Demonstration

This is an exploration of which factors affect the force of friction when one surface slides across another and which do not.

Apparatus and Materials

• Forcemeter, 30 N, large size for easy class viewing
• Plank wit screw eye 91 m x 15 cm x 1 cm), smooth
• Block with screw eye (25 cm x 12 cm x 1 cm), smooth
• Blocks without screw eye (25 cm x 12 cm x 1 cm), 3
• Rollers, 1 cm diameter and 20 cm long, 10
• Crank assembly
• G-clamp to fix crank assembly to bench

Health & Safety and Technical Notes

Read our standard health & safety guidance

Dimensions are for guidance and are approximate.

The faces of the block must be of equal smoothness.

Procedure

1. Place the plank on the bench and drag the block along it with the forcemeter. It is difficult to obtain a fixed forcemeter reading. Repeat the process a number of times. Estimate the average force that is needed to pull the block. This is equal to the force of friction acting between the surfaces of the block and the plank.
2. Use a crank to pull the plank so that it slides beneath the block. This will let you make a more reliable measurement of the force of friction. Place the plank on rollers. Pull it along at a constant speed using the string and crank.

3. Ask students to predict the effect of dragging the surfaces across each other at different speeds. Show them what happens.
4. Press down against the plank with a finger and again drag the plank along. This increases the frictional force between the surfaces of the plank and block. The forcemeter shows an increased force.
5. Add an equal block on top of the first so that the force pushing the block on to the plank is doubled. This force acts at 90° to the surfaces. Call it the normal force. Measure the frictional force. Increase the load with two, three and four blocks and see how the frictional force increases.
6. Ask students to predict the effect of change in contact area of the surfaces. Turn the block on its side and drag it as before to demonstrate what happens.

Teaching Notes

• In 2, the block is held at rest by the forcemeter while the plank moves under it. The frictional force is balanced by the tension in the forcemeter.
• Students should be able to discuss conclusions about the force of friction and three other variables: speed, area of contact, and the normal force. The force of friction is independent of area of contact (provided that the surfaces are equally smooth) and of speed (at low speeds). It is proportional to the normal force.
• Normal force: Since you have varied this by using the weights of 1, 2, 3 and 4 blocks, a simple quantitative conclusion is possible.
• Ask students if doubling the normal force has the effect of doubling the frictional force, or whether the behaviour is more complex than that. You could go further, asking whether they believe that the frictional force always changes by the same proportion as the normal force, thus bringing in the idea of proportionality of variables.
• A graph of frictional force (y axis) against normal force (x axis) provides a visual presentation that quickly reveals the nature of the relationship. However, measurements in this activity are subject to a fairly high degree of uncertainty, or error. Students' conclusions should acknowledge this uncertainty.
• You can show that force of friction is greater between static surfaces than between moving surfaces. Leave the plank at rest, not on the rollers, and just pull the block. The forcemeter reading increases from zero up to a maximum value before the block slips and the reading falls.
• Try placing sheets of different materials (wood, plastic, paper) between the block and the plank. (With a bit of ingenuity you can also try ball bearings, plastic beads and/or oil, using suitable tracks.)
• How Science Works Extension: This experiment provides a good opportunity to discuss the nature of variables. As described above, the experiment looks at a categoric variable (the nature of the surfaces) and a discrete variable (the normal force, determined by the number of equal blocks). Contact area is also a discrete variable, since it is altered by turning the block on its side.
• Discuss whether these variables could be changed to become continuous variables. Students should suggest that, by adding weights to a single block, any value of normal force can be obtained. This then opens the way to a discussion of how more values of contact area can be obtained. Blocks of different areas could be made and the contact force adjusted to a standard value by adding suitable weights.
• You can then discuss the independent, dependent and control variables in each part of the experiment. See our general guidance on Variables.

This experiment was safety-checked in October 2004

Class practical

An object falling through water reaches a constant terminal velocity after falling relatively small distances. This terminal velocity can be seen and measured.

