H. Bradt, 37-581, M.I.T.
ELECTRIC CHARGES
1
H. Bradt, 37-581, M.I.T.
X1. Two kinds of charge: Tinsel on rod - 1W
Purpose: Show creation of charge by rubbing amber with fur, glass with silk. Show repulsion of like
charges and existence of 2 types of charge.
Equipment: Amber and glass rods, fur and silk, tinsel hanging from stand.
Procedure:
Charge amber rod negative (a) by rubbing with fur;
Show charge by touching rod to tinsel; tinsel flares out (c)
Recharge amber, bring close to tinsel, rod repels tinsel (d)
Bring fur close to tinsel, fur (positive) attracts it;
Charge glass rod positive, bring close to tinsel (still charged); glass rod attracts it.
Shows 2 kinds of charge
HB touch tinsel to discharge
Repeat with glass-silk: charge tinsel positive by touchingwith rod, etc.
(Glass harder to charge; should be clean; always pick up at same end; use clean silk.)
X1
'Christmas-tree'
tinsel
Amber
rod
(a)
(b)
TOUCH!
(c)
(d)
2
H. Bradt, 37-581, M.I.T.
X2. Coulomb attraction & repulsion; induction; Balloon and Charged Rod - 1W
Purpose: Demonstrate induced charge on conducting balloon; show Coulomb forces.
Equipment: Conducting balloon filled with helium; aluminum sprayed outside and graphite inside;
fine conductor connects outside to inside(?); balloon preparation is non-trivial. Glass and amber rods
with silk and fur.
Procedure:
Charge amber (plastic) rod with fur.
Rod attracts balloon (induction)
Balloon touches rod , then balloon repelled (maybe)
Can be flaky; I've always wondered how conducting the surface of Al sprayed paint is.
(Is it covered w. thin coat of non-conductor(?); probably not.)
Let balloon bump rod several times at different places on balloon; then works OK.
Ref: wl video V1, Tape 1; 01:00:23
_____________________________________________________________________
3
H. Bradt, 37-581, M.I.T.
_____________________________________________________________________
4
H. Bradt, 37-581, M.I.T.
5
H. Bradt, 37-581, M.I.T.
6
H. Bradt, 37-581, M.I.T.
X3. Electroscope, 2 kinds of charge, induction - 1W
Purpose: Show operation of electroscope with charged rods.
Equipment: Leaf Electroscope (LE) with amber and glass rods; 'scooper' (small piece of sheet metal
on plastic handle).
Procedure:
Bottom of LE grounded - brings ground closer.
Charge LE directly by 'scooping' charge from amber rod with flat 'scooper' (touching rod to LE
directly will blitz the LE); this shows rod charged; leaf moves up.
Discharge LE by touching it; leaf collapses
Explain following steps carefully with 2 transparencies (reproduced here).
Charge by induction:
(a) rod neutral
(b) bring rod close to LE do not touch LE);
(c,d) touch shaft of LE (not knob) briefly with finger;
(e) move rod away, leaf opens (charged positive)
bring rod close again, then away, close, etc. (leaf closes, opens, closes etc.).
(If rod closer than it was when touched by finger, get opposite charge and leaf opens)
Discharge LE by touching.
Charge neg.directly (again) with amber and scooper
(f) Bring up glass rod: get more charge.
Alternate bringing up amber and glass rods
See 2 kinds of charge; leaf descends with amber, rises with glass.
(a)
(b)
7
H. Bradt, 37-581, M.I.T.
8
H. Bradt, 37-581, M.I.T.
(c)
9
H. Bradt, 37-581, M.I.T.
10
H. Bradt, 37-581, M.I.T.
(d)
(e)
11
H. Bradt, 37-581, M.I.T.
(f)
12
H. Bradt, 37-581, M.I.T.
13
H. Bradt, 37-581, M.I.T.
14
H. Bradt, 37-581, M.I.T.
X4. Number of electrons to repel pith balls - 1W
Purpose: Show charge repulsion between two pith balls and calculate number of electrons required.
Compare Coulomb force to gravity.
Equipment: Two Pith balls (or conducting ping-pong balls) hanging on ~1 meter strings from
common point; each is a pendulum. Amber rod and fur.
Procedure:
Arc light to shadow project them
Charge pith balls with amber rod
Note length of string and angle (~ 100)
Calculate # of electrons:
Ans: ~ 1 x 1012 electrons on each ball for 10o (angle from vertical) and m = 1 gm
Compare to number of atoms (protons) in ball, and note gravity between balls is negligible.
