School Science Lessons
(UNPh09)
2024-07-24
Energy
Contents
9.2.0 Conservation of energy
9.1.0 Kinetic motion, size of particles
9.3.0 Pumps, syringes
9.4.0 Work and energy
9.1.0 Kinetic motion, size of particles
9.1.1 Aluminium powder twinkles
9.1.2 Clay soil suspension
9.1.3 Heat lumps of wax containing lead shot
9.1.4 Hot and cold water drops
9.1.5 Molecular dimensions, size of a molecule
9.1.6 Particulate matter and dilution
9.1.7 Rattle tin of stones
9.2.0 Conservation of energy
9.2.1 Conservation of energy, work, energy, kinetic energy and potential energy
9.2.2 Bicycle ergonometer
9.2.3 Braking distance and stopping distance, tyres
9.2.4 Bow and arrow ballistic pendulum
9.2.5 Children's swing
9.2.6 Drop a golf ball inside a car tyre
9.2.7 Flywheels
9.2.8 Hammer lead and iron
9.2.9 Height of a ball
9.2.10 Hot wire current meter
9.2.11 Kinetic energy
9.2.12 Loop the loop
9.2.13 Nose basher pendulum
9.2.14 Big yo-yo
9.2.16 Prony brake
9.2.17 Rebounding balls
9.2.18 Roll-back jar
9.2.19 Rolling down an U-shaped track
9.2.20 Rotating washer
9.2.23 Toy spring jumper
9.2.24 Transform electromagnetic energy to kinetic energy
9.2.25 Transmission of compressive energy, motion of coins, dominoes
9.2.26 Vertical ballistic pendulum
9.2.27 Wave propagates energy
9.2.28 Weight of a pendulum, break a pendulum wire, stopped pendulum
9.3.0 Pumps, syringes
9.3.1 Force pump
9.3.2 Lift pump
9.3.3 Syringe lift pump
9.3.4 Test-tube force pump
9.4.0 Work and energy
9.4.1 Energy, potential energy, kinetic energy, Gravesande's experiment
9.4.2 Potential energy
9.4.3 Power
9.4.4 Renewable energy, biomass, hydroelectric, solar, wind
9.4.5 Solar panels, CSP, CPV
9.4.6 Solar water heater
9.1.1 Aluminium powder twinkles
To observe the relation between the twinkle of aluminium powder and its size, add aluminium powder to a beaker of tap water with a few drops of detergent.
Stir the mixture.
Make the room dark and shine a strong light through the liquid.
Observe that the smallest suspended particles twinkle like stars, but the larger particles do not twinkle.
The reason is that the water molecules hit the smallest aluminium particles and turn them over so that they reflect flashes of light and cause the twinkling.
Water molecules cannot turn over the larger particles, so they do not twinkle.
9.1.2 Clay soil suspension
Shake a little clay soil with water in a test-tube.
Leave this to settle.
Note the humus layer at the tiny particles of rock and mineral at the bottom.
Filter the liquid.
Students will observe that the filtrate is still cloudy, this is because the clay particles have passed through the filter paper.
Do students understand why the suspension particles do not settle, even after a few days?
The size of colloidal particles is about between 1 and 100 mu.
Divide the filtrate into parts in test-tubes.
Keep one as a control.
To the other add a few drops of barium chloride solution, or some aluminium salt solution.
Note what happens in half an hour and in one hour.
The same effect occurs when a clay suspension in a river meets the salts contained in sea water.
In many hot countriesm, salt is crystallized from pans built on the clay beds near the mouths of rivers.
9.1.3 Heat lumps of wax containing lead shot
To imitate differences of movements of solid, liquid and gas particles, push very small lead shot into wax or petroleum jelly.
The lead shot is like the particles of a solid that cannot move about.
Put the mixture into a container and melt the wax at the bottom of the container.
Heat the container.
The wax melts and the lead shot can move around each other limited.
The lead shot is like the particles of a liquid.
If you burn away the wax and shake the container that lead shot can move in straight lines at random.
The lead shot is now like the particles of a gas.
9.1.4 Hot and cold water drops
Use a pin to make identical holes in the bottoms of identical paper cups.
Mount the cups over drinking glasses.
Fill one paper cup with hot water and the other with cold water.
Observe the drops of water dripping from the paper cups.
The hot water leaks faster than the cold water, because the particles have more kinetic energy.
So it is easier to overcome the forces of adhesion around the holes in the bottoms of the paper cups.
9.1.5 Molecular dimensions, size of a molecule
Use oil molecules. because oil has a density less than water.
The oil will float on the surface and not dissolve in the water.
If the water has a large enough surface area, that thin oil will spread out in a layer one molecule thick, a monomolecular layer and not form little "hills" of molecules.
If the volume of oil and the surface area that it forms is known, calculate the thickness of a monomolecular layer by dividing the volume by the area.
Use a water container > 30 cm2 so as not to restrict the oil film.
Sprinkle the surface of the water with a very fine light powder such as talc powder.
When the oil is put on the water, it will push the powder away and the area covered by the oil will be seen clearly.