Apparatus and Materials

For each student group:

• Gas jars
• Chinagraph pencil or water-based pen
• Styrocell beads
• Stopwatch or other timer
• Other objects and materials (e.g. Plasticine pieces, glass beads and wood)

Health & Safety and Technical Notes

Unexpanded beads are now classified as a dangerous substance because of the pressure generated when they expand. Teachers must control access to the unexpanded beads and supervise this activity carefully to see that the beads are not removed for unauthorized experiments.

Read our standard health & safety guidance

Styrocell beads are the raw material for making expanded polystyrene. There should be enough of them to provide each student group with between thirty and fifty.

They contain an expanding agent, which causes them to expand to forty or sixty times their original volume when heated up. You can expand them by boiling in water for five minutes. If you then allow them to fall freely in air, they will approach a terminal velocity in about 30 cm. They behave like slow motion raindrops. The unexpanded beads should be stored in a cool place, preferably in a sealed polythene bag. Expanded beads should be thrown away.

A drop of a wetting agent in the water will help to prevent beads from clinging to the surface of the water in the gas jar.

Harbutt's original Plasticine is no longer manufactured: modern products have different names.

Procedure

1. Fill a gas jar, to near the top, with water.
2. Use a chinagraph pencil or water-based pen to make marks at regular intervals, such as 2 cm, on the outside of the gas jar. Put the top mark close to the water surface, at the height from which you drop the beads.
3. Release a Styrocell bead (unexpanded) on the water surface.
4. Find out whether the bead travels at the same speed all the way down. To do this, see if the beads pass the marks with a steady rhythm. You'll probably need to try this more than once.

Teaching Notes

• The density of the unexpanded Styrocell is close to that of cold water.
• Students should find that the motion quickly approaches an almost constant speed, or terminal velocity. They should discuss why, with gravity continuously acting, the beads do not accelerate all the way down. The answer relates to resistive force, which depends on speed.
• Initially, when the bead is released and has no speed and there is no resistive force:
• Downwards force = weight of bead
• Upwards force = upthrust (flotation force)
• The weight is slightly larger than the upthrust so the forces are not quite balanced, and there is a small net or unbalanced downwards force. After falling for a short distance, when the bead has acquired speed and experiences resistive force:
• Downwards force = weight of bead
• Upwards force = upthrust + resistive force
• Upwards and downwards forces become balanced as soon as resistive force becomes big enough, and then acceleration is zero.
• You can use other objects and materials, such as small ball bearings, glass beads, Plasticine pieces of various sizes and loaded pieces of wood. Denser objects need a greater distance before they reach their terminal velocity. The upthrust they experience is less similar to their weight, so they need larger resistive force before they can reach the balanced force condition.
• How Science Works Extension: This experiment provides opportunities to consider sources of error and how they can be reduced by considering experimental design. Ask your class to suggest how the terminal velocity of a bead can be measured. One approach is to time a bead as it falls between two marked points on the jar. Start a stopclock as the bead passes the first mark, and stop it at the second mark. How far apart should the marks be? The further apart the better, but the upper one must not be in the region where the bead is still accelerating. How should the marks be made? Chinagraph pencil lines have a thickness; it might be better to use coloured adhesive tape and try to start/stop the clock as the bottom of the bead reaches the top edge of the tape.
• Now ask an individual student to make repeated measurements of beads falling between two markers. How consistent are the timings? This is a useful way to assess the random error in the measurements, but there may be a systematic error caused by the student’s reactions. (This could be reduced or eliminated by making measurements over different heights.)
• Now ask another student to repeat the measurements. Is one experimentalist more consistent than another?
• This can lead to a discussion of the desirability of automated measurements (perhaps using light gates), but it is important to stress that these must also be assessed for errors.

This experiment was safety-checked in September 2004

Class practical

Ideas about terminal velocity and about streamlining can be quite difficult. This gives students some practical experience on which to build their understanding.

Apparatus and Materials

• Table-tennis ball
• Sheets of paper (A4 is suitable)
• Parachute, improvised with paper or cloth

Health & Safety and Technical Notes

Students must NOT be allowed to stand on stools or benches in this activity. Serious accidents have happened as a result. Parachutes can often be dropped down stairwells providing pupils are adequately supervised.

