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Electrostatic ExperimentsPicture shows the assembled electroscope with a 'counter electrode' - this is the foil sphere on the blue straw passing through the upright pillar
Some of the accessories: (left to right)
- green straw - test negative charge
- orange balloon and dropper for charged water drops
- yellow straw with drawing pin as proof plane
- rectangular foil 'cap' (alternative to blue can at top)
- card with cottons and fuse wire
- plasticene with pins
- pink balloon for source of charge
- foil sphere (ping-pong ball) with orange straw handle
The constructional and experimental details are supplied on the understanding that they are NOT COPIED or used for commercial benefit. The may be copied by individuals for private use, and by educational establishments for teaching purposes only. Please acknowledge the source of your information as Douai Abbey, Woolhampton, Berkshire. Please send an E-mail to the author, Rev Wilfrid Sollom OSB, at Douai Abbey to register your interest in case you need further advice which should be sought by e-mail.
Be assured that this is a serious instrument developed after years of teaching A-level Physics, and that with it electrostatics experiments can be convincingly demonstrated to a large audience.
DEMONSTRATOR
See Figure 1.
Try hair instead of pin in the 4mm socket.
Put a pin, head first, into the 4mm socket.
Charge the food BAG. Hold the bag at its edge.
Be gentle! Don't damage the BAG.
Draw the charged side slowly over the pin.
Remove the BAG.
Pointer remains deflected.
Put a finger between pointer and pillar.
Finger attracts pointer.
Also try between pointer and wire frame.
Let finger touch the pointer.
Deflection collapses to zero.
For 'hair' use 2 short cotton threads 2-3cm.
Hair waves at the BAG to collect charge.
Pointer is insulated so holds the charge.
Verify that deflection remains steady!
Finger is earthed so charge is lost.
Points may collect charge without touching.
Silent discharge and sparks may occur.
The pin represents a lightning conductor.
The charged polythene bag represents a cloud!
DEMONSTRATOR
If straw is uncharged - negligible effect.
Set a good deflection on the pointer. as in Expt.1
Hold a straw by one end, test with its other end.
Keep hand well away from pointer.
Put straw in the space between pillar and pointer.
If straw is gently rubbed/stroked first: repulsion!
Repulsion is the 'acid-test' for two like charges.
Discharge straw by a gentle wipe with damp cloth.
Repulsion means that two like charges are present.
This cannot happen by induction.
Attraction requires two unlike charges present.
This can happen in induction by only one charge.
The next Experiment investigates this.
How does distance affect force of repulsion?
A smaller distance d gives a greater force. Also force is greater if charge is greater, either on pointer Q1 or on straw Q2, or both!
(Coulomb's Law: force = Q1.Q2/d2)
DEMONSTRATOR
One attracts (loves!), other repels (hates!)!
Touch the BALLS together.
Pointer charged with good deflection.
Charge the BAG.
Place it, charge upward, on an insulating stand.
- e.g. inverted plastic cup. See Fig.2.
Holding end of handle, place a BALL on the BAG
Similarly touch second BALL on top of the first.
Remove second BALL.
Remove first BALL. Keep BALLS apart.
Test each BALL near the pointer.
Observe which ball does what.
Test again. Dead! Apart from slight attraction.
Earth the BALLS. Test again. Same - Dead!
Call charge on BAG 'negative'.
This will agree with plus and minus on batteries.
-ve BAG attracts +ve charge to first BALL,
and repels -ve charge to second BALL.
Separate the BALLS, charges remain.
Charges on BALLS are equal and opposite.
Charges neutralise when BALLS touch.
Why do the charges separate?
The two BALLS when touching become one conductor. The potential (voltage) must be the same at all points on a conductor or a current would be flowing (Ohms Law). But this is not a circuit. There is a 'field' (region of influence) or potential gradient (volts/metre) between the BAG and Earth,in all directions, so there is a potential difference between the two BALLS due to the field. This causes charge in the BALLS to flow momentarily until the potential difference is zero. The displaced charges set up their own fields which counteract the original field. This requires opposite sign charge on the BALL which is in the higher potential position to lower the potential there, and same sign on the further BALL to raise it there.
