Projectile Motion
Forces and Motion

Momentum in two dimensions

for 14-16

Momentum, a vector quantity, is conserved in interactions such as collisions and explosions. Often, to understand such processes, it is necessary to resolve the momentum of objects in two (or more) dimensions. These experiments show students how non-linear interactions can be analyzed.

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Two dimensional momentum interchanges with pucks

Collisions
Forces and Motion

Two dimensional momentum interchanges with pucks

Practical Activity for 14-16

Demonstration

Multiflash photographs of momentum interchanges with pucks.

Apparatus and Materials

  • Co2 pucks kit
  • CO2 cylinder (syphon type)


  • Dry ice attachment
  • Camera
  • Motor-driven stroboscope
  • Gantry for CO2 pucks kit
  • Bright lamps, 2

Health & Safety and Technical Notes

When using CO2 and dry ice it is essential to have good ventilation to the room.

Remember to wear heat-insulating gloves when handling dry ice.

Avoid flash frequencies between 15-20 Hz and avoid red flickering light which makes some people feel unwell. Rarely, some people can experience photosensitive epilepsy.

Read our standard health & safety guidance


In all of the following experiments, the magnetic pucks should be pushed gently to avoid their being chipped and thereby damaged.

The glass plate must be very carefully cleaned with ethanol and carefully levelled using the wedges supplied with it. The CO2 pucks are themselves the best ‘spirit level’ when adjusting the wedges.

About two or three cm 3 of solid CO2 are placed underneath the puck, which will float on a layer of gas as the solid CO2 evaporates.

To avoid troublesome condensation of water vapour on the glass plate, it is advisable to work with the laboratory windows open.

The camera is attached to the supporting frame set up over the glass plate. The motor-driven stroboscope is put in front of the camera.

Alternatively, a camera gantry can be improvised using laboratory stools and a trolley runway.

Two bright lamps (up to 500 W) are used to illuminate the pucks at a glancing angle from the floor. They should not cast a reflection from the glass plate through the strobe disc into the camera.

An alternative way of illuminating the pucks is to place a small flashing light on the top of the puck. The home-made flashing light used to take the photograph has a 'lop-sided' mark space ratio so that the time for which the light is off can be marked accurately. The time intervals for the light off period can be calculated from the electronics or measured using a xenon strobe. For most measurements it is enough to know that the time intervals are constant before and after the collision and so the number of light offs is proportional to the time taken.

A metre rule needs to be visible in the photograph if measurements are to be taken, however, the distances moved are proportional to the length of the light streaks and so actual measurements are not really necessary.

The pucks can be either:

  • ring magnets, of the kind used in TV sets, with a white lid fixed to them and solid carbon dioxide placed in the hole underneath them so that the magnet glides on a cushion of carbon dioxide. The motion is practically frictionless. Too much humidity (breathing students!) causes condensation on the cold plate, which then freezes the puck to the glass plate.
  • motorized pucks which pump an air cushion underneath themselves so that the puck rides on the air.

Procedure

  1. Obtain a multiflash photograph of a single magnetic puck moving freely across the plate. The photograph should show constant velocity.
  2. Photograph a magnetic ring making a head-on collision with another ring of the same mass, originally at rest.
  3. Repeat step 2 with the second puck twice the mass of the moving one. This is realized by putting one of the brass rings on top of the magnetic ring as all the rings provided have the same mass. A number of variants of these head-on collisions can be tried by varying the masses of both the pucks.
  4. Photograph some collisions that are not head-on, so the colliding puck and the stationary puck afterward move off in different directions. Try to show a collision between two equal masses (one stationary) so that after the collision the pucks move off at a 90 ° angle.
  5. Photograph collisions between two pucks which are already moving. This is more difficult but rewarding.

Teaching Notes

  • In studying linear collisions with the CO2 pucks, it is helpful to put magnetic strips across the glass plate, as illustrated, in order to confine the motion to one dimension. The repulsion between the strip and the pucks keeps the pucks’ motion linear.
  • Demonstrations 4 and 5 involve velocities and therefore momenta in two dimensions. Momentum is a vector quantity which must be added by the parallelogram rule. The diagram shows the result of pucks colliding and moving off after the collision at an angle. These collisions may also show a 90-degree angle between the tracks after collision.
  • Multiflash photographs can be analyzed by projecting a photograph onto a screen and separating the screen from the projector so that an actual puck just covers the image of the puck on the screen. In this way scaling factors are avoided.
  • Knowing the time intervals and the distances travelled by a puck (centre to centre), the velocities can be calculated, the momentum of the pucks calculated and the angles between the colliding pucks measured. Taking account of the vector nature of the movement, then the momentum before collision can be compared with the momentum after collision.
  • A scale drawing can also be used to analyze the motion before and after the collision. This will eliminate difficult mathematics. Use a photo of a puck colliding with another puck of the same mass producing a right angle collision (see below).
  • Place an acetate sheet over the photograph. Draw lines through the centres of the pucks and mark the centres of the pucks on the lines. This can then be scanned and projected on a whiteboard.
  • Line A is the incoming puck which strikes a stationary puck that bounces off along line C, while the incoming puck travels on along line B. If you are lucky enough to get a right angle, then linear momentum is conserved. If it is not a right angle and the masses are really equal, then some momentum will have been carried off by the pucks gaining angular momentum.
  • Resolving along the two sides of the right angle for convenience and simplicity, then only track A has to be resolved along lines D and E. The distances travelled in equal time intervals along lines B and D are equal, representing equal momenta before and after the collision. The analysis is similar along lines C and E.
  • The other photos (see below) can be analyzed in a similar way but there is no convenience of a right angle for resolving the momenta. In this case resolve along and perpendicular to the incoming puck. Remember that the masses are different when calculating the momentum before and after collision.