Read our standard health & safety guidance

Procedure

1. Hold a sheet of paper horizontally and let it fall to the ground. Watch how it moves.
2. Fold the paper to make a tray, with sides about 2 cm high. Drop the tray and watch it fall. Try it both ways up.
3. Discuss the reasons why the paper falls in different ways.

4. Drop a small ball.
5. Make a parachute from cloth or paper, with threads hanging from its edges.
6. Tape the ball to the parachute. Drop the ball and its parachute. Discuss how the parachute makes a difference to how the ball falls.

Teaching Notes

• The plain sheet of paper flutters down when allowed to fall but the tray falls more steadily. As a tray, the 'right' way up, it not only presents a smaller surface area to the air but also encourages a more even air flow past it. It is more 'streamlined'.
• Forces of resistance in fluids, sometimes called forces of fluid friction, increase with speed. They bring a falling body to a constant speed, the terminal velocity.
• When terminal velocity is achieved then the upwards forces (due to the resistance to motion and due to any flotation effect of the fluid) are equal to the weight of the body. (This is what happens with parachutes and raindrops. More massive bodies need to fall longer distances before they reach terminal velocity.)
• There are in fact at least two different kinds of fluid resistance: the kind associated with streamline motion that depends on the speed of the body, and the kind that leaves a wake of vortex motion and involves resistance that varies with the square of the speed.
• Students can devise their own additional investigations, which could include:
• dropping a tray from different heights
• working with trays carrying different loads, or with holes in the tray, or trays of different cross-sectional area (or even with the paper screwed up as a ball)
• dropping different numbers and hence different weights of paper cups, or different sizes of paper cup (as used for baking cupcakes)
• varying the area of the parachute, the number of holes in it, the number of strings and the shape.

This experiment was safety-checked in October 2004

Demonstration

The higher the viscosity of a liquid the more it resists motion of a body through it. The result can be very low terminal velocity.

Apparatus and Materials

• Measuring cylinder or tall and fairly wide glass tube, 1,000 ml, with firm stopper
• Glycerine, heavy oil or liquid detergent
• Ball bearings (approximately 3 mm and 1.5 mm)
• Chinagraph pencil, water-based pen, or elastic bands
• Eye protection

Health & Safety and Technical Notes

Glycerine (glycerol or propane -1, 2, 3-triol) will irritate eyes, so eye protection should be worn. Its properties will change if it is allowed to absorb water vapour from the atmosphere so it must be kept in a closed container. Waste engine oil is carcinogenic and must not be used.

Read our standard health & safety guidance

A tall glass tube allows a greater distance of fall than a measuring cylinder. Seal the bottom end firmly with a stopper and rest this on a surface so that it cannot fall out. Do not over-tighten any clamp that you use to hold such a tube.

Use the pencil, pen or elastic bands to provide equally spaced markers on the measuring cylinder or glass tube. Do this before the lesson.

Place the ball-bearings in a dish of the same liquid before use. This reduces the occurrence of air bubbles, which will affect the motion of the ball bearings.

Retrieve ball-bearings from the liquid with a magnet outside the jar. This is a messy activity to clear away, especially if many ball bearings are allowed to fall and must then be retrieved.

Procedure

1. Set up the measuring cylinder or tall glass tube, filled with the viscous liquid, so that it is illuminated from above by a bright source. In an otherwise darkened room (full blackout is not necessary) the ball-bearings then appear as bright points of light.
2. Release a ball-bearing from just above the liquid surface.
3. Ask students to clap as the ball-bearing passes each marker. This is sufficient to show that the time intervals become the same, and thus that the ball-bearings quickly reach their terminal velocity.

Teaching Notes

• You could use a more sophisticated timing system, but the point here is to demonstrate terminal velocity rather than to make precise measurements.
• Advanced level students could determine the viscosity of the liquid, using Stokes' law. Or they could investigate the relationship between the radius of a falling ball and its terminal velocity. When a ball bearing is moving at terminal velocity, the forces acting on it are balanced.
• Frictional force acting upwards = weight - upthrust

• where _η_ = viscosity
• α = radius of the ball bearing
• ν_ο = terminal velocity
• γ = gravitational field strength
• ρ = density of the bearing material
• σ = density of the liquid

This experiment was safety-tested in April 2006

Class practical

Science can reveal simple patterns with considerable predictive power. This is of huge practical value. Knowing what affects the turning of a balanced beam leads to understanding such a pattern.