Experiment balances Coulomb force against gravity of entire earth!
Conclude: Coulomb force very strong compared to gravity
1987: Did not seem to work well - must experiment with different pith balls, or
use the 2-ball assembled apparatus with conducting wires and charging knob on top
(ketch) which works quite well. The balls are ping-pong (?) balls, painted with
conducting
paint..
B4.
Apply charge here
15
H. Bradt, 37-581, M.I.T.
X5. Static Discharge with Human - 1W
Purpose: Demonstrate buildup of static charge when 'beating' student with fur by discharging high
voltage through fluorescent lamp.
Equipment: Plastic stool, glass plate, fur, 2 ' home fluorescent lamp
Procedure:
Student (S) sits on plastic stool and holds tinsel to show charge
Lecturer (L) stands on plastic or glass slab, also holds tinsel.
L hits student repeatdely with fur; tinsel flares out as both charge up.
L and S touch hands; tinsel discharges. (very effective)
Dim hall lights to initiate dark adaption
L picks up 2 foot fluorescent lamp in one hand and beats S again until S charged up (tinsel
flares),
Turn hall lights off completely; wait a bit for dark adaption
L holds lamp toward S who touches opposite end.
Bulb flashes; discuss voltage required for breakdown.
Ref: wl video V2: tape 1, 01:02:14; note changes: tinsel, slab, and large tube.
X5
Tinse l
Tinse l
Fur
Pla stic stool
Glass Plate
16
H. Bradt, 37-581, M.I.T.
ELECTRIC FIELDS
17
H. Bradt, 37-581, M.I.T.
X6. Electric Field Lines Shown; EM Waves Anticipated; Movie - 1F
Purpose: Illustrate graphically he 'existence' of field lines.
Anticipate subsequent development of course (e.g. transverse EM waves)
Equipment: Movie #20 "Electric Fields of Moving Charges?" Constant Velocity, Starting and
Stopping, Sinusoidal Motion. (Do not show Circular Motion.)
Procedure:
Show Movie #20: Moving charge: v = const., starting, stopping.
Shows electric field lines; "you can see electric field lines!"
Note relativistic bunching of field lines ("will not be covered in course")
Note kinks in field lines due to accelerations
Field Lines point back to location of charge assuming constant velocity.
Kinks result in 'transverse' E vectors moving outward.
18
H. Bradt, 37-581, M.I.T.
X7. Dipole Electric Field with Sisal Fibers - 1F DANGER! EXPOSED HI V0LTAGE
Purpose: Show the form of the dipole electric field by orienting floating fibers with a strong electric
field.
Equipment: Dish, carbon tet, mineral oil, 2 terminals (flat cylinders), adjustable AC high voltage
supply (variac and transformer, >1000 ? volts), sisal fibers (snipped from a rope with scissors),
overhead projector.
Procedure:
Sisal fibers at carbon tet - mineral oil interface
Dish sits on projector so fibers are shadow projected.
Dish about 1/3 full of Carbon Tet and 1/3 full of min. oil.
Terminals buried in liquid - at interface where sisal is.
High AC voltage on terminals provides dipole field
(DC does not work well!; net charge on fiber will repel or attract?)
Two terminals constitute dipole
Turn on voltage, tap dish; fibers align along dipole field.
Why fibers align is not clear; induced dipole moments?
(This can be quite effective, but effect depends on depth of mineral oil and carbon tet, height
and
spacing of poles, and fiber lengths; must optimize or will be disappointing!)
Could repeat with same sign charge on each terminal (Lewin did this.)
Ref: wl video V6; tape 1; 01:13:21
19
H. Bradt, 37-581, M.I.T.
X7
Sisal fibers
Mineral
oil
SIDE VIEW
Carbon tet.
Electrodes
Dish
Sisal fibers
TOP VIEW
20
H. Bradt, 37-581, M.I.T.
X8. Charges, Electric Fields (and Operation of Van de Graaff Generator?) - 1F
Purpose: Show effects of a very high voltage generator (~100,000V). Presented as a 'black box'.
(Principle of operation to be presented later; experiment X18.)
Equipment: Small desk-top van de Graaff generator (radius ~ 10 cm), confetti, bundle of tinsel
Procedure:
First introduction of van de Graaff; chose small one as build up to large one to be seen later.
Place onfetti (paper) on top, turn on, confetti flies off (a)
Lecturer L holds tinsel, stands on glass plate, touches v deG; tinsel flares up, shows charging.