Up next

Momentum interchanges with other objects

Collisions
Forces and Motion

Momentum interchanges with other objects

Practical Activity for 14-16

Demonstration

Collision between long pendulums.

Apparatus and Materials

  • Steel ball with hook, 2.5 cm diameter
  • Steel balls with hooks, 5 cm diameter, 2
  • 1 steel ball with hook, 1.25 cm diameter
  • Plasticine

Health & Safety and Technical Notes

If multi flash photography is used, avoid flash frequencies between 15-20 Hz and avoid red flickering lights, which makes some people feel unwell. Rarely, some people experience photosensitive epilepsy.

Read our standard health & safety guidance


For this experiment to be effective, it is necessary to suspend the steel balls from a rigid support in the ceiling so that they form really long pendulums.

Multiflash photographs of these events could be taken, but the experiments are better shown directly. Photographs can be taken by illuminating the spheres strongly and positioning the camera and strobe disc in a direction at right angles to the direction of collision.

Procedure

Demonstrate collisions in this order:

  1. Head-on collisions between two equal masses (large balls).
  2. Head-on collisions between the medium size and large mass.
  3. Oblique collision between a moving mass and an equal mass at rest (large balls). Try and show that the angle between the paths after collision is 90 degrees.
  4. Collisions between a very small ball and a large heavy one.

Teaching Notes

  • These collisions are more obviously friction-free than the collisions of trolleys and pucks.
  • In step 3, make the link between the colliding steel balls of equal mass and the collision between alpha particles and helium nuclei in a cloud chamber.
  • In step 4, the small ball colliding with a large ball shows the small ball bouncing off the large one at almost the same velocity as its incoming velocity and little recoil from the large one. When the large ball strikes the small one then the small one bounces off the large one at a speed greater (up to twice the speed) than the large ball was moving towards it. The large one continues its motion with little change in velocity.
  • The collisions described are almost perfectly elastic. Inelastic collisions can be produced by putting wax or Plasticine on the ball at rest in such a way that the balls stick together on collision. Although such inelastic collisions are very important this particular demonstration is not one that students understand clearly and easily.
  • You could invoke the principle of Galilean relativity, though students may just take it for granted. Newton's laws of motion and the events that they describe are independent of uniform motion of the observer or apparatus. The same laws of motion will be observed in a steadily moving train as in a laboratory test.
  • Using the concepts learnt when the large and small pendula collided try this story, a thought experiment: An elephant on roller skates.
  • "Consider a collision between a table tennis ball and an elephant on roller skates. Throw the ball at the stationary elephant’s head at 5 m/s. The elastic ball will bounce back at almost 5 m/s and the elephant will recoil backwards very slowly, barely noticeably. Now hang the ball at rest by an imaginary thread in mid-air and let the elephant rush towards it at 5 m/s. When the elephant hits the table tennis ball what motion will the ball take?"
  • "It seems quite difficult to answer this until we try the following trick. Imagine the elephant surrounded by fog so that the rider on his shoulders has no idea that he is moving along the road. Likewise the elephant is moving smoothly on his roller skates and the rider knows nothing at all of his motion. Then in the fog the rider sees a table tennis ball ahead. What will the rider think that the ball is doing? He will think that the ball is moving towards him at 5 m/s. He still does not know that the elephant is sliding along through the fog. Seeing the ball rushing towards him at 5 m/s to hit the elephant’s head, he knows what it will do. It will bounce away at 5 m/s from the head of the elephant."
  • "Now instead, suppose there is no fog and an observer standing on the ground watches what is happening. The outside observer sees the ball bounce away from the elephant’s head at 5 m/s relative to the elephant but he also sees that the elephant is moving at 5 m/s so the ball moves at 10 m/s relative to the ground."
  • This result has applications in sport and, for example, the kinetic theory of gases. Whenever a massive bat hits a stationary ball of much smaller mass, making an elastic collision, the ball moves away at double the speed of the bat. When gas molecules hit a stationary piston head-on they rebound, on average, with an equal speed in the opposite direction. However when they are hit by a moving piston that is approaching them they rebound with a gain in speed which is twice that of the moving piston. Bicycle pumps get hot.

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Collisions between balls

Collisions
Forces and Motion

Collisions between balls

Practical Activity for 14-16

Demonstration

Further collisions similar to snooker balls colliding.