To find the volume of oil, pour thin oil into a burette.
Use a thin petroleum distillate.
Find the volume of fifty drops by running oil from the burette drop by drop and counting the drops.
Allow one more drop to fall on a piece of plastic.
Touch the oil drop with the point of a glass rod and then touch the prepared water surface.
The oil spreads out.
Make an approximate measurement of the area over which it spreads.
Estimate what fraction of oil was removed by the glass point by using the glass point to remove successive fractions from the drop until it has all been used.
Calculate the volume of oil put on the water and estimate made of the thickness of the oil layer, about 10-6 mm.
This is an approximate dimension of a single molecule of the oil.
9.1.6 Particulate matter and dilution
Put one crystal of potassium permanganate in a test-tube.
Add 1 mL water.
Dissolve the crystal completely by shaking vigorously, keeping your thumb over the end of the test-tube.
Then add water to a total volume of 10 mL.
This is a "10 times dilution".
Pour this 10 mL of purple solution into a 100 mL beaker and then fill the beaker with water.
This is now "100 times" dilution.
Fill the 10 mL test-tube with this solution and throw the rest away.
Dilute this again in the beaker to 100 mL.
It is now a"1, 000 times" dilution.
Note how often the solution can be diluted by a factor of 10 before the colour is so pale that it is only just visible.
The final dilution factor shows that if matter is particulate, the size of the particles must be small.
9.1.7 Rattle tin of stones
To imitate the heat movement of gas particles and know the energy of it, put some small stones in a tin with a lid.
Replace the lid and shake the tin.
You can feel and hear the stones rattling inside the tin.
The stones are knocking against the walls of the tin.
If you shake much harder, the stones can knock the lid off the tin and burst out.
The movement of gas particles in a closed container is similar to the movement of the stones.
If you heat the gas particles they move faster and can burst the closed container.
Heating a liquid or a gas in a closed container is very dangerous.
9.2.1 Conservation of energy, work, energy, kinetic energy and potential energy
See diagram 9.2.0: Work at angle θ to the direction of the force
Energy conversion, convert potential energy to kinetic energy, potential energy and gravity, transfer energy, conservation of mechanical energy in a closed system, problems involving the conversion of mechanical energy GPE <--> KE, EPE <--> KE, gravitational potential energy in its general, non-uniform form, GPE = mgh, GPE = (- G m1 m2 / d) and its application to escape velocity.
When an object is moved by a force, F, through a distance in the direction of the force, s, the work done is F × s.
The unit of work is the joule, newton metre.
Work = Fs
However, if the direction of displacement of the object is at angle to the direction of the force Work = Fscos
When work is done on an object the energy of the object changes as stored energy, potential energy and / or as change in speed, kinetic energy.
Unit energy is expended when unit work is done.
The unit of energy is the joule, newton.metre.
(James Joule, 1818-1889, England)
The joule is the SI unit of energy and work, equal to the work done by a force of one newton, when its point of application moves one metre in the direction of the force.
Work done = change in energy so energy is the capacity for doing work.
9.2.2 Bicycle ergonometer
Use pedal power to turn a car generator with a load of two 24V 250W globes as a load.
Estimate the power generated with a voltmeter and ammeter.
Power = V × I
1 HP = 746 Watts
9.2.3 Braking distance and stopping distance, tyres
The "braking distance" of a moving vehicle involves the work that must be done to bring the vehicles to rest, W = Fs,
i.e. how much force must be applied to stop the vehicle in a certain distance.
So it refers to the distance a vehicle travels from the time when the brakes are applied to the time when the vehicle stops.
Braking distance is affected by the speed of the vehicle, the coefficient of friction between tires and road surface, the rolling resistance of the tires,
and the air drag on the vehicle.
The "total stopping distance" includes braking distance and also the reaction distance = speed × driver reaction time, e.g. 1.5 seconds.
Stopping distances can be affected by, experience and age, average deceleration of the vehicle, vehicle condition and braking capacity of the vehicle,
condition of the tyres, weather conditions, road surface, weight of the vehicle.
During a tyre change pit stop for racing cars, the tyres are heated by the changers, because tyres don't "work" until they reach their designed optimal temperature range.
The rubber and other compounds are selected to perform within a specific temperature range.
If the tyres are too cold, there is less grip causing spins or having to travel slower on corners.
Travelling slower is a problem for grip as the car aerodynamics are such that they can only effectively corner at higher speed, pressed onto the road.
Drivers are more confidant taking a corner at speed than slowly, because the grip is so much greater if the tyres are sufficiently hot.
9.2.4 Bow and arrow ballistic pendulum
The relation between bending of the bow and the velocity of the arrowwas found to be linear.
9.2.5 Children's swing
1. Ride on a child's swing.
Note the original height of the swing above the ground.
Let yourself swing to the other side.
The height reached on the other side is almost the original height.
2. Swing a pendulum.
Note the original height of the bob.
Let the bob make a full swing.
The height it reaches is almost the original height.
3. To study the transfer and conservation of mechanical energy, observe a child on a swing.
The heights raised at the two sides are always the same.