(I wouldn't do the latter with the large generator.)
Remove hand from vdeG, tinsel shows L still charged.
Step off glass plate onto floor; tinsel immediately collapses. (May depend on floor and shoes.)
Entire generator must be very very clean; otherwise will not charge up
Small VdeG is good prelude to later introduction of large VdeG.
See demo X18 for principle of VdeG
Ref: wl video V3 and L4; tape 1, 01:05:10 (uses large VdeG and tinsel flakes; confetti better)
21
Confetti (pa per)
H. Bradt, 37-581, M.I.T.
Van de Gra aff
Sm all
va n de Graaff
Tinsel
Glass pla te
22
H. Bradt, 37-581, M.I.T.
X9. Torque on Electric Dipole; small Van de Graaff gen. - 1F
Purpose: Demonstrate torque of field on a dipole using small Van de Graaff generator and dipole
hanging from a 'fishpole'.
Equipment: Small van de Graaff gen. (with small grouned ball removed); dipole of two styrofoam
spheres (r ~ 4 cm) covered with conducting paint (one gold and one silver) hanging from a 'fishpole';
shorting wire mounted on stand at height of vdeG sphere and close to sphere, but not too close (avoid
breakdown).
Procedure:
Charge dipole in field of VdeG by resting dipole on shorting wire.
Lift dipole free of shorting wire; remove shorting wire (may not be necessary)
Show torque by moving dipole all around VdeG at height of sphere.
Same pole will always be oriented toward VdeG.
If attracted pole gets too close to VdeG sphere, voltage arcs over
Changes sign of dipole; that pole immediately is repelled.
Demonstration follows discussion of torque in a uniform field
(Did not derive torque for a dipole in a radial field
vdG must be very very clean to develop high voltage.
Ref: wl video V5, tape 1; 01:10:05
23
H. Bradt, 37-581, M.I.T.
X9
Plastic tube
Strin g
Small
Van de Graaff
plastic ro d
Styrofoam
balls w. con ductin g
paint
Shorting wire
( sh ould be at same
level as Vd eG sphere)
24
H. Bradt, 37-581, M.I.T.
X10. Dipole Field lines and Charging of Balloon - 1F
Purpose: Demonstrate motion of constrained (with string) conductor in a dipole field; balloon
bounces between VdeG and human head.
Equipment: Small van de Graaff, helium-filled conducting balloon on string; grounded human head.
Procedure:
Place head about 3 feet from vdeG sphere and hold onto grounding sphere; head is one pole
Balloon on string between 'poles'.
Balloon is attracted to closest pole (by induction), picks up opposite charge, is repelled to other
pole, picks up original charge, moves back to original pole, etc. back and forth etc.
Can control balloon direction by moving head during bounce.
Balloon can go to side and 'follow' curved field lines. (not so convincing - lots of intertia)
Non-trivial to do this well; must try it and optimize.
The corn may outweigh the so-so clarity (I skipped this in 1986,7)
Ref: wl video V7, tape 1; 01:15:47
25
H. Bradt, 37-581, M.I.T.
Helium-filled
conduc ting ba lloon
X10
+
(a)
+ +
+
+
+
+
+
+
+
+
Neutral polarized
ba lloon m oves
toward v de G and
bounce s off it,
picking up c harge .
Grounded sphere
(b)
+
+
+
+ +
+
+
+
+
+
+
+
+
Grounded sphere
26
Charge d balloon moves
towa rd forehe ad, lose s
charge and 'bounc es' ba ck
toward V de G
H. Bradt, 37-581, M.I.T.
X11. Ping Pong Ball Bouncing Between 2 Spheres: Induction, Charge Transfer - 1F
Purpose: Fascinate students with seemingly 'magic' demonstration of induced charge and coulomb
attraction. A suspended ping-pong ball oscillates between two conducting spheres as charged amber
rod induces charge.
Equipment: Two conducting spheres (r ~ 4 cm), 'A' and 'B', on stands; ping pong ball (PPB) with
conducting paint hanging with thread between spheres. Amber rod and fur.
Procedure:
Neutralize spheres and PPB by touching them; charge on each is zero.
Let PPB come to rest
Bring up charged amber rod close to outside of one sphere (A) and hold it there during next
steps.