DEMONSTRATOR
Earth (touch )the CAP with one hand.
Earth the cap (touch it)
Repeat, leaving pointer deflecting.
Lay charged straw on CAP.
N.B. The earth does not have to be present all the time, but only momentarily when the charge is closest to the Cap. Deflection increases as charge approaches, collapses to zero when earth applied, and returns on removal of the charge source.
Fit the CAP in the 4mm Socket.
Zero the pointer.
Charge the BALLOON or BAG.
Slowly bring the charge close to the CAP,
(from above).
Pointer deflects more and more.
Remove the charge.
Pointer returns to zero.
Keep it earthed while bringing up charge.
No deflection of pointer.
Remove earth keeping charge still.
No deflection of pointer.
Remove the charge.
Pointer deflects and stays deflected.
Deflection goes to zero.
Test with charged straw.
Attraction! (Contrast lightning conductor)
Deflection reduces!
The CAP provides good capacitance to the source of charge. With an insulated CAP and pointer, induction causes opposite charge to move to the CAP and like charge to move to the pointer. This happens because the metal must all be at the same potential, but the metal is in the field of the external charge. Its average potential must become that due to the field.
N.B. No charge is transferred to the metal. Deflection indicates potential (difference between pointer and earth). So deflection returns to zero when field is removed.
While touching the CAP its potential must remain at earth potential (no deflection). But to keep the potential at zero, opposite sign charge has come from earth to the CAP to neutralise the field there (induced charge). If the earth connection is removed before the charge, the charge on the CAP becomes re-distributed over the insulated metal in the absence of the field. The pointer is now charged and has a potential of its own. The induced charge remaining is of opposite sign to the inducing charge.
The approach of the straw to the cap sets up a field which opposes that due to the charge there and lowers the potential: deflection reduces. The straw placed between pointer and earth increases the potential difference (or potential gradient - i.e. field strength) there and deflection increases.
DEMONSTRATOR
Recharge the CAP after each test.
Try a wooden ruler similarly.
Many plastics like the uncharged STRAW cause no appreciable loss of charge, though a temporary small drop in deflection may be noted.
Charge the pointer by any means.
Get a good steady deflection of the pointer.
Discharge it by touching the cap.
See the time for deflection to drop by half.
Hold a spill of paper at one end.
Touch the CAP with the other end.
Compare the time to half-deflection.
Try wire, cotton, polyester.
The capacitance of the instrument is about 10 pico-farads. The half life of the charge (50%) is similar to the 'Time Constant' (37%) which is the product of resistance and capacitance (T = R.C). Deduce that the resistance of a length of cotton is about 10,000 megohms if the half life is one second!.
The initial deflection does not have to be identical for each test. Half-life time is the same whatever the starting point of the decay.
The moment of inertia of the pointer, and particularly its balance adjustment, limit the speed at which it can respond to changes in charge, but the experiment is qualitatively good.
DEMONSTRATOR
Allow drawing pin to touch the pointer.
Gauge repulsion force by push of deflection.
Don't earth drawing pin between tests.
Compare repulsion forces for charge collected from CAP, wire frame, pivot pin, etc, testing sharp corners, ends, flat surfaces, pin points.
Repulsion force depends on charge sampled at each place. Total charge stays constant. Excess charge on drawing pin returns to pointer, or more is collected at each test.
Charge the pointer: good deflection.
Hold the Proof Plane by the end of handle.
Move drawing pin close to end of pointer.
Slight attraction.
N.B. The handle must be uncharged!
Repulsion between pin and pointer.!
Touch it onto different parts of the wire frame.