Apparatus and Materials

For each student group

• Beam with regular markings, simple
• Wooden prism block
• Metal loads, square and identical

Health & Safety and Technical Notes

Read our standard health & safety guidance

These items of apparatus are available from educational suppliers as part of a "lever kit".

The square metal loads should have approximately the same diagonal length as the width of the beam.

Procedure

1. Place the wooden prism block underneath the centre of the wooden beam. The block acts as a pivot.
2. Rest a paper clip on the beam and move it closer to or further from the pivot to balance the beam as well as you can. You won't be able to get the beam to balance exactly. When it is very close to being balanced then it tips one way as easily as the other. Fix the paper clip under the beam with a little Sellotape.
3. Place one square load, 1 unit of length along the beam from the pivot, placing the diagonal of the square along the line of the first distance mark on the beam.
4. Balance this, roughly, with one square load on the other side of the pivot. Make a written record of the number of square loads and their distances from the pivot.
5. Add a square load on top of one of the others, to make a pile of two on one side of the pivot. Move the single square load on the other side so that the beam is roughly balanced again. Make a record of the numbers of loads and their distances.
6. Add another load to make a pile of three. Move the single load again, to roughly balance the beam.
7. You will find it useful to put your records into a table. A table is easier to make and use than a long list.
8. Make the pile of square loads four high and five high. Move the single load each time so that the beam is roughly balanced. Record all of your results. There is a pattern in the result. Try to spot it.
9. OPTIONAL: Put two loads at two positions on one side of the pivot and try to balance them with one load on the other. Repeat a few times, at different distances and try to spot the pattern in these results.

Teaching Notes

• The pattern in the results can be described in several ways.
• A student who says words to the effect that, 'doubling the load on one side requires the distance on the other side to be doubled' has spotted the pattern. One who says that, 'the product of load and distance is the same on both sides of the beam when it is balanced' has provided a more general description that can be used to make predictions. In other words, the beam balances when the anti-clockwise moment equals the clockwise moment.
• Different students will require different amounts of support in this. The most able will not only identify a pattern but will see for themselves that they can use it to make predictions of load position in order to achieve balance. Others will not see a pattern at all unless it is directly pointed out to them. It is worth explaining that the pattern is important because of its predictive power, which can be applied in many practical situations.
• Students' application of the predictive power of their new learning can be tested by moving the multiple loads to two marks from the pivot, and asking them to say where the single load must be placed for balance.
• The number of loads here provides a measurement of weight, or force.
• The product of the force and its distance from the pivot is a measure of its turning effect, and is called the --moment-- of the force.
• For balance, the sum of the clockwise moments is the same as the sum of the anticlockwise moments. Large forces on one side of the fulcrum can be balanced by smaller forces on the other, provided that the smaller force is further from the fulcrum.
• To illustrate the turning effect of a force, demonstrate with the classroom door. Try pushing it at the edge, then close to the hinge, then at intermediate positions. Compare the effects. You could try pushing near the hinge while a pupil pushes (from the other side) farther out. If you do this then take care that fingers cannot be trapped if the door closes.
• An interesting variation is to replace the variable load by a weak spring such as an expendable steel spring.
• To do this, place the pivot near the edge of the table. Tie the upper end of the spring to the beam with thread, one space from the pivot. Attach the lower end of the spring to an anchoring mass resting on the ground. The spring should be a little bit stretched. Adjust the tension by altering the length of cord used in securing the spring to the lever or the anchor.
• Again there is a clockwise turning effect, or moment, and an anticlockwise moment, which should be the same for balance to be achieved. You can use this system to measure the force exerted by the stretched spring, in terms of weight of the loads. Weight = mass x gravitational field strength (w=mg).
• Why is a weak spring needed? If the lever is only a short distance above the table, it can only tilt a little and the pull of the string will be practically constant. To ensure such constancy, the stretch of the spring must be large compared with the change when the lever tilts.