Apparatus and Materials

  • Flexible curtain-rail
  • Steel balls (or glass marbles), 6
  • Newton's cradle

Health & Safety and Technical Notes

Read our standard health & safety guidance


Support the curtain-rail by gluing a 70 cm wooden lath to each end of the underside of it. One end is conveniently held with a retort stand and clamp.

Set up the plastic curtain-rail with its main section horizontal but one end inclined slightly.

To make a Newton's cradle, hang each steel ball by a bifilar suspension. Each ball must have two small hooks soldered to it and the suspensions must be adjusted so that the balls all hang in line at the same level. If any ball is displaced laterally from the rest, or is too high or too low, the demonstration fails. The apparatus is a delight to play with, but much play is apt to upset the adjustment.

Procedure

  1. Let one of the balls roll down the slope to hit another ball on the horizontal section and observe what happens.
  2. Repeat with several balls on the horizontal section.
  3. As an alternative demonstration, use a line of steel balls as in Newton’s cradles. Use a commercially-produced version if at all possible, as home-made versions are notoriously tricky to adjust successfully.
  4. Students might try a simple form of this with a line of pennies, but that does not deserve much time. If they have some form of runway for common marbles, they can try this experiment on their own with marbles.

Teaching Notes

  • The rolling ball might upset the simple collision story because of its angular momentum.
  • Behind the simple story of the momentum of the impinging ball travelling through a line of balls and sending the front ball of the line forward there is a complicated story of the propagation of a compression wave through a ball. The complete transfer of momentum from one ball to the next is even more surprising when one thinks of it in terms of compression waves.

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Two-dimensional collisions with ball bearings

Collisions
Forces and Motion

Two-dimensional collisions with ball bearings

Practical Activity for 14-16

Demonstration

Another interesting study of momentum conservation.

Apparatus and Materials

  • Collisions in two dimensions kit, with steel balls and marbles of 1.5 cm diameter
  • G-clamps
  • Sheet of carbon paper (or use coloured paper with the balls covered in chalk dust)
  • Large sheet of white drawing-paper
  • Marbles, 2 identical
  • Marbles of different mass, ball bearing (OPTIONAL)

Health & Safety and Technical Notes

Read our standard health & safety guidance


Set up the ramp at the edge of a bench, fixed with a G-clamp.

Procedure

  1. Place a large sheet of white drawing-paper on the floor and cover it with carbon paper (carbon side downwards). The point of impact will be marked on the white paper by the carbon paper.
  2. Release a marble so that it is projected horizontally off the edge of the table onto the floor.
  3. Find the point on the drawing-paper vertically below the point of projection with a plumb line or a falling marble, and mark it on the paper. The distance between this point and the point of impact is proportional to the horizontal velocity of the ball on projection.
  4. Balance a second marble on the support, which is attached to the bottom of the ramp using a small piece of Plasticine. Arrange it so that the ball is exactly opposite the end of the ramp.
  5. Release a marble so that a collision occurs at the bottom of the ramp and both balls hit the paper. Investigate whether momentum is conserved in the collision.
  6. Offset the support to show oblique impacts use two spheres of the same mass.

Teaching Notes

  • The velocities are estimated by measuring the horizontal distance travelled by each ball in the time of vertical fall. It is essential to measure from the projection point found in step 3. As a measure of reliability, repeat each reading a few times.
  • Note that, at the moment of collision, the difference in position of the two balls equals the sum of their radii.
  • The projected marble takes the same time to reach the floor as a marble falling vertically.
  • The horizontal motion of the projected marble is not affected by its vertical motion and so the horizontal component of its velocity is constant. The horizontal distance the projected particle moves is proportional to the time for which it is falling and the horizontal component of its velocity.
  • In step 5, the first marble collides with the one at the end of the ramp. The first marble stops and just topples off the ramp to fall vertically. The one which is originally stationary is projected outwards.
  • If the collision is elastic then it should be projected outwards as far as the marble in step 1. Momentum is conserved in the collision. The masses are the same and so the horizontal speed of the first marble before the collision is the same as the horizontal speed of the second one after collision.
  • You can also try collisions between marbles of different masses, or a marble and a steel ball bearing.
  • Both marbles are projected outwards. Allow for the diameter of the spheres. The positions where they both reach the floor are joined to the position just below the starting point for the projection. A line representing the marble falling down the ramp can be drawn from step 3. A scale drawing will show momentum is conserved.
  • Compare the paths of the oblique collisions with cloud chamber photographs.

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Collisions with coins

Collisions
Forces and Motion

Collisions with coins

Practical Activity for 14-16

Class practical

A simple experiment to illustrate the conservation of momentum.

Apparatus and Materials

  • Paper or thin cardboard
  • Coins, 2 identical

Health & Safety and Technical Notes

Read our standard health & safety guidance


Make a launching ramp from an exercise book cover or some other thin sheet of cardboard. Prop it up against some books on the table to make a curving ramp, of smaller and smaller slope until it becomes horizontal when it reaches the table.

Procedure

  1. Choose a standard starting-place at the top of the ramp and allow a coin to slide down the ramp and out along the level table. It will travel some distance, decelerating before it is brought to rest by friction. Note the horizontal distance it travels along the table before coming to a stop.
  2. Repeat the experiment with another coin placed at the bottom of the ramp. There is a collision and the two coins move along the table until friction brings them to a stop. Again measure the distance travelled.