The slight attraction of any uncharged and insulated metal surface is due to induction. The separation of charges locates the opposite charge near to, and the like charge remote from, the charge. The attractive force is therefore much greater than the repulsive force, and the result is attraction.
The same is true for plastics and other insulators ("dielectrics") in a field. Due to polarisation (alignment of molecules with the field) the surfaces have induced charges which result in some overall attraction. It may be difficult to distinguish this from residual charges on plastics which are supposedly uncharged. Try the gentle damp wipe!
DEMONSTRATOR
Touch the FOIL.
Restrain the foil with a straw and
Pointer deflects again.
Repeat, testing with proof plane:
On the CAP place a layer of polythene sheet
On top of this put a square of FOIL.
Above this bring up a charge (BAG / BALLOON)
See Figure 3 for general arrangement.
Pointer deflects.
Pointer goes to zero.
remove the charge.
Earth the CAP.
Pointer to zero.
Remove FOIL: pointer deflects.
When first deflection is obtained -
pointer is -ve (test with STRAW)
Top of foil is +ve
Under foil, above polythene, is -ve
CAP (under polythene layer) is +ve
Induction occurs even in a FOIL!
After earthing the foil the attraction by BAG / BALLOON charge is greater, and FOIL may try to adhere on removal of the charge.
Effectively there are three capacitors in series: BAG to FOIL, FOIL to CAP, POINTER to EARTH. Potential difference across each is smaller if the capacitance is greater. The pointer deflects according to its share of the total potential. No charge is given to the instrument until the CAP is earthed.
DEMONSTRATOR
Bring a charged BAG / BALLOON close above foil.
No deflection of the pointer.
Support foil on polythene layer on another stand
Plenty of charge on top, none underneath foil.
Use the layer of polythene and foil as in Expt.7
Stick a length of fuse wire to underside of foil.
(use PVC tape for such attachments)
Connect other end of wire to EARTH.
(stick a drawing pin in top edge of a foot of stand,
and wind the wire round the pin)
Keep the wire away from pointer, cap and frame.
The earthed foil has screened it.
e.g. inverted plastic cup. Foil still earthed.
Charge the pointer with a good deflection.
Bring a charge close above the foil as before.
Test with proof plane the top and bottom of foil.
Maintaining the Earth connection ensures that the potential of the foil is always that of Earth.
Charge from Earth covers the side of the foil nearest the charge to counteract the field at its surface due to the other charge.
Screening increases the capacitance... more on that later.
A hand is an Earthed conductor. It may screen the CAP if placed in the space between a charge and the CAP, reducing deflection.
The hand's presence near an insulated charge increases the capacitance, lowering the potential.
Again less deflection.
A finger near the charged pointer will have induced charge attracting the pointer.
DEMONSTRATOR
Both pointer and CAN will have same sign charge.
Test edges, rim, side, bottom screw, inside.
Add a pin to the screw at the bottom of the CAN.
Support the CAN on its side on an insulating stand.
e.g. inverted plastic cup .. test its insulation!
The top of the pillar will do if nothing else.
Secure the CAN with small bits of plasticine.
Use the CAP and charge the pointer by induction.
Similarly charge the CAN on its stand by induction.
Test around the CAN with proof plane.
Strength of charge is shown by force of repulsion.
Nothing inside! (right inside, not just near rim!)
Stick its head to the side of the screw thread.
Use plasticine. Ensure metallic contact.
Point pin outwards from the bottom of the CAN.
Test with proof plane again after recharging.
Maximum charge is obviously on pin point.
The CAN is an excellent shape for performing Faraday's "Ice-Pail" experiments. It is sufficiently deep, and has no sharp external edges. Its top is curved towards the opening which reduces edge effects at the rim where the lid was removed. It is tested here first as an isolated hollow conductor to avoid any effects due to its being part of the instrument.
The inside of the empty CAN is a space in which the potential is everywhere the same.
The pear-shaped conductor illustrated in many text books can be tested in this way. The insulated stand must not leak away the charge appreciably during the experiment.