In field of charges on rod, spheres and PPB have induced surface charges
PPB pulls slowly toward sphere of largest polarization (A), touches it, gets charged neg.,
is repelled back toward B, hits it, picks up + charge, and is then repelled back to A PPB
thus bounces very rapidly back and forth between 2 spheres with dribble sound.
Oscillations cease when potentials are nearly equalized. (Rod is still being held close to A.)
Analog: The PPB momentarily shorted (incompletely) the spheres.
Each sphere now has a net charge and PPB is at rest or nearly so.
Now move rod away from A
PPB again pulls toward one sphere and then bounces rapidly back and forth between spheres.
Bouncing stops when spheres back to charge zero (almost).
Move rod into close position again; ball starts bouncing again.
Repeat, again and again, moving rod in and out; PPB does its thing each time.
Seems magical: spheres never touched by human hands or battery!
Explain with a transparency (I only made a rough sketch.)
[Leaf electroscope can be attached to one sphere to show potential changes, but this is another
complicating and confusing element; [just where is the charge?); I did not use it.]
[The PPB may seem recalcitrant to pull over to sphere the 1st time; must hold rod very close to
sphere. Hanging PPB a bit closer to sphere A helps. Thereafter, the rejuvenation of the
motion is easy because of the residual motion of the PPB pendulum and possibly the r
esidual charge on the PPB.]
Excellent demonstration: a seeming puzzle.
27
H. Bradt, 37-581, M.I.T.
B11.
_
_
_ _
_
_
_
A
Ball pulls to left
B
_
_
_ _
(It touches A)
28
_
_
_
A
B
Ball bounces back and for
between A and B
th
H. Bradt, 37-581, M.I.T.
GAUSS'S LAW
29
H. Bradt, 37-581, M.I.T.
X12. Principle of Electroferous and Charged Pith Balls - 2M
Purpose: Explain operation of the electroferous; we will use it for Gauss's Law demonstrations;
Equipment: Electroferous complete with metal plate with insulated handle, plastic base and fur. Two
pith balls hanging from threads. Shadow project pith balls.
Procedure:
Explain operation of electroferous with hb transparency
Rub plastic with fur, put metal plate on it (charges induced), short it with finger (leaving net
charge on plate), and then lift (charged) plate away. [Assumes leaky shoes and floor.]
Charge pith balls; see them bounce around and repel each other - like 2 kittens playing.
Shadow projected pith balls quite dramatic.
Can charge an object to high potential by repeatedly charging plate and touching it to the object.
[Why doesn't charge flow off plate onto plastic? Not so obvious. Our plate does not have
insulator coating on lower side. Possibly because most of plate not in direct contact with
plastic.]
30
H. Bradt, 37-581, M.I.T.
31
H. Bradt, 37-581, M.I.T.
32
H. Bradt, 37-581, M.I.T.
X13. Electric Field Outside and Inside Metallic Sphere - 2M
Purpose: Demonstrate that the field inside a charged sphere is zero and that it is finite outside as
predicted by Gauss's Law.
Equipment: Hollow metal sphere (r ~ 10 cm) with hole at top with plastic sleeve around opening;
projected electroscope (ES); 2 small brass spheres (BS, d = 1 cm) on plastic-rod handles;
electroferous (EF).
Procedure:
Charge sphere with electroferous; charge EF and touch it to sphere; repeat several times
(a) Show there is a field outside sphere
Neutralize BS by touching them both.
Touch BS briefly together outside of, but very near (~2 mm), large sphere;
The 2 BS are positioned radially when making contact; see sketch (a).
In presence of field, the BS are charged by induction
Touch them one at a time to ES: find they are oppositely charged.
Leaf rises when 1st touches and falls when other touches.
[If leaf motion is not substantial, hold BS closer to sphere when making contact and try
reversing order of touching BS to ES.
(b) Show E = 0 inside sphere
Touch brass spheres inside hollow sphere to charge by induction (b,c)
Contact should be off center to verify no field everywhere inside.
Maybe students can hear click of touching through mike
Must avoid arcing while putting spheres inside (takes a bit of practice)
Insert one BS into sphere at a time.
Touch brass spheres to ES one at a time; find no charge (hopefully)
If BS were charged; breakdown occurred; try again.
Ref: wl video V9, tape 1; 01:23:00
33
H. Bradt, 37-581, M.I.T.
X13
(a)
(b)
+
+
+
+
+
+
+ +
+
Hollow m eta l sphere
with hole mounted on
insulator and charged
with elec trofe rous.