Teaching Notes

  • Students could do this experiment at home.
  • Measurements of distance travelled are used to indicate the velocities just before and after collision. However the velocities are not directly proportional to those distances. You could pose the relationship between velocity and distance as a problem.
  • If the coins have rotational momentum, a simple calculation of linear momentum will not illustrate momentum conservation.

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Collisions with electrostatic forces

Collisions
Forces and Motion

Collisions with electrostatic forces

Practical Activity for 14-16

Demonstration

An illustration of a collision with invisible forces.

Apparatus and Materials

Health & Safety and Technical Notes

Do not add extra capacitance to the Van de Graaff generator.

Van de Graaff generator safety


Read our standard health & safety guidance


Coat the two ping-pong balls with Aquadag to make their surfaces conduct. Using the fine nylon thread, suspend them one diameter apart, preferably from the ceiling but in any case on threads as long as possible.

Procedure

  1. Switch on the Van de Graaff generator and charge each of the balls by allowing them successively to touch its sphere.
  2. Study collision processes between the charged balls by drawing back one of the threads and allowing the balls to swing together. The 90° angle between paths after collision may be seen.
  3. You could also demonstrate collisions of the charged ping-pong ball with the Van de Graaff sphere.

Teaching Notes

Step 3 provides a useful introduction on the scattering of alpha particles in gold foil.

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Expansion cloud chamber

Ionising Radiation
Quantum and Nuclear | Forces and Motion

Expansion cloud chamber

Practical Activity for 14-16

Demonstration

This is a useful demonstration to introduce students to what they can expect to see in the diffusion cloud chamber, and possibly to see forked paths resulting from collisions.

Apparatus and Materials

  • Expansion cloud chamber
  • Source of alpha radiation (if it is not part of the cloud chamber)
  • EHT power supply (NOT an HT one)
  • Large forceps or pliers, if required
  • Bicycle pump or other device for producing expansion
  • Illuminant (lamp, lens and power supply)

Health & Safety and Technical Notes

See guidance note:

Radioactive sources (UK guidelines)


The radioactive sources supplied with some expansion cloud chambers screw into the base of the chamber and are moderately strong. Handle them with large forceps or pliers.

Read our standard health & safety guidance


Various types of expansion cloud-chamber are available commercially. They differ greatly in effectiveness and clarity for small groups of students so it's advisable to try before buying. Much depends on the illumination. Some require alcohol, but those which work with water are preferable.

The above diagrams show a Princeton University design used (i) for a few students viewing directly; or (ii) for projecting a large image by an overhead projector.

The above diagram shows another design.

All types need an electric field to sweep away ions left by earlier events. In each case the manufacturer's instructions should be followed carefully.

Expansion cloud chambers are relatively expensive so schools are unlikely to have more than one.

By contrast, the Taylor diffusion cloud chamber is inexpensive and can be used for class experiments. Also, it maintains a constant state of supersaturation:

Taylor diffusion cloud chamber


Procedure

  1. Expansion cloud chambers are all slightly different in their operation, and you will need to refer to the manufacturer's instructions. The position of the lamp is often critical.
  2. Some have an evacuation mechanism, such as a bicycle pump, which removes the air whilst others change the pressure of a water column.

Teaching Notes

  • The tracks produced in cloud chambers are always fascinating and a forked track even more exciting. If the cloud chamber is filled with helium gas then alpha particles will produce a 90° forked track. Alpha particles in a chamber filled with air will be deflected at more than 90° (oxygen and nitrogen have almost the same atomic mass, about four times that of alpha particles).
  • Display collections of cloud-chamber photographs, keeping them on view for some time. Include examples of fork-tracks resulting from collisions and in particular an example of a 90° fork of an alpha-particle collision with a helium nucleus.
  • The first cloud chamber was invented in 1895 at the Cavendish Laboratory by the Scottish physicist CTR Wilson. He was trying to imitate the way that clouds formed as wet air cooled on expansion. Initially, he used it to examine clouds and condensation rather than to track ionising radiation. He invented it to save him having to climb up to the observatory on the top of Ben Nevis. It wasn’t until 1911 that he developed the cloud chamber as a measuring device and photographed the paths of alpha and beta particles. For this he was awarded the Nobel Prize in 1927.
    • The Wilson cloud chamber works by expanding a fixed volume of wet air. The air cools as it expands, forming a supersaturated vapour. The vapour will condense into droplets if it is provided with condensation nuclei, such as ionised air molecules. In this way, the cloud chamber produces a visible trail of droplets left behind by, for example, an ionising alpha particle.
    • The air will soon warm up again. So the results are fleeting. For this reason, the Wilson cloud chamber is sometimes known as the pulsed cloud chamber.
    • Cloud chambers and even their successors, bubble chambers, are now no more than historic curiosities. Analysis of cloud chamber photographs assumes that momentum is conserved in nuclear events. A uniform magnetic field applied to a cloud chamber perpendicular to the direction of motion of the charged particles produces tracks which are bent into a circle. From this, the momentum of the particle can be calculated and hence its velocity if its mass is known.
    • The kinetic energy before and after collision can then be calculated and if it is not conserved either it was a rare inelastic collision or the target nucleus was wrongly identified (due to impurities or the liquid used to make the drops). If it was an inelastic collision then a nuclear transformation has been effected by the alpha particle.