The addition of a pin to the pointed end (screw) of the can exaggerates the effect of most of the charge accumulating on a point and makes the demonstration more convincing.
DEMONSTRATOR
Remove the BALL carefully, not touching the CAN.
Test the BALL near the pointer.
Recharge the BALL from the BAG in same way.
Under good conditions pointer deflects about one scale division more for each BALL-full of charge.
Fit the CAN instead of the CAP in the 4mm socket.
Charge the BAG
Lay it charge side up on an insulated stand.
e.g. inverted plastic cup. See Figure 4.
Take a foil covered BALL by its handle.
Press it lightly onto the BAG with other hand.
You are Earthing it so inducing a charge on it.
Lower the charged BALL into the can.
Pointer deflects a little.
Touch the bottom of the CAN with the BALL.
No change in deflection.
No change in deflection.
Slight attraction only. BALL is uncharged!
Repeat: lower BALL into CAN.
Touch BALL to bottom of CAN.
Remove BALL without it touching CAN.
Pointer deflects a bit more.
Since all charge is removed from BALL each time, there is no limit to the charge that can be built up on the Pointer. But scale is not linear and deflection must limit at 90 degrees.
There may also be a limit set by corona (or silent) discharge from points and sharp corners of the pointer and wire frame.
This experiment demonstrates the progressive filling of a capacitor by a current flowing into it. BALL-fulls of charge per minute is the unit of current here. The BAG is not discharged by the induction process so each BALL-full should be about the same amount of charge.
DEMONSTRATOR
Pointer deflects. See Figure 5.
Earth the CAN.
Note that this provides an efficient way to get a charged pointer of which ever sign is required.
Fit the CAN in the 4mm socket.
Deflection of pointer at zero.
Roll up the BAG to make a tube of it.
Charge its outside surface by rubbing/stroking it.
Insert it into the CAN and leave it there.
Test pointer is -ve by using the STRAW.
Deflection goes to zero.
Remove the BAG.
Deflection returns, +ve this time.
Since the total charge on the BAG is placed inside the CAN, the induced charge is greater than that usually obtained by a charge placed near or on the CAP.
The outside of the CAN may also be charged by induction by bringing a charge near it in the same way as the CAP but this does not make use of the special property of the hollow CAN.
There is no nett charge inside the hollow conductor. If a charge is placed inside, there is an exactly equal opposite charge induced on the inside of the CAN. This can only have come from its outside, leaving there a charge equal to the one placed inside.
If the charge placed inside was carried on a conductor (as the BALL in the previous experiment) touching the two together inside results in a nett charge of zero inside, and the BALL can be withdrawn uncharged.
DEMONSTRATOR
The reservoir provided is a balloon with narrow neck thet fits the capillary tube. Any other container could be used, perhaps with a bored cork or binding of PVC tape to make a water-tight fit. A 'Fairy'-liquid plastic bottle is excellent! A toy water-can, kettle, teapot, etc might be considered.
Fit the polythene capillary tube.
The deflection is slow to start, but quite appreciable after 20-30 drops, if all is well! The charges on the DROPS are all transferred when they touch the inside of the CAN.
If water leaks from the screw at the bottom of the can, try tightening the nut or smearing a little plasticine around the washer. Or put the plastic cup into the can, and drip water into that.
CAN fitted, deflection at zero.
Fill the water dropper reservoir with water.
Dry its surface thoroughly.
Charge its tip by rubbing e.g.with cotton hankie.
If water gets on the outside, dry and recharge!
Gently drip DROPS (not a stream or dribble) into the CAN from about 15 - 30cm above it.
The water is charged by induction. The polythene tip induces a charge in the water (a conductor!). When the drop separates it carries the charge with it. If Earth does not supply the induced charge directly (as with balloon or plastic bottle), the capacitance between the hand holding the reservoir and the water inside, will become charged.
The analogy using flowing water for electric current is commonly used in explanations. Here the analogy is particularly relevant!