It shows that the potential energies of the swing at the two peaks are equal.
If you sit on the swing, you may experience the short rest feeling while reaching the peak, then the velocity will become faster and faster.
At the top point the swing and person only possess potential energy, but no kinetic energy.
At the bottom point their kinetic energy reaches the maximum, but the potential energy becomes the minimum.
When the swing reaches the other peak, the kinetic energy completely changes into the potential energy again.
9.2.6 Drop a golf ball inside a car tyre
See diagram 9.2.2: Ball inside car tyre
1. Hold the ball inside the tyre and note the original height above the ground.
Let the ball go.
It runs down inside the tyre and up the other side.
The height it reaches in the other side is almost the original height, but never more.
Repeat the experiment using different original heights.
2. Let an outer tyre stand upright on the ground and fix it well.
Hold a golf ball with your hand above the tyre and loosen your hand at a certain height and let the ball roll in the groove inside a tyre.
The ball rolls from the original height to the bottom of the tyre, then rolls upwards reaching the original height, but not more than the original height.
Repeat the experiment by dropping the ball at different heights.
The small ball will keep its energy changeless at the whole process of moving if neglect the energy loss due to friction and other reasons.
When the small rolls down from the top point to the bottom of the tyre, its gravitational potential energy changes into the kinetic energy.
When it rolls up from the bottom point to the top, its kinetic energy changes into the gravitational potential energy again so it comes back to the original height.
9.2.7 Flywheels
A flywheel is a heavy-rimmed wheel attached to a revolving shaft to store momentum and usually to regulate the movement of machinery.
A flywheel stores energy by accelerating a cylindrical assembly, a rotor or flywheel, to a high speed and maintaining the energy as rotational energy.
The energy is released by slowing down the flywheel.
The flywheel is like a mechanical battery that spins to store energy that is instantly available when needed.
When absorbing energy, the flywheel's motor acts like a load and draws power from the grid to accelerate the rotor to a higher speed.
When discharging, the motor is switched into generator mode, and the inertial energy of the rotor drives the generator,
which, in turn, create electricity that is then injected back into the grid.
Performance is measured in energy units - kilowatt-hours (kWh) or megawatt-hours (MWh), to show how much power is available for a given period of time.
At 16, 000 rpm a flywheel can store and deliver 25 kWh of extractable energy.
Flywheel-based energy storage systems, unlike fossil-fuel power plants that are used on the grid for frequency regulation, are sustainable "green" technology solutions
that consume no fossil fuel, nor produce carbon dioxide or other emissions during operation.
Flywheels can operate reliably for many years with little or no maintenance.
9.2.8 Hammer lead and iron
Transform kinetic energy to internal heat energy
See diagram 9.2.5: Hammer lead sheet
1. Use a small piece of lead sheet wrapped around one end of a piece of thin iron wire.
Hold the other end of the wire.
Hit the lead several times.
You can feel the temperature rise.
2. Use a thin sheet iron, with thickness not more than 0. 3 mm.
Cut a 3 cm width strip off the sheet iron.
Wrap the sheet iron strip three times around an end of a stick.
Hold the other end of the stick with your hand and place the end of the stick with the sheet iron on wooden stool or tree stump.
Touch the strip of sheet iron with your other hand to experience its temperature.
Beat the strip sheet iron with a mallet until it is hot.
Feel the heat radiating from the strip of sheet iron.
The thinner the strip of sheet iron, the more quickly the temperature rises.
The faster the strip of sheet iron is beaten with the mallet, the more quickly the temperature rises.
If a metallic stand is used instead of a wooden stool or tree stump, the temperature of the sheet iron will not increase so quickly.
9.2.9Height of a ball
High bounce paradox
A device to project a ball upward at different known velocities to show dependence of kinetic energy on the square of velocity height of a ball.
A steel ball is launched upward by a stopped spring for which the initial velocity is calculated.
Flip a half handball inside out and drop on the floor and it bounces back higher than the height from which it was dropped.
9.2.10 Hot wire current meter
See diagram 9.2.7: Current meter
The electric current to be measured passes through a platinum alloy hot wire, AC.
The current heats the wire so it expands and loosens an attached phosphor bronze wire, BD, that is insulated from heat.
One end of a silk strip is attached to the phosphor bronze wire.
The silk thread winds around a pulley, E, attached to a pointer then the other end is attached to a spring metal strip that keeps the silk thread tight.
When the silk strip becomes loose the spring metal strip moves out to the left, tightens the silk strip that then turns the pulley so the pointer turns to the right.
Electrical energy transforms into heat energy into the kinetic energy of the pointer and potential energy of the spring metal strip.
9.2.11 Kinetic energy
Kinetic energy, K.E., energy due to velocity = 1/2 mv2.
So kinetic energy can be measured by the work an object could do in coming to rest.
If an object moving with velocity u has its velocity increased to velocity v by application of an uniform force F, then the work done is equal to the change in energy.