This experiment was safety-tested in April 2006

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Applications of the conservation of momentum

Conservation of Momentum
Forces and Motion

Applications of the conservation of momentum

Practical Activity for 14-16

Demonstration

Apparatus and Materials

  • Water rocket
  • CO2 capsule rocket
  • Spare CO2 capsules will be required.

Health & Safety and Technical Notes

See the manufacturer’s instructions.

With CO2 capsule rockets: Wear eye protection. Make sure the capsule is firmly fixed to the toy truck. Do not stand in front of the truck.

Read our standard health & safety guidance


Various kinds of water rocket are obtainable from toy shops. Follow the manufacturer’s instructions.

Also available from toyshops are CO2 capsule rockets. A simpler arrangement is to attach a CO2 capsule (as used for soda syphons) to the top of a toy truck. The capsule should be horizontal with its neck facing the rear of the truck. Sellotape is satisfactory for fixing but it may be better to attach an aluminium tube and then fix the capsule inside it.

The capsule can be broken with a round nail and a sharp blow from a hammer. The truck will then move at high speed across the floor.

Other examples include:

  • toy cars driven by inflated balloons
  • a sausage-shaped balloon taped to a straw which is then threaded onto a horizontally fixed thread. When the balloon is released it travels along the thread.

Teaching Notes

  • How do the rocket motors push the rocket to make it accelerate? The hot gases in space rockets, or the gas or water in toys, simply travels out of the back of the rocket. Any molecules that happen to be travelling backwards go from the tail and any molecules travelling forward hit the front wall of the rocket motor inside the rocket, bounce off it and are then travelling backwards and will then probably escape. All molecules which escape carry away momentum and the rocket gains an equal amount of forward momentum. The molecules which bombard the front wall propel the rocket.
  • The rocket plus its fuel do not form a closed system because it is attached to the Earth by the gravitational field. You can take account of that by subtracting the weight of the rocket from the upward thrust produced by the motors.

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Van de Graaff generator - the basics

Voltage
Electricity and Magnetism

Van de Graaff generator - the basics

Teaching Guidance for 11-14 14-16

Principle of operation

Some insulating materials when separated from the surface of others, leave those surfaces electrically charged, each with the opposite sign of charge and with a high potential difference (p.d.).

A machine to make charges was invented in 1929 by a young American called Van de Graaff. Huge machines, some over 30 m high, based on his ideas have been built to produce extremely high potential differences.

Belts and rollers

A flexible belt made from an insulating material and running continuously over two rollers can, by the same process, produce a supply of charge where the surfaces separate. The two rollers have to have different surfaces (often acrylic and metal) and together with the belt-rubber, are chosen by experiment.

Combs

Charges are “sprayed” on to and removed from the moving belt by “combs” situated adjacent to the rollers. Actual contact between the combs and the belt is not essential because of the high potential differences. Combs can be simply a stretched wire, or a sharp or serrated edge: action depends on very high potential gradients due to their small radii (similar action to lightning conductors).

The lower comb is maintained at or close to earth potential and is a drain for negative charge, leaving the belt with positive charges that are carried up to the top comb.

Collecting sphere

The top comb is connected to a collecting sphere which, having inherent electrical capacity (proportional to its radius) will collect and store the charge on its outer surface until discharged either by breakdown of the surrounding air as a spark, or by conduction to an adjacent earthed object.

Charging current

So long as the belt continues to move, the process continues, the drive (motor or manual) supplying the power to overcome the electrical repulsion between the charges collected on the sphere and those arriving on the belt.

The charging current is usually a few mA and potential difference achieved by “junior generators” will be 100-150 kV and by “senior” generators up to about 300 kV.

The whole apparatus

The mechanical arrangement of the belt/roller system is very simple. The lower roller is driven either manually or by motor. The former usually involves a hand wheel and pulley with belt-drive; this pulley can be mounted directly on the motor spindle. In “junior” models, fixed speed, shaded-pole induction motors are usual; “senior” models often incorporate small H.P. variable-speed (sewing-machine) motors, with carbon brushes, control being by either a simple rotary rheostat or a solid state circuit. The motors, control switches and mains input socket are housed in a metal or plastic enclosure, although some junior models have used a transparent plastic cake-cover.

The support column for the collecting sphere can be a simple PVC plastic rod or acrylic tube or a pair of acrylic strips with separators. In some models the belt is enclosed within a plastic pipe with “windows” along its length. Not all generators have means of adjusting the separation of the upper and lower rollers i.e. the belts have to be tailored for a particular machine.

Since the diameter of the collecting sphere determines the maximum p.d. (voltage) achievable, large spheres are mounted on taller columns to be more remote from the earth motor and control box.

Machines are usually supplied with a “discharger", often another, smaller, sphere mounted on a metal rod that has to be earthed to draw sparks from the collecting sphere.