DEMONSTRATOR
Put the two BALLS inside the CAN, making the slack cotton fall inside first, leaving the handles of the BALLS protruding.
Mount the can on the 4mm socket.
It is essential that this deflection stays steady (a good dry day outdoors!)
Lift the Balls out carefully by their handles, and raise them apart above the CAN. See Figure 6.
Replace the BALLS inside the CAN with their cottons.
Original deflection is restored!
Attach about 30cm of conductive cotton to each BALL using PVC tape.
Attach the other ends of the cottons to the inside of the rim of the CAN with PVC tape.
Charge the CAN from its outside by induction, with a good deflection of the pointer.
Deflection reduces a bit.
When inside the CAN the BALLS have no charge. When outside it and connected to it, they share the charge on the CAN.
There is more room for the charge to spread out because the connected external surface area is greater. More room means less compression of charge, so less potential, and less deflection of pointer.
Total charge is constant. When the BALLS are replaced inside, all the charge on them again goes to the outside of the CAN.
The Capacitance of a conductor is a physical property depending on its shape and surface area, and the proximity of Earth.
If the BALLS on their cottons are brought down close to Earth their capacitance would be increased giving further reduction of potential.
Mention should be made of work done by the field in spreading charge over a wider area, and against the field by forcing the charge on the BALLS back into the CAN against the force of repulsion. If work is done by the field, the charge loses potential energy. If work is done against the field, the potential energy of the charge increases. Note the definition of the VOLT is a JOULE (of work) per COULOMB of charge.
The Capacitance of a conductor is defined by the amount of charged placed on it by a volt of electromotive force. The units are Volts (V), Coulombs of charge (Q), and Farads of capacitance (C). C = Q/V, or Q = C.V
The capacitance of the CAN, wire frame, pointer
assembly is about 10 picofarads, (10 x 10-12 farad).
This may be estimated by connecting a capacitance that roughly halves the deflection of the pointer. Insert the 6cm wire into the polythene capillary. Wrap some foil, 4cm wide, round the middle of the polythene to make a 3pF coaxial capacitor. Hold the foil. Touch the wire to the CAP or wire frame.
DEMONSTRATOR
Earth the BALL by a cotton or wire thread to a drawing pin in the top edge of one of the feet of the stand. When the pointer deflects it touches the BALL and earths itself so goes back to zero.
Go through the process of charging the CAN by induction placing the rolled-up BAG inside it.
As the charged BAG is inserted slowly into the CAN, the pointer 'ticks' against the BALL maybe 10 or more times. With a wire Earth to the BALL, the ticking stops when movement of the BAG stops. With cotton Earth, the time constant delays the ticking for some seconds after movement stops.
Remove the BAG and the ticking repeats as the induced charge on the CAN flows (in ticks) back to Earth.
Thread the handle of a BALL through the holes in the pillar of the stand roughly opposite the end of the pointer. Adjust the BALL so that it is about 1cm from the pointer
This is a vivid demonstration of the movement of the induced charges. Current is here measured in 'ticks' per second. The voltage reached before discharge of the pointer (set by the gap) and the capacitance of the instrument, determines the amount of charge transferred per 'tick'. One tick per second is probably about 20 millionths of a milliampere!
DEMONSTRATOR
Slowly and steadily drip charge into the CAN.
Set up the pulse electroscope, with very small gap.
Prepare the water-dropper to give charged drops.
As long as charge flows into the CAN, charge 'ticks' out at the pointer.
Here is a demonstration of continuous current flow using equipment for static electricity. Really there is nothing 'static' unless there is no physical movement of charge, and time constants have been fully satisfied. The use of cotton threads to slow down effects, and of extremely good insulators for eliminating leakage, results in a convincing set of demonstrations.
revised 7/7/02 by WS
Wilfrid Sollom OSB, Douai Abbey, Upper Woolhampton, Reading, Berks. RG7 5TQ