Work = 1/2 mv2 -1/2 mu2
9.2.12 Loop the loop
Energy well track, ball in curved tracks, triple track energy conservation
See diagram 9.2.14: Ball in curved track
1. Release the ball near the top of the track.
The energy loss makes the minimum height necessary to complete the loop significantly higher than the calculated value.
2. A ball rolls down an incline and then around a vertical circle.
Vary the initial height of the ball.
A water stream loop the loop shows the effect of centripetal forces much more dramatically then when a ball is used water.
The reverse loop-the-loop is placed on a cart hooked to a falling mass that produces an acceleration,
just large enough to make the ball go around backwards into the cup.
A ball can escape the energy well when released from a point above the peak of the opposite side.
Balls are rolled down a series of curved tracks of the same height, but different radii.
Balls released from three tracks with identical initial angles rise to the same height independent of the angle of the second side.
9.2.13 Nose basher pendulum
See diagram 9.2.12: Nose basher
1. Hold a bowling ball suspended from the ceiling against your nose and let it swing with pushing it or in any way giving it any additional velocity.
It will not bash your nose, because it must return to the original height on the back swing.
2. Stand against the wall.
Bring the bowling ball up to your nose or chin.
Release the ball without giving it any initial velocity.
Stand very still and wait for the return.
This is an example of conservation of energy.
3. Each time a pendulum swings it does not swing back quite as far as it did the last time.
Make a large pendulum consisting of stout cord and a large coffee can containing sand or a brick.
Invite a person to stand back, pull the coffee can back towards the nose, and release the coffee can so it swing forwards and back.
Do not push away the coffee can.
The person will be afraid that the coffee can will swing away then return to hit the nose.
However, some energy will be lost during the swing by drag, air resistance friction, and at the pivot point attachment to the ceiling.
The energy lost will be converted into heat to be lost to the system.
The energy loss in the swing can be measured from the decrease in amplitude of pendulum swing.
9.2.14 Big yo-yo
A large yo-yo is hung from bifilar threads wrapped around a small axle.
The string unwinds on the way down and rewinds on the way up.
Low cost yo-yo made with cardboard sides and paper towel centres.
9.2.16 Prony brake
See diagram 9.2.19: Prony brake
The Prosy brake (Gaspard de Prony, 1821) rotates a shaft against a constant frictional resistive force.
Rotary power (newton-metres per second, = 2 π × lever length× revolutions per second × measured force (newtons, N).
Horsepower, HP = 2 π × length (feet) × weight (pounds) × rpm (revolutions per minute / 33, 000
The Prony brake is uded to measure the torque produced by an engine.
Power as brake horsepower = torque X rotational speed.
9.2.17 Rebounding balls
To observe the transfer and loss of mechanical energy, hold a ball with a strong elasticity, record its height, then let the ball falls freely from rest.
Record the heights the ball rebounds several times after the ball falls on the cement floor and find the relationship among the heights.
Use balls with different bounce characteristics, e.g. tennis ball, baseball, squash ball, cricket ball, golf ball, steel ball bearing, plasticine ball, silly putty ball.
Do the experiment again on a sandlot, instead of the cement floor.
Observe the change in shape of the sand as well as the rebounding heights.
9.2.18 Roll-back jar
Come back can, boomerang tin
Come back can, elastic potential energy and kinetic energy
See diagram 9.2.3: Roll-back jar
1. Use a 10 cm plastic jam jar with a screw-on plastic lid.
Drill two small holes, each 2 cm from the centre, along the diameter of the bottom of the jar.
The distance between the two holes is 4 cm.
Drill the same two holes in the cap of the jar.
Push an elastic band through each of the two holes in the bottom of the jar.
Tie the two rubber bands together outside the bottom of the jar.
Pull the ends of the two elastic bands into the jar to cross over then push the two ends through the two holes in the cap.
Attach a 50 g weight to one end of a thin wire.
Tie the other end of the wire to the rubber bands together where they cross over.
Pull on the rubber bands that passed through the cap and tie them so that the weight does not touch the sides of the jar when the jar is horizontal on its side.
Hold the jar horizontally and turn it over many times so that the thin wire becomes shorter and shorter.
When you hold the jar horizontally and turn it you store potential energy as the thin wire became shorter and the weight became higher.
Also, the elastic potential energy is stored in the elastic bands when they are twisted.
So the rotating kinetic energy of the jar changes into gravitational potential energy and elastic potential energy.
Put the jar on a flat surface.
Push the jar ahead and it rolls back.
Put the jar on a slight slope.
The jar rolls up the slope.
2. Cut two slits, 1 cm × 0.5 cm, in the middle of the bottom and the middle of the lid of a round biscuit tin.
Cut a strip of bicycle inner tube rubber length 1 cm longer than the depth of the tin and 1 cm wide.
Pass the strip of rubber through the slits and fasten each end outside the biscuit tin with pins through the ends.
Attach a heavy machinery nut from the middle of the rubber strip with a wire paper clip.
Roll the tin several rotations forward, then let it go, and it will roll back.
When you roll the biscuit tin forward, the heavy nut remains hanging down due to the force of gravity, so the elastic becomes twisted tighter with each rotation.