Demonstrations and accessories

Certainly the Van de Graaff generator can produce striking demonstrations. The usual experiments are:

Faraday’s cylinder to show electric charge resides on the outer surface of a charged hollow conductor.

Bouncing ball. Suspend a conducting ball a non-conducting thread. When the ball touches the charging sphere, it will become charged and be repelled away from the sphere. If the ball is then allowed to discharge (touching an earthed surface, or leaking charge to the air) it will be attracted once more to the sphere, to be recharged ... and so the process continues.

The head of hair is another demonstration of repulsion. Real hair or shredded paper strips bunched at one end are used and provide a sensitive means of detecting charge.

The electric wind is produced by release of ions at the end of a pointed conductor and is enough to deflect a candle’s flame.

Hamilton’s mill utilizes the electric wind at the pointed ends of four arms to cause rotation about a pivot. This is similar to the action of a lightning conductor, which allows charge transfer at sharp points.

Kinetic theory model You can show random motion of metallic balls continuously affected by repulsion and loss of charge within a transparent vessel.

Neon indicator shows luminous discharge from the gaseous excitation by the high electric fields near the generator.

An apparatus note on the Van de Graaff generator gives information about good housekeeping and repairs:

Van de Graaff generator


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Multiflash photography

Conservation of Energy
Energy and Thermal Physics | Forces and Motion

Multiflash photography

Teaching Guidance for 14-16

Multiflash photography creates successive images at regular time intervals on a single frame.

Method 1: Using a digital camera in multiflash mode

You can transfer the image produced direct to a computer.

Method 2: Using a video camera

Play back the video frame by frame and place a transparent acetate sheet over the TV screen to record object positions.

Method 3: Using a camera and motor-driven disc stroboscope

You need a camera that will focus on images for objects as near as 1 metre away. The camera will need a B setting, which holds the shutter open, for continuous exposure. Use a large aperture setting, such as f3.5. Digital cameras provide an immediate image for analysis. With some cameras it may be necessary to cover the photocell to keep the shutter open.

Set up the stroboscope in front of the camera so that slits in the disc allow light from the object to reach the camera lens at regular intervals as the disc rotates.

Lens to disc distance could be as little as 1 cm. The slotted disc should be motor-driven, using a synchronous motor, so that the time intervals between exposures are constant.

You can vary the frequency of ‘exposure’ by covering unwanted slits with black tape. Do this symmetrically. For example, a disc with 2 slits open running at 300 rpm gives 10 exposures per second.

The narrower the slit, the sharper but dimmer the image. Strongly illuminating the objects, or using a light source as the moving object, allows a narrower slit to be used.

Illuminate the object as brightly as possible, but the matt black background as little as possible. A slide projector is a good light source for this purpose.

Method 4: Using a xenon stroboscope

This provides sharper pictures than with a disc stroboscope, provided that you have a good blackout. General guidance is as for Method 3. Direct the light from the stroboscope along the pathway of the object.

In multiflash photography, avoid flash frequencies in the range 15-20 Hz, and avoid red flickering light. Some people can feel unwell as a result of the flicker. Rarely, some people have photosensitive epilepsy.

General hints for success

You need to arrange partial blackout. See guidance note

Classroom management in semi-darkness


Use a white or silver object, such as a large, highly polished steel ball or a golf ball, against a dark background. Alternatively, use a moving source of light such as a lamp fixed to a cell, with suitable electrical connections. In this case, place cushioning on the floor to prevent breakage.

Use the viewfinder to check that the object is in focus throughout its motion, and that a sufficient range of its motion is within the camera’s field of view.

Place a measured grid in the background to allow measurement. A black card with strips of white insulating tape at, say, 10 cm spacing provides strong contrast and allows the illuminated moving object to stand out.

As an alternative to the grid, you can use a metre rule. Its scale will not usually be visible on the final image, but you can project a photograph onto a screen. Move the projector until the metre rule in the image is the same size as a metre rule held alongside the screen. You can then make measurements directly from the screen.

Use a tripod and/or a system of clamps and stands to hold the equipment. Make sure that any system is as rigid and stable as possible.

Teamwork matters, especially in Method 3. One person could control the camera, another the stroboscope system as necessary, and a third the object to be photographed.

  • Switch on lamp and darken room.
  • Check camera focus, f 3.5, B setting.
  • Check field of view to ensure that whole experiment will be recorded.
  • Line up stroboscope.
  • Count down 3-2-1-0. Open shutter just before experiment starts and close it as experiment ends.

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How clouds form

Ionising Radiation
Quantum and Nuclear

How clouds form

Teaching Guidance for 14-16

Clouds form in the atmosphere when warm, wet air is pushed up. As the air goes higher its pressure decreases and it gets colder. The colder air cannot contain so much water vapour and the air becomes supersaturated with vapour. Given the right conditions, some of the vapour condenses to form water droplets in a cloud.

The droplets need something to form around. An extremely small droplet of just a few molecules cannot form a cloud all by itself. Its molecules would escape from each other again. If the air contains attractive particles (usually minute particles of salt) that the water can wet, a droplet will start to form. The salt particles serve as condensation nuclei on which larger drops can grow.