This experiment was safety-tested in August 2007

Class practical

This is a fun challenge, ideal as an extension activity or just for some mind-stretching for its own sake.

Apparatus and Materials

For each student group

• Beam with regular markings, simple
• Wooden prism block
• Metal loads, square and identical

Health & Safety and Technical Notes

Read our standard health & safety guidance

These items of apparatus are available from educational suppliers as part of a lever kit.

The square metal loads should have approximately the same diagonal length as the width of the beam.

Procedure

1. Start with the beam balanced with no square loads on it.
2. Take several square loads and arrange them on the balance, at marks on the balance, so that the beam is balanced. Make a sketch of that pattern.
3. Now put all the square loads in a single pile above the pivot.
4. Move two square loads, so that the beam is balanced again. You can put them at the marks on the beam, but not in between marks.
5. The object of the game is to reproduce the pattern that you sketched. You can only move two loads in each move. At the end of each move the beam must be balanced. The person who can do this in the smallest number of moves is the winner.

Teaching Notes

• This game was devised by a mathematical physicist, and can be absurdly simple or extremely hard. It is very suitable as an extension activity.
• It is a good idea to demonstrate the game first to would-be competitors, starting with a very simple pattern, so that the rules about moves are clear.
• Faster students will quickly learn how best to play the game. They shouldn't be allowed to discourage those who take a bit longer.

This experiment was safety-tested in October 2004

Demonstration

Principles of physics are not just abstract. They have practical value.

Apparatus and Materials

• Plank, wooden
• Brick or wood block
• Metre rule
• Masses, 1 kg to be used as 10 newton weights, 16

Health & Safety and Technical Notes

If the weighing device is used to find the weights of students then a steadying rail must be available to avoid falls.

Students should be disciplined to avoid misbehaving if an attempt is made to find the mass of a student or teacher.

Read our standard health & safety guidance

The wooden plank should be 2 to 3 m long, at least 20 cm wide and 2.5 cm or more thick.

Procedure

1. Place the plank on the brick as the pivot or fulcrum.
2. Place the body that is to be measured at a distance of, say, 0.5 m from the brick.
3. Add 10 newton weights (1 kg masses) at the other end until balance is achieved.

Teaching Notes

• Encourage students to design and build a weighing machine to find weights of everyday objects such as parcels or letters.
• You could highlight distinctions between mass and weight here. Weight is a force, a force due to gravity, and is measured in newtons. The weight of a body can vary from place to place, such as on the Earth and on the Moon, and in deep space it is zero. The mass of a body is a measure of the quantity of material, and is measured in kilograms.
• The mass of a body doesn't change unless material is added to it or taken away. The distinction is less important in everyday life provided that this takes place exclusively on the surface of the Earth, where the weight of a body does not change too significantly from place to place. In a more universal context, and in science, the difference is very real.

This experiment was safety-tested in October 2004

Demonstration

Springs tend to return to their original shapes when they are stretched, compressed, or, in this case, turned.

Apparatus and Materials

• Clock-spring, large, mounted on a board at the spring centre
• Retort stand, boss, and clamp
• Loads to be hung from spring arm, e.g. thread

Health & Safety and Technical Notes

Read our standard health & safety guidance

If loads slip along the arm of the spring, use small pieces of modelling wax or Plasticine to hold them steady.

Procedure

1. Clamp the board vertically to a retort stand, for convenience of demonstration.
2. Hang loads from the arm of the spring to apply force to it. The spring tends to return to its original shape, and provides a force which opposes the applied force.
3. Hang different loads at the same place on the arm, to show that weight is a factor in the size of the turning effect due to the load.
4. Hang a load at different positions along the arm, to show that distance is a factor in the size of the turning effect due to the load.

Teaching Notes

• The force due to a spring, in opposition to an applied force, is a restoring force.
• An additional demonstration could show a screw jack (or a rotating chair with a spiral thread), raising a load by pushing on the arm of the screw jack with a very small force. The longer the arm, then the smaller the force.