The biscuit tin rolls back to release the force of tension accumulated in the rubber strip.
9.2.19 Rolling down an U-shaped track
When a ball or wheeled object rolls down an U-shaped track it will roll up to the same height as it was released.
If it rolls down onto a flat track, then meets the second half of the U, it will almost roll up to the same height as it was released.
An object rolling down from the top of a hill will have enough energy when it gets to the bottom of the hill to then roll up the next hill to the same height, with some corrections for friction.
9.2.20 Rotating washer
See diagram 9.2.4: Rotating washer
1. Use a dowel (round stick) 1 m long and a rubber or plastic washers 2.5 cm diameter.
The inner diameter of the washer is just larger than the diameter of the round stick.
Hold the dowel vertically and attach the bottom end to a table.
Hold the washer just above the top of the dowel.
Let the washer fall down the length of the dowel.
Estimate how long it takes to fall.
2. Hold the washer just above the top of the dowel.
Use your thumb and first finger to make the washer spin then fall down the length of the dowel.
Note that the rate of fall slows and the speed of rotation increases.
The spinning washer at the top of the dowel has rotational kinetic energy and gravitational potential energy due to its height.
As the spinning washer falls some of the gravitational potential energy is converted to rotational potential energy so it spins faster.
However, some of its gravitational energy is lost to friction with the dowel, so it falls more slowly.
9.2.23 Toy spring jumper
Compress a spring under a toy held down be a suction cup
9.2.24 Transform electromagnetic energy to kinetic energy
Compass and coil
See diagram 31.66.1: Compass in a coil
To observe the transformation of electromagnetic energy to kinetic energy, place a compass in the tray of a match box.
Wind 10 turns of copper wire around the tray so that the wire just covers the compass.
Leave two ends of the wire.
Rotate the match box so that the compass needle is parallel to the wire.
Connect one end of the wire to one terminal of a 1.5 V dry cell or low voltage DC power supply.
Briefly touch the other end to the other terminal of the dry cell or power supply.
Touch the other terminal again after a few seconds.
Note how the compass needle behaves at the moment of touching.
The compass needle deflects to align itself with the magnetic field produced by the current in the coil.
9.2.25 Transmission of compressive energy
Motion of coins, dominoes
See diagram 9.2.8: Coin motion, domino motion
Longitudinal waves produce compress and stretch in the medium.
With the propagation of the form of movement, the energy of the wave propagates.
The form of movement is the same to the velocity of energy of propagation under the condition of no chromatic dispersion.
It can reach thousands of metres in a second, far surpassing the value of general moving body.
Experiments
1. Arrange several same coins in a line on the table.
You can fix them with adhesive tape.
Place one coins in front of the line and place another coin at the end of the line.
The last coin touches the one in front of it, but is not connected.
Shoot the first coin quickly to make it strike the second.
Then observe the movement of all the coins.
When the first coin hits one end of the line of coins the last coin moves back very rapidly.
The energy produced by the first coin transmits rapidly along the line of coins.
2. Use a set of dominoes, or some similar small squares of wood, e.g. mah-jong pieces, on their edges in a long row.
Place them face to face.
The distance between two dominoes should be shorter than height of them.
Push the first one A rapidly.
Observe the movement of all the dominoes.
As shock dominoes A, it is propagated a small pulse of energy to cause it topples.
It knocks against the following domino, which knocks the third and so on until finally the end domino falls.
It can be seen if you observe that although every domino falls just at its original place and the distance of its motion is very small.
The propagation of this pules energy is so quick that B has begun to fall down before A falls down completely.
During propagation of compressive energy, the behaviour of solid particles is like that of the dominoes.
Solids consist of atoms, ions and molecules arranged in a row closely together.
When one end of a solid obtains the compressive energy due to impact, the particles there produce compressive deformation,
the compressive energy is transformed to neighbour particles, and so on.
The process of above happens among particles in turn that reaches the other end in a short time,
and from the particles here propagates the compressive energy to the outside, finally cause the last coin in experiment A shoots forward.
In the process above, although every particle's motion is very weak,
they are much like that connected with small springs going on forward with energy in a form of waves.
It has been shown from the dominoes experiment above and analysis about the motion of particles inside the solid,
that the speed of propagation surpasses far from that of each object when they are connected.
Sometimes you can see the similar phenomenon to the dominoes in daily life.
3. If you have visited a train dispatch yard, you can notice the situation of marshalling.
As one carriage is connected with the whole train, it collides the last carriage of the train.
With a series of reactions that every carriage moves slightly in turn, such energy in a form of shock waves propagates throughout the train rapidly,
which makes you feel as though the whole train begins to move almost instantaneously.
4. When traffic jam happens in a highway the cars run in a row.
At this time if the last car cannot stop in time to collide the back of the car in front of it, the shock wave energy may be propagated forward rapidly,
which lead to a series of accidents.
It is too late to brake the car for all car drivers.
9.2.26 Vertical ballistic pendulum
A ball is dropped into a box of sand suspended from a spring and the extension of the spring is measured.