+
Two sm all
bra ss spheres
on insulating
rod
34
(c)
H. Bradt, 37-581, M.I.T.
X14. Poster with Gauss's Law - 2M
Poster with Gauss's Law on it hidden behind lab desk
Pull to top of hall via string over a high horizontal rod.
(Have poster and magic marker available earlier; I'll draw it.)
Congratulate class on knowing 1st Maxwell Equation.
35
H. Bradt, 37-581, M.I.T.
X15. Charge Inside and Outside Metal Enclosure of Arbitrary Shape - 2W
Purpose: Demonstrate field of a charged bucket of arbitrary shape is zero inside and finite outside.
Show that the surface charge is zero inside and finite and outside.
Equipment: Bucket with foil cover containing hole, 2 metal spheres on rods, electroferous, scooper
(small round piece of sheet metal on plastic handle), electroscope (projected).
Procedure:
Similar to X13 above
Charge bucket with electroferous
Use 2 spheres to show (for any crazy shape)
E field outside is finite
E field inside is zero
Use scooper to "scoop" charge from inside and outside
Touch scooper to electroscope to show
finite charge outside
no charge inside
[Not a trivial experiment; must practice.]
(I never got around to this in 1987. )
Refs: . Expt. X13; wl video V13, tape 1, 01:36:32 [uses small Van de Graaff to charge bucket; also
shows different signs of charge on opposite sides of (charged?) bucket when bucket
is in field of
VdeG but not in contact with it. I did not get the latter to work.)
X15
Al foil
(a)
(b)
brass
spheres
(c)
(d)
Scooper
Plastic
handle
36
H. Bradt, 37-581, M.I.T.
ELECTRIC POTENTIAL
VAN de GRAAFF GENERATOR
HIGH VOLTAGE BREAKDOWN
37
H. Bradt, 37-581, M.I.T.
X16. Work and Potential; 'External Agent' - 2F
Purpose: Demonstrate the concepts of work and potential. First introduction of Potential. Distinguish
work done by electric field and work done by an 'external agent'.
Equipment: Large van de Graaff (not turned on); ball painted red or orange with prominent +Q on it
(to represent charge); sign labeled 'EXTERNAL AGENT' to be worn over shoulders; black paper
mustache to be stuck to upper lip.
Procedure:
Van de Graaff is symbol of high potential; assume that it is charged with plus charge.
VdeG never turned on (We'are leading up slowly to actually using it (next lecture).
Put on sign and mustache. (Mustache makes lecturer look like 'secret agent'; get it?)
Carry ball (+Q) toward 'plus' charge on VdG; look like it is hard work.
Point out that work done by lecturer (external agent) is positive; that by E field is negative.
from definition of work and noting relative directions of force and displacement.
Point out that potential energy change is negative of work done by field;
thus potential energy change is positive.
Could then carry ball away from vdeG and discuss again; all signs are reversed.
Leads directly to idea and definition of electric potential: potential energy per unit charge.
[The cornball sign etc. is meant to dramatize the distinction between the work done by the
electric field and that done by the 'external agent'. Students are often very casual about signs in
defining work, potential, etc. Also, it is easy to go too fast through any discussion of this topic
because of this confusion.]
X16
+Q
EXTER NAL
Large
va n de Graaff
Gene rator
38
AGENT
H. Bradt, 37-581, M.I.T.
X17. Charge Bucket to 30,000 V; Principle of Van de Graaff Generator - 2F
DANGER! EXPOSED HIGH VOLTAGE
Purpose: Illustrate principle of the Van de Graaff generator by carrying charge repeatedly from a
4000V
supply to the inside of a metal cavity.
Equipment: Metal sphere (r ~ 5 cm) with plastic handle; metal bucket; 4000 V DC supply; meter to
measure potential of bucket or field from it with scale projected.
Procedure:
Touch sphere to 4000 V supply and then touch it to bucket
(a) First, carry charge with many passes from 4000V supply to outside of bucket
Note rise of meter to a fixed value to a maximum value beyond which it will not go.
This value is 4000V; this serves to calibrate the meter (unless previously calibrated).
Short bucket to ground; show meter goes back to zero (hopefully).
(b) Repeat procedure, but carry charge to inside of bucket
Meter goes much higher than before, possibly up to ~30000 V
Again, short bucket to see if meter goes back to zero. (Sort of, as I recall.)
Illustrates van de Graaff principle
[Tried to calibrate meter by shorting 4000V supply to bucket; did not work (why?)]