A similar process is put to use in a cloud chamber containing alcohol vapour. Ions in the air can serve as excellent condensation nuclei. The alcohol molecules are electrically ‘oblong’ with positive and negative charges at the ends, so they can cluster easily around a charged particle. The other small particles can be cleared out. This stops droplets forming into unwanted clouds (even when the air is supersaturated). Instead, the droplets form on the trail of ions left behind by ionising radiation, typically an alpha particle.

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Alpha particle tracks

Ionising Radiation
Quantum and Nuclear

Alpha particle tracks

Teaching Guidance for 14-16

Nuclear bullets from radioactive atoms make the tracks in a cloud chamber. They hurtle through the air, wet with alcohol vapour, detaching an electron from atom after atom, leaving a trail of ions in their path. Tiny drops of alcohol can easily form on these ions to mark the trail.

The trail of ions is made up of some ‘air molecules’ that have lost an electron (leaving them with a positive charge) and some that have picked up the freed electrons, giving them a negative charge.

There is no sighting of the particle which caused the ionisation, because it has left the ‘scene’ before the condensation happens. If you count the number of droplets an alpha particle might produce 100,000 pairs of ions by pulling an electron from 100,000 atoms.

When the alpha particle has lost all its energy in collisions with the ‘air molecules’ it stops moving and is absorbed.

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Evidence for the hollow atom

Ionising Radiation
Quantum and Nuclear | Forces and Motion

Evidence for the hollow atom

Teaching Guidance for 14-16

The main and first evidence for the hollow atom came from...

Rutherford's alpha scattering experiment


However, the first evidence students see for a hollow-atom often comes from cloud-chamber photographs. Although this may be historically back to front, it is reasonable to use the cloud chamber photographs as the first indication that atoms are mainly empty.

Chronology of evidence

Rutherford had devised his model of a nuclear atom by 1910, before alpha particle tracks were photographed in cloud chambers (c1911). However, Rutherford and Wilson worked in the same laboratory so it is likely that Rutherford had seen tracks in cloud chambers.

The evidence provided by cloud chamber photographs and the inferences that can be made are extremely useful whether you present them as preparation for the Rutherford model or follow-up support for it.

Evidence from cloud chambers

Most of the time there is just a straight track produced when an alpha particle passes through the cloud-chamber, producing ions. Mostly, these ions are produced by inelastic collisions with electrons in neutral particles. An alpha particle will have around 100,000 inelastic collisions before it no longer has energy stored kinetically. The number of collisions shows that electrons are easily removed.

The straightness of the tracks shows that:

  • an electron has a mass that is much smaller than the mass of an alpha particle (now known to be about 7000 times smaller).
  • the atom is hollow: each straight track represents about 100,000 collisions without any noticeable deviation. All of these collisions missed anything with significant mass. During a session, the class might observe 1000 tracks between them – all of which are straight.

Therefore, in all of these 100 million collisions with atoms, the alpha particles never hit anything with significant mass. So most of the atom is empty.

However, students will see photographs that show large deflections of alpha particles. These are rare events (requiring thousands of photographs to be taken). They show that:

  • there is something in an atom that has a mass that is similar to the mass of an alpha particle; only a target with a comparable mass could cause a large deviation.
  • this mass is very concentrated; the rareness of the forked tracks shows that most alpha particles miss this massive target.

Evidence from alpha particle scattering

The hollowness of the atom is treated more quantitatively in the Rutherford scattering experiment. In this, 99.99% of the alpha particles are undeflected. This gives an indication of how tightly the positive charge of the nucleus is packed together.

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Managing radioactive materials in schools

Ionising Radiation
Quantum and Nuclear

Managing radioactive materials in schools

Teaching Guidance for 14-16

Countries have national laws to control how radioactive materials are acquired, used and disposed of. These laws follow internationally agreed principles of radiological protection.

The following principles apply to schools:

  • There should be a person designated to be responsible for the security, safety and proper use of radioactive sources.
  • Sealed radioactive sources should be of a safe design and type suitable for school science.
  • Sealed sources should be used whenever possible in preference to unsealed sources. Unsealed sources can only be justified when the scientific demonstrations would not be practicable using sealed sources.
  • Records of all radioactive sources should be properly kept, showing what they are, when they were bought, when and by whom they have been used, and eventually, how they were disposed of.
  • Radioactive sources should be used only when there is an educational benefit.
  • Radioactive sources should be handled in ways that minimize both staff and student exposures.
  • Sealed sources should be carefully checked periodically to make sure they remain in a safe condition.
  • The school should have a suitable radioactivity detector in good working order.

UK regulation & guidance

Generally, school employers will insist you obtain their permission before acquiring new radioactive sources.

You must follow your employer’s safety guidance relating to the use the radioactive sources. Most school employers will require you to use either SSERC or CLEAPSS safety guidance, as follows:

In Scotland, safety guidance for use of radioactive sources in schools is issued by the Scottish Schools Equipment Research Centre (SSERC) and is available to members through their website.