9.2.27 Wave propagates energy
See diagram 9.2.9: Waves propagate energy
Waves propagate not only the vibration state of the source, but also the energy of it.
Prepare a water vessel, pour water into it and put a cork by the vessel.
Hold a long, narrow wooden rod and make it move up and down in another end of the vessel.
Observe the motion of water wave and notice the motion of cork.
Increase or decrease the motion and observe what happens about water wave and cork.
Increase or decrease the frequency of the motion and observe the situation again.
The vibration of a wooden rod caused by hand makes it become a vibration source.
The state of its motion and energy propagate out by means of water.
Thus forms the water waves.
So you can see the wave crest and trough in water.
The energy carried by water wave is propagated to the cork causing it vibrate up and down.
The cork is up as crest comes, down when a trough comes.
The energy of rod vibration propagates to the cork through the medium role of water waves.
So the wave propagates not only the state of a vibration source, but also the energy of it.
Increasing the extent of a shake by hand is increasing the vibration energy of source and water wave increases with it.
All this shows that the more the energy the source has, the more energy it propagates and vice versa.
If you increase the frequency of a shake by hand, the wave shape varies with it and the distance between the wave crest or that of crest and trough decreases.
The vibration frequency of the cork increases too.
Waves are the propagators of energy that carried the energy produced by source to all places it arrives.
9.2.28 Weight of a pendulum, break a pendulum wire, stopped pendulum
Suspend a pendulum from a double beam balance with a small block placed under the opposite pan to keep the system level.
Swing the pendulum so it just lifts a weight off the stopped pan.
Suspend a heavy bob on a weak wire, as the ball descends in its swing the wire breaks.
A pendulum is started at the height of a reference line and returns to that height even when a stop is inserted stopped pendulum
9.3.1 Force pump
See diagram 12.4.4: Force pump
Downstroke:
Valve A is closed, valve B is forced open, air passes out of the chamber into the outlet pipe
Upstroke:
Valve A is open, valve B is closed,.
The pressure in the chamber is reduced and water rises in the inlet pipe caused by atmospheric pressure on the water surface.
Downstroke:
Valve A is closed, water is forced through valve B into outlet pipe
Upstroke:
Water in outlet pipe closes valve B.
More water enters the chamber through valve A.
The air chamber causes the flow of water to continue during the upstroke.
On the downstroke, water enters the air chamber compressing the air inside it.
On the upstroke, the air expands again to force some air up the outlet pipe.
The atmosphere can support a column of mercury 76 cm vertical height So h in the diagram can be no more than 76 × 13.6 (relative density of density of mercury) = 1, 030 cm.
9.3.2 Lift pump
See diagram 12.4.3: Lift pump
Downstroke 1:
Valve A closes and valve B opens.
Air passes from lower chamber to upper chamber through valve B.
Upstroke 1:
Reduced pressure in lower chamber causes valve B to close and valve A to open.
Pressure in inlet pipe is reduced, so atmospheric pressure acting on water surface cause water to rise into lower chamber.
Downstroke 2:
Valve A is closes, valve B is open.
Water passes from lower chamber to upper chamber.
Upstroke 2:
Valve A is open and valve B is closed.
Water is lifted up and passes out through outlet pipe and more water enters through inlet pipe.
The atmosphere can support a column of mercury 76 cm vertical height, so h in the diagram can be no more than 76 × 13.6 (relative density of density of mercury) = 1, 030 cm.
9.3.3 Syringe lift pump
[Some school systems do not allow the use of syringes in the classroom.]
See diagram 4.233: Syringe lift pump: A Glass case, B Rubber valve, C Cork piston, D Holes, D Intake tube
1. Drill a hole through the centre of a cork, B, that makes a tight fit inside the glass tube of the syringe body, A.
Use a piece of hot wire to burn two small holes through the cork, C, on either side of the centre hole.
Pass a metal rod through the centre hole in the cork then expand the end after it has passed through the cork.
Cut a circular piece of flexible plastic, D, to the exact size of the cross-section area of the glass tube of the syringe body.
Cut a hole in the centre of the flexible plastic to allow the metal rod to pass through it.
Attach the inner edge of the plastic to the cork with glue.
The piston consists of the cork and metal rod.
The inlet valve is the piece of plastic.
The inlet is the nozzle of the syringe.
Push the piston down, then place the nozzle of the syringe under water.
Raise the piston.
During the upstroke, the inlet valve should remain closed and water is drawn into the lower body of the syringe by reduced atmospheric pressure.
Lower the piston.
During the down stroke water moves up through the side holes while the inlet valve remains open.
Raise the piston.
During the upstroke the inlet valve should remain closed,
the water above the piston is raised and water is drawn into the lower body of the syringe by reduced atmospheric pressure.
See diagram 12.4.3.2: Syringe lift pump
2. Wrap a string around one cork to make a tight fit inside the glass or metal tube.
The other cork, with a piece of glass, bamboo or strong tubing, acts as an intake.
Then drill a hole in the cork and push a piece of glass, bamboo or strong tubing through the hole ensuring a tight fit and push the assembly into the end of the glass This will be the intake.