This experiment was safety-tested in October 2004

Class practical

Surface tension in a liquid is an intrinsically interesting phenomenon. It also provides evidence in the human-sized world of the unseen inter-particle forces that cause it.

Apparatus and Materials

For each student or group of students

• Wire, bendable
• Thread
• Paper clips, small
• Detergent solution
• Chalk
• T-piece or Y-piece and rubber tubing

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

1. Make a loop of wire about 5 to 10 centimetres across. Tie two pieces of thread across it fairly loosely.

2. Dip the loop into detergent solution to make a soap film.
3. Touch the film between the threads with a piece of chalk, to burst the film only between the threads. Watch what happens.
4. Make a square frame of wire with three sides. Tie a thread across the open end. Tie a second thread to the centre of the loose thread. Dip the frame into detergent solution to make a soap film. Hold the frame with the centre thread hanging down. Try pulling gently on the hanging thread, and letting go. Watch how it behaves.

5. Hang the small paper clip from the hanging thread. Make a new film. See if the film will support the weight of the paper clip.

Teaching Notes

• These are 'surface tension' effects, which arise from the forces between the particles of the liquid. Particles of the liquid attract each other. Particles below the surface experience forces in all directions. Particles on a surface do not. Unlike in a solid, particles in a liquid respond to the effects of forces by changing their location in the material. Each particle on the surface of a liquid is also attracted by its neighbours, but its neighbours are not distributed all around it being 'below' it in the liquid. Particles on a liquid surface tend not to stay there for very long.
• One result is that the surface of a liquid becomes as small as circumstances allow. Small quantities of liquid form spherical drops.
• (A flock of penguins make an amusing analogy. On a cold Antarctic night, penguins on the outside of a flock would prefer to be on the inside of the huddle. The result is that the shape of the huddle or flock becomes such as to minimize the size of the perimeter. The shape will change if there is a strong wind from one direction, of course.)
• Soap films, have double surfaces. They also tend to become as small as possible. This produces an effective tension in the surface which pulls on the threads and can even balance the weight of a small paper clip.
• The following are suitable for demonstration.
• Dip two arms of the Y-tube or T-tube into detergent solution. Pinch the rubber tubing of one arm and blow a bubble at the end of the other. Then pinch the tubing of the other arm and blow a second bubble. Allow both lengths of the rubber tubing to be open. Watch what happens. Ask the class to explain it. The tendency of surface tension to reduce bubble size is balanced by the pressure of the air inside.

• Float three matchsticks on water, so that they form a closed triangle. Touch the water between them with the corner of a bar of soap. What happens can be explained in terms of the change in inter-particle forces (and hence in surface tension) that is caused by the presence of soap in the water.
• Pour some molten wax onto a scrap of wood to give it a waterproof surface. Put a drop of water onto the surface. Look at it carefully and then touch it with a matchstick which has been dipped in detergent.

This experiment was safety-checked in October 2004

Demonstration

It is quick and simple to demonstrate that surface tension gives droplets a spherical shape.

Apparatus and Materials

• Microscope slides, 2
• Wetting agent (or liquid detergent)
• Bulb pipette
• Paraffin wax
• Matchstick
• Mercury

Health & Safety and Technical Notes

The mercury experiment is best done as a teacher demonstration using a flexcam and screen, since working in a mercury tray obstructs the students' view. A mercury spill-kit should be to hand.

Read our standard health & safety guidance

The waxed slides are difficult to clean for re-use; best to throw away.

Procedure

1. Clean a microscope slide carefully so that there is no grease or oil on it. Use a strong detergent and rinse well.
2. Coat a second microscope slide with paraffin wax by dipping it into molten wax (e.g. 150 g of clean paraffin wax in an old saucepan, heated until it is very hot) or brushing molten wax on with a clean, cheap paint brush.
3. Fill a bulb pipette with water and drop pools of water about 1 cm diameter onto each slide. Compare the drops.
4. Dip a match stick into a wetting agent such as Manoxel-OT or liquid detergent (not as good) and touch the drop on the waxed slide.
5. Repeat steps 3 and 4 with mercury drops.

