compact
  • Chemical balance
  • Balloons, 3
  • Candle
  • Beakers, 500 ml, 3

Health & Safety and Technical Notes

Be aware that compact light sources using tungsten-halogen bulbs without filters are significant sources of UV. Ensure that no-one can look directly at the bulb.

If gas cylinders are used, care must be taken with handling to comply with the Manual Handling Regulations. See CLEAPSS Laboratory Handbook section 9.9. Staff also need instruction in the use of regulators and (for hydrogen} the needle valve.

Read our standard health & safety guidance


You will need a small but intense source of light, to cast a shadow on the wall of the laboratory or on a screen.

If the balloons are being used, they need filling with air, carbon dioxide and hydrogen so their volumes are about the same. They must not be left too long before being used with the balance, or significant diffusion will take place.

If you are filling balloons with an aspirator, see the experiment:

Filling balloons


Procedure

  1. Arrange the lamp so that light falls on a wall or screen several metres away. Place a beaker in the beam and about half a metre from the lamp.
  2. Pour carbon dioxide down into the beaker.
  3. Show hydrogen moving upwards into an inverted container.
  4. If the gas comes from a cylinder, position a rubber tube from that cylinder so as to release the gas horizontally into the light beam.
  5. Show carbon dioxide moving down a cardboard gutter into a beaker containing a lit candle.
  6. Place, in turn, balloons filled with air, carbon dioxide, and hydrogen, on a balance to show the difference in density quantitatively.

Teaching Notes

  • As the light passes through the different gases, differences in refraction cause shadow effects on the wall or screen.
  • Think about weighing a balloon of air and then letting out the air to weigh just the balloon. The balloon when air-filled weighs slightly more. This is because the walls of the balloon squeeze the air, making it slightly more dense than the air outside the balloon.
  • If intermediate or advanced level students are familiar with the kinetic theory equation pV = 1/3 Nmv 2 , and see that Nm / V is density, you could give them values for the densities of different gases at atmospheric pressure. They could then calculate average molecular speeds.
  • Carbon dioxide, is denser than hydrogen and has a smaller molecular speed. Hydrogen has a higher molecular speed than carbon dioxide at the same temperature and pressure.
  • N = the number of molecules
  • m = mass of a molecule
  • v 2 = average of the molecular speeds, squared
  • V = volume
  • p = pressure

This experiment was safety-tested in July 2006

Up next

Diffusion through a porous pot

RMS Speed
Properties of Matter

Diffusion through a porous pot

Practical Activity for 14-16

Demonstration

These are demonstrations of gaseous diffusion through porous pots.

Apparatus and Materials

  • Porous pots, 2
  • Beakers, 2, large, one containing hydrogen, the other carbon dioxide
  • Rubber bungs to fit the pots, each fitted with glass tubes and connected to manometers
  • Coloured water for the manometers
  • Stands, clamps and bosses

Health & Safety and Technical Notes

If the gases are obtained from cylinders, the heavy cylinders must be handled safely. See CLEAPSS Laboratory Handbook section 9.9 and staff must be instructed in the correct use of regulators and for hydrogen the needle valve.

If the gases are generated chemically, see the relevant Hazcards.

Read our standard health & safety guidance


For the porous pot to be used with hydrogen, the glass tube could be part of the manometer.

For the pot to be used with carbon dioxide, a rubber tube needs to connect to a manometer.

Procedure

  1. Put coloured water into each manometer, and insert the bungs into their porous pot. Allow enough time for diffusion to cause the air pressures inside and outside the pots to be more or less equal.
  2. Invert the beaker of hydrogen, hold it in a clamp, remove its cover, and raise the porous pot into the jar.
  3. When the pressure difference is a maximum, remove the beaker and show the process reversing.
  4. Remove the cover of the beaker of carbon dioxide and lower the other porous pot into it.

Teaching Notes

  • Students - and teachers! - always enjoy these demonstrations since they can seem counter-intuitive. However, they may need careful explanations for less able students.
  • In step 2, the manometer will show the pressure in the pot increasing as the lighter, faster hydrogen molecules diffuse in faster than the air diffuses out.
  • The subsequent reduction in pressure inside the pot in step 4, is caused by the heavier, slower carbon dioxide molecules diffusing into the pot more slowly than the air diffuses out.

This experiment was safety-tested in July 2006

ONLINE COMMUNITY FORUM

Have a Physics Teaching Question?

Want to ask it in a safe, friendly, knowledgeable environment? TalkPhysics is an online community for anyone involved in the teaching of pre-19 physics.

Visit TalkPhysics