To make the piston drill a hole through the other cork to attach the metal rod.
Use a piece of hot wire burn two more small holes through the cork on either side a parallel to the hole for the metal rod.
Burn two holes through the piston with a hot wire and fit a thin piece of leather above them to act as a valve that closes on the up-stroke
and yet allows liquid to pass through on the down stroke.
To make the piston, drill a hole through the other cork to attach the metal rod.
Then, with a piece of hot wire, burn two more small holes through the cork on either side, parallel to the hole for the metal rod.
Put the glass, bamboo or strong tubing, acts as an intake, into water.
Trim a piece of leather or rubber to the same size as the internal diameter of the syringe body.
Cut a hole in the centre of the piece of leather or rubber to allow the metal rod to slide through it.
Assemble the piston.
Raise the piston, sucking water up under the piston into the syringe.
To operate raise the piston up sucking water up under the piston into the syringe.
Then push down the piston then the water raises above the piston of the syringe.
Finally, raise the piston up then the water streams out of the top of the syringe.
9.3.4 Test-tube force pump
See diagram 4.234: Test-tube force pump
1. Use a small and a large test-tube.
Heat the bottom of each test-tube with an alcohol burner.
When the bottoms are soft, but not melting, punch a hole with a metal awl.
Fit a ball bearing or small marble in each test-tube to act as a valve.
Fit a cork with a bent glass tube at the end of the small tube.
Put the small test-tube into the large tube.
Wrap string around the inner test-tube so that it fits tightly in the outer test-tube, but can still slide up and down.
Secure the inner tube to act as a piston of a force pump.
Put the end of the large tube into water in a beaker.
Raise the piston then water is sucked up the large tube through the ball valve of the large tube.
Push down the piston then the water raises the small tube through the ball valve of the small tube.
Raise the piston then the water streams out of the small tube.
Raise and push the piston continually then water streams out of the bent tube continually.
2. Use a thinner test-tube and a thicker test-tube.
Heat the bottom of each of test-tube with a burner then punch a hole with a nail when the glass in the bottom is soft, but not melting.
When the test-tubes are cool, fit a ball bearing or marble to sit in the holes in the bottom to act as valves.
Fit a cork with a bent glass tube passing through it into the end of the thinner test-tube.
Wrap string around the thinner test-tube so that it fits tightly in the thicker test-tube, but can still slide up and down.
Put the thinner test-tube into the thicker test-tube.
The thinner test-tube can act as a piston of a force pump.
Put the end of the thicker test-tube into water.
Raise the piston.
During the upstroke water moves up through the ball valve of the thicker test-tube, because of reduced atmospheric pressure.
Lower the piston.
During the downstroke water moves up through the ball valve in the thinner piston while the ball valve in the thicker piston remains closed.
Continue to raise and lower the piston until water streams out of the bent tube.
9.4.1 Energy, potential energy, kinetic energy, Gravesande's experiment
Potential energy, PE or EP, is energy deriving from position.
A stretched spring has elastic potential energy, PE = kx2 /2.
An object raised to a height above the Earth's surface has gravitational potential energy, PE = mgh.
Other sorts of potential energy include electrical, nuclear and chemical potential energy.
Kinetic energy, K E, is the energy of moving objects, K E = mv2 / 2.
The experiment to show that Ek = 1/2 mv2 was first done by Willem Jacob Gravesande, 1688-1742, Netherlands,
who dropped brass balls with different velocities into soft clay.
Conservation of energy.
Energy can be converted from one form to another, but the total quantity stays the same.
Transfer of energy
GPE (Gravitational Potential Energy) and KE (Kinetic Energy)
Conservation of energy: KE translational (1/2 mv2) = GPE (mgh)
9.4.2 Potential energy
Potential energy, P.E. = change in position or change in state, energy due to position or strain.
If an object weight mg newton is raised through a vertical height, h, there is an increase in gravitational potential energy.
Potential energy, P.E. = mgh
Change in potential energy may result in a change of state.
Steam at 100oC has greater potential energy that water at 100oC.
A spring under tension in a jack-in-a-box has the more potential energy than a slack spring.
A large molecule, e.g. glucose, has more potential energy than the component molecules.
Coal is an example of a store of chemical potential energy.
The sum of kinetic energy and potential energy of a body falling freely under gravity is a constant throughout its path.
9.4.3 Power
Power, watt, W, J s-1
Watt, the unit of power in the International System of Units (SI) = 1 joule of work performed per second.
1 kilowatt (kW) = 1000 watts (W).
Electromagnetic watt is when a current of one ampere flows across an electrical potential difference of one volt.
or, the power dissipated in an electrical conductor carrying one ampere current between points at one volt potential difference.
Power is a form or source of energy available for application to work, or applied to produce motion, heat or pressure.
Power, (Latin posse to be able), is the time rate of doing work.
P = w / t
Average power = work done / time taken.
If the rate is constant, power = work / time = work done per second.
If a body is moving with uniform velocity, and work is being done to overcome a constant resistance, the rate of work done is constant and the power = done per second = force × distance travelled per second = force × velocity.