Teaching Notes

• Students should note that small water drops coming from the dropper are spherical. A sphere gives the smallest surface area for a given volume. When drops become larger, gravity deforms their spherical shape
• When the drop of water falls onto the clean glass slide, it forms a circular patch of water. Surface tension pulls it into a circular shape but gravity pulls it flatter. Surface tension is a cohesive force resulting from attraction between molecules in the liquid.
• A drop of water on the waxed slide stands higher because the adhesive force between water and wax modifies its angle of contact. The water does not wet the wax. A little wetting agent added to the water reduces its surface tension and the drop collapses back onto the waxed slide, resembling the patch of water on the clean slide.
• 'Waxing' the surface of a material is the basis of waterproofing it.
• Mercury has a greater surface tension, so larger drops will maintain their spherical shape.
• An additional demonstration: Make a tray from perforated zinc or other metal. Dip it into molten candle wax. Will it float or hold water?

This experiment was safety-checked in January 2005

Demonstration

In this experiment aniline drops grow in water. Apparently weightless (because aniline density is almost the same as water), the drops are perfect spheres.

Apparatus and Materials

• Aniline, bottle of
• Separating funnel
• Retort stand and clamp
• Glass beaker, 500 ml
• Translucent screen and lamp or projector

Health & Safety and Technical Notes

Wear eye protection when handling aniline (phenylamine). If aniline gets on the skin, wash it off with soap and cool water.

Do not attempt to pour directly from the stock bottle to the separating funnel or burette. Pour into a small beaker with a good pouring lip, and then from the beaker into the separating funnel which, is standing in a sink.

Alternatively, use a larger beaker and a small funnel in the top of a burette, which is in its stand on the floor. After the demonstration, safe disposal is essential. Pour off the water to leave a puddle of aniline at the bottom of the large beaker.

Add about six times its volume of 2 M hydrochloric acid, stir and leave it in a safe place for 24 hours. Pour carefully down the foul-water drain (toilet) and flush it away.

Read our standard health & safety guidance

A burette can be used in place of the separating funnel.

Aniline darkens with age so old stock is best. Aniline damages most plastics. If you use a plastic beaker, place a small glass beaker inside it to catch the aniline drops.

An alternative would be olive oil drops in alcohol, but aniline drops are more clearly visible.

Procedure

1. Put about 10 ml of aniline in a glass separating funnel with a tap. Clamp it so that the end of the funnel dips into a tall beaker of water.
2. Release aniline to form a drop at the end of the funnel. Students should observe that a narrow neck forms before the drop breaks away.

Teaching Notes

• Students will need to be nearby to see the drops forming, so ask them to come forward in small groups.
• Alternatives: Either project the experiment, using a bright lamp or projector behind the beaker and a translucent screen in front of it. Or take a series of photos of a drop forming with a digital camera or, even better, make a short video clip. Project the result onto a screen.
• Surface tension is caused by short range molecular attractions and very short range repulsions. The energy and force fields of surface molecules are very different from those of molecules deep inside the liquid. Therefore, their behaviours are different too. Both sets of molecules are in equilibrium.

This experiment was safety-checked in December 2004

Class practical

Experiment to flatten the water curvature inside a capillary tube.

Apparatus and Materials

• Capillary tube
• Trough

Health & Safety and Technical Notes

Read our standard health & safety guidance

Use clean apparatus and water. Wash in strong detergent, rinse in clean water, then in distilled water and take care not to put grease from your fingers on it.

The height of the capillary tube should be greater than the height to which water will rise inside it.

Procedure

1. Fill the trough with water and place a capillary tube in it. The water will rise inside the tube, to a height h above the water surface in the cup (diagram a).

2. Observe the concave curvature of water inside the capillary tube. The curvature forms an angle, say θ, on the inside wall of the capillary tube.
3. Now, lower the capillary tube such that the height of the capillary tube above the water surface in the beaker is less than h (diagram b).
4. Observe the concave curvature starts reducing and the angle at which the water surface meets the inside wall of the capillary tube is greater than θ.
5. When the tube is fully lowered to the surface level of water in the cup you will notice that the water surface flattens (diagram c).