In the rest of the UK and British Isles Crown Dependencies, guidance is available from CLEAPSS, the School Science Service. Their guidance document, L93, is freely available from their website, even to non-members.

In the UK...

  • In classes where children are under the age of 16, the use of radioactive material shall be restricted to demonstrations by qualified science teachers, (which includes newly qualified teachers). However, closer inspection of devices containing low-activity sources such as diffusion cloud chambers is permitted provided the sources are fully enclosed within the devices and not removed during the inspection.
  • Young persons aged 16 and over may use radioactive sources under supervision. Although the use of radioactive material is regulated, it should not be used as an excuse to avoid practical work. As the ASE points out, "Using the small sources designed for school science gives a good opportunity to show the properties of radioactive emissions directly, and to discuss the radiation risks. Just as importantly, it is an opportunity to review pupils' perception of risks, as they are likely to have constructed their own understanding from a variety of sources, including science fiction films and internet sites. If the work is restricted just to simulations, it may reinforce exaggerated perceptions of risk from low-level radiation.”

Summary of legislation (UK)

Updated October 2008

The following summarizes the somewhat complicated legislative framework in which schools are expected to work with radioactive sources in the UK. However, teachers do not need to obtain and study this legislation; this has been done by CLEAPSS and SSERC, and it is incorporated into their guidance in plain English.

In the European Union, member states have implemented the 1996 EU Basic Safety Standards Directive (as amended) that in turn reflects the 1990 International Commission on Radiological Protection recommendations. In the UK, this has been done through the Radioactive Substances Act 1993 (RSA93), which controls the security, acquisition and disposal of radioactive material, and the Ionising Radiations Regulations 1999 (IRR99) which controls the use of radioactive material by employers. Transport of radioactive material is controlled by The Carriage of Dangerous Goods and Use of Transportable Pressure Equipment Regulations 2007.

There are exemptions from parts of the RSA93 and schools can make use of The Radioactive Substances (Schools etc.) Exemption Order 1963, The Radioactive Substances (Prepared Uranium and Thorium Compounds) Exemption Order 1962, and others. These exemption orders are conditional and to make use of them and avoid costly registration with the Environment Agency (or SEPA in Scotland, or the Environment and Heritage Service in Northern Ireland) you must adhere to the conditions. Note that currently, these exemption orders are being reviewed.

The way in which these laws are implemented in England, Wales, Northern Ireland and Scotland varies. The Department for Children, Schools and Families (DCSF) has withdrawn its guidance AM 1/92, and the associated regulations requiring this have been repealed. Consequently, purchase of radioactive sources by maintained schools in England is no longer regulated by the DCSF. The DCSF commissioned CLEAPSS to prepare and issue ‘Managing Ionising Radiations and Radioactive Substances in Schools, etc L93’ (September 2008) and has commended it to schools in England. Similar regulations relating to other educational institutions in the UK have not changed; English institutions for further education remain regulated through the Department for Innovation, Universities and Skills. Similarly, schools in Wales should follow the guidance from the Welsh Assembly Government Department for Children, Education, Lifelong Learning and Skills. Schools in Scotland should follow the guidance from the Scottish Government Education Directorate and associated guidance issued by SSERC. Schools in Northern Ireland should follow the guidance from the Department of Education Northern Ireland (DENI). The Crown Dependencies Jersey, Guernsey and Isle of Man are not part of the UK and schools and colleges should follow the guidance from their own internal government departments responsible for education.

In the UK, if an employer carries out a practice with sources of ionising radiations, including work with radionuclides that exceed specified activities (which is 100 kBq for Co-60, and 10 kBq for Sr-90, Ra-226, Th-232, Am-241 and Pu-239), the practice must be regulated according to the IRR99 and the employer must consult with a Radiation Protection Adviser (RPA). Since 2005, the RPA must hold a certificate of competence recognized by the Health and Safety Executive. Education employers are unlikely to have staff with this qualification, so the RPA will usually be an external consultant. Education employers need to notify the HSE 28 days before first starting work with radioactive sources. This is centralized at the HSE’s East Grinstead office.

Note: For higher risk work with radioactive material, the IRR99 requires designated areas, called controlled areas and supervised areas, to be set up if special procedures are needed to restrict significant exposure – special means more than normal laboratory good practice. It should never be necessary for a school to designate an area as controlled, and only in special circumstances would it be necessary to designate an area as supervised. The normal use of school science radioactive sources, including the use of school science half-life sources, does not need a supervised or controlled area.

Disposal of sources in the UK

Sources that become waste because they are no longer in a safe condition, or are no longer working satisfactorily, or are of a type unsuitable for school science, should be disposed of. In England and Wales, the Environment Agency has produced a guidance document through CLEAPSS that explains the available disposal routes. Similarly, SSERC has produced guidance for schools in Scotland. Schools in Northern Ireland should refer to DENI.

Health and safety statement

See the health and safety notes in each experiment. This is general guidance.

Health and safety in school and college science affects all concerned: teachers and technicians, their employers, students, their parents or guardians, and authors and publishers. These guidelines refer to procedures in the UK. If you are working in another country you may need to make alternative provision.

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