If a body is moving with uniform acceleration, the power is not constant, but at any instant power = force × velocity.
9.4.4 Renewable energy, biomass, hydroelectric, solar, wind
The sources of renewable energy:
1. Bagasse gas
Waste plant materials, the residual fibrous waste from raw cane sugar processing can be burned to generate electricity.
2. Biomass energy
Landfill gas, mainly methane gas, (CH4), produced by decomposing organic matter, is captured and burned to produce electricity.
This method also prevents methane, and other landfill gases, from becoming a potential greenhouse gas in the atmosphere.
3. Hydroelectric energy
Falling water to drives turbines to generate electricity.
4. Solar energy
Solar energy converts energy from sunlight to electricity, an average of 1366 watts per square metre per hour.
5. Wind energy
Wind drives turbines to generate electricity
9.4.5 Solar panels, CSP, CPV
A crystalline silicon cell used in solar panels for solar power has an efficiency of about 22% for the conversion of light to electricity (16% for the cheaper cells in household solar panels).
A concentrated solar power system (CSP) use lenses or mirrors to concentrate solar thermal energy, onto a small area to heat water in a steam turbine and generate electricity.
A concentrated photovoltaic system (CPV) focus sunlight onto photovoltaic materials to generate electricity directly.
Cadmium telluride thin films in solar cells to replace silicon panels Indium tin oxide, ITO, optically transparent front electrode for each pixel in flat screen televisions, touch screens, solar cells)
Pyrite, FeS2, iron (II) disulfide, can be used in solar cells instead of silica.
Experiments
1. Measure current produced when the collector is perpendicular to rays during the day, noting the angle of incidence of sunlight.
2. Measuring the effect of the angle of incidence on the flow rate (i.e. power output) of a electrical water pump powered by photoelectric cells.
3. Use layers of shade cloth to measure the effect of shade on the output of a solar panel.
4. Use coloured cellophane to measure the effect of light of different wavelengths and to note whether solar cells are sensitive to all wavelengths.
Note that not all different coloured cellophane sheets have the same percentage transmission!
5. Dust on a solar panel
See diagram 39.1.0: Power from solar panels
Use bentonite clay powder, fine sand, or icing sugar to measure the effect of dust on the output of a solar panel.
Use and incandescent bulb as the constant light source.
Adjust the solar panel so it is working at maximum power.
In the circuit use variable resistance from 0 ohm to 300 ohm to plot a graph to find the maximum power before adding the "dust".
Then Efficiency = P dust/ Pno dust x 100%.
6. An array of solar panels produce 17 to 19 kW on cloudy to cloud free days or average heat to hot days.
The sunlight falling on the panel changes usually peaks at noon, and as the temperature of the panel rises its efficiency changes.
Most panels produce maximum power at 25oC.
Measure the V and I across a resistor, e.g. 56 Ω, connected to a solar panel.
Repeat the experiment with constant illumination and with the solar panel over a hotplate.
Keep the temperature constant and vary the illumination with and electric bar heater in front of the solar panel.
9.4.6 Solar water heater
After Davis, Peter and Fries, Peter, Australian Science Teachers Journal 33 (4)
1. You do not need a pump to circulate the water to the plastic bottle storage tank.
Water in the blackened collector tube absorbs the sun's energy and gets hotter.
This hot water is less dense than the rest of the water so it rises out of the collector into the storage tank.
The cold water at the bottom of the bottle then flows into the bottom of the collector and the cycle begins again.
This process is called thermo-syphoning.
You need water based black plastic paint, -10oC to 110oC thermometer, adhesive tape, roof and gutter type silicon sealer,
or "Blue Tack", plastic bottle with lid for the water storage tank, dry cleaning wrap or "Glad Wrap" plastic film, plastic funnel, roll of aluminium foil, tie wire.
Paint inside the collector black and paint the hose.
Attach clear plastic wrap, clear acrylic or glass.
Use the blue tack or silicon sealer to attach the collector to the plastic bottle (tank).
Place the plastic bottle above the collector.
2. To test the solar heater:
* Put cold water in your solar water heater, e.g. 4 litres.
* Record the temperature of the cold water.
* Put your solar water heater in the sun or use a strong lamp.
* Record water temperatures every 5 minutes for 1 hour.
* Calculate the amount of heat energy gained by the water in your heater in 1 hour.
Heat (Joule) = m × c × DT, where m = mass of water = (volume in cm3 for water), c = (specific heat, 4.2 Joule / g for water),
DT = temperature increase (final temperature - initial temperature).
How could you make improvements to increase the efficiency of your solar heater?
Calculate the efficiency of the solar collector:
Percentage Efficiency = (Energy input / Energy output) × (100 / 1).
Energy input from the sun falling on 1 m2 = 800 × number of seconds collector exposed to sun, J per m2.
Energy input after 1 hour in the sun = (800 × 60 × 60 J per m2) = (2, 880, 000 J per m2) = (288 MJ per m2).
Adjust this figure for the surface area of your collector.