Teaching Notes

• Water rises inside the capillary tube due to adhesion between water molecules and the glass walls of the capillary tube. This adhesion, together with surface tension in the water, produces an effect called capillarity, with a characteristic concave surface.
• The water rises until the weight of the column equals the vertical component of the forces of adhesion.
The weight W of the water column = π r²h ρ g where h is the maximum height of the liquid column ρ is the liquid density g is the gravitational field strength The vertical supporting forces around the circumference of the liquid surface = γ cos θ x 2 π r where γ is the surface tension of the liquid θ is the angle of contact between liquid and glass r is the internal radius of the tube
• This means
π r² h ρ g = 2 π r γ cos θ and h = 2 γ cos θ /( r ρ g )
• The narrower the tube, the higher the water will rise. Nature uses this effect to carry water up from roots to leaves in plants, including trees.
• When the tube is lowered so that the water surface inside the tube is at any height less than h, then θ becomes larger so that the weight of the water column and the forces of adhesion remain balanced.
• When the water surface inside and outside the capillary tube are level, the surface is shaped by surface tension alone.

This experiment was submitted to the site by Shivaji Chelladurai who is a software architect working for Cognizant at their Chennai, India office.

This experiment was safety-tested in July 2007

Class practical

In a streamline flow, fluid pressure will be reduced in a region where the flow velocity is increased, for example due to a constriction.

Apparatus and Materials

• Bernoulli tubes (also called Venturi tubes).
• Rubber tubing
• Funnel
• Air pump
• Ping pong ball or other light ball

Depending on the supplier, ‘Bernoulli tubes’ may come as a single tube with a narrower cross-section in its middle, or as two tubes, (one with a constant cross-section and the other with a narrower cross-section in its middle). Each tube will have connectors, to which you attach manometers at 3 positions. The tubes and manometers will need to be clamped to stands, near a sink for water source and drain.

Suppliers of Bernoulli tubes include the following: EDU-LAB, Eisco Labs, and PASCO Scientific.

Health & Safety and Technical Notes

Take care when handling and clamping the Bernoulli tubes, fragile glassware. Be careful not to get water on the floor, which can create a slip hazard.

Read our standard health & safety guidance

Procedure

1. If you have tubing of constant cross-section, run water through it and discuss the relative heights of the 3 manometers.

2. After asking students to predict (with some explanation) what might happen with the tubing that has a narrower cross-section in the middle, run water through it. The middle manometer will be lower than the other two.

3. Ask students to predict what may happen if air is blown out through the funnel, above a light ball. Then try it.

Teaching Notes

• In steps 1 and 2 water flows steadily along the tube with no turbulence. In step 3 air similarly moves smoothly through the funnel. In all cases, the flow is described as ‘streamline’.
• In step 1, with a tube of uniform diameter, the levels in the three manometers decrease slightly along the direction of flow. This shows that there is a slight decrease in pressure along the tube. This pressure difference is needed to overcome viscous drag along the tubing walls.
• In step 2, the mass flow rate of the water is the same in the narrow central section of the tube as in the outer sections. The level in the central manometer is lower than that in the other two manometers, showing that the pressure is less where the water is flowing faster.
• The pressure difference between the first two manometers is needed to accelerate the water as it enters the narrow tube. Similarly, the increase in pressure between the second and third manometers causes the water to decelerate as it enters the wider tube.
• Bernoulli’s equation relates the pressure P and speed v of a fluid along a streamline. For horizontal flow (no change in gravitational potential energy) we have: P 2 < P 1 because v 2 > v 1
• In step 3, the air slows down and its pressure increases as it moves from the narrow tube to the wider funnel. The ball is pushed from the area of higher pressure towards the area of lower pressure.
• Note that many controversial or false demonstrations of the Bernoulli principle are prevalent, for example aerofoil lift, balls in curving flight, or lifting a sheet of paper by blowing across its upper surface.

This experiment has yet to undergo a health and safety check.