Watch out for meter charging up, zero shifting; short bucket and zero meter just before starting,
carry out charging rapidly, and then short bucket and check zero again. Some rise is
tolerable.
Used transistorized 'E-field meter' in 1987, could not get to 30,000 V
Good demo despite problems.]
Refs: wl video V25, tape 2; 02:23:13
hb transparency of VdeG schematic taken from an old book (Black?)
X17
Large brass
sphere on
plastic handle
'F ield Meter'
Sh orting Lead
40 00 V
Plastic
Stoo l
P.S.
39
H. Bradt, 37-581, M.I.T.
X18. Large Van de Graaff Generator Demonstrated (First Time) - 2F
CAUTION! VERY VERY HIGH VOLTAGE - ALBEIT LOW CURRENTS.
Purpose: Show large van de Graaff generator in operation.
Equipment: Large v de Graaff generator, confetti.
Procedure:
Explain principle of vdeG if not done before:
Put confetti on top of large v de Graaff
Turn on v de G and watch confetti fly off.
Develop sparks, turn lights off, etc.
Voltage deduced from length of spark drawn.
~ 200,000 V for 110 mm spark for 10 cm diam. ground probe (check its diameter).
(ref. HdbkChemPhys 'Spark Gap Voltages')
Breakdown in kV/mm a function of electrode diameter and actual voltage.
At 30 kV, ranges from ~ 3 kV/mm for spheres to 1.1 kV/mm for Needle point
At 200 kV, "
" 2.6 kev
"
"
" 0.56 kV/mm. " "
"
Ref: : wl video V3, tape 1, 01:05:10
Experiment X8 (small v deG demonstrated)
Expt. X21: "Breakdown, Lightning, etc."
SEE FIG. X8 FOR CONFETTI
40
H. Bradt, 37-581, M.I.T.
X18
ZO T!
Aluminum rod
Van de Graaff
Ground
strap
41
H. Bradt, 37-581, M.I.T.
42
H. Bradt, 37-581, M.I.T.
X19. Large Potential Differences near Large VdeG generator - 2F: CAUTION!
Purpose: Demonstrate that electric potential differences exist in space . Potential difference is created
by v de G, and it is detected with a long fluorescent tube.
Equipment: Large v de Graaff generator; long fluorescent tube
Procedure:
(a) Turn lights out; hold tube toward VdG; tube lights up (very dramatic)
Shows large potentials (breakdown voltage in tube is ??)
(b)Tube can be suspended horizontally from string and spun
It flashes on and off twice each rotation (on when oriented radially).
[What is return current path in this case? Maybe none required if light flashes only momentarily.
CHECK VIDEO TAPE OR TRY IT; I had forgotten about this part so never did it.)]
Ref: wl video V12, tape 1, 01:32:27 (also uses small neon tube).
43
H. Bradt, 37-581, M.I.T.
X19
String
Tu be sp in s
abo ut vertical
axis.
Fluore scent
tube
Van de Gra aff
Ground here
is safe r
Grounding
thru shoes
may be OK;
bu t do leave your
pacemaker h ome
44
H. Bradt, 37-581, M.I.T.
45
H. Bradt, 37-581, M.I.T.
X20. Charge Density on Equipotential a Function of Curvature - 3W
Purpose: Show that charge density is greater on surface of a metal at places of highrest curvature and
vice versa.
Equipment: Ice-cream-cone shaped metal on insulated stand, electroferous, scooper, electroscope
Procedure:
Charge cone with electroferous
Scoop charge off rounded (pointed) end.
Touch e.s. to get measure of charge density.
Difference very pronounced.
Ref: wl video V15, tape 1, 01:46:31 (used VdeGraaff and cooking pan.)
B20.
46
H. Bradt, 37-581, M.I.T.
X21. Lightning, Breakdown near Spheres, Corona Discharge, Lightning Rod - 3W
CAUTION: MORE VERY VERY HIGH VOLTAGE
Purpose: Demonstrate that fields are strong near a metal surface with small radius of curvature
and weaker for metal with large radii of curvature. Show principle of Lightning Rod.
Equipment: Large Van de Graaff; long metal rod with threaded end to accept spheres and pointed tip;
2 metal spheres (r = 25 mm and 50 mm) and metal pointed tip, each threaded to permit attachment to
rod;, Braun electroscope meter off to side ~ 2m from VdeG (shadow projected and grounded?)
Procedure:
Electroscope shows charge and discharging of VdeG.
Rod is attached to ground strap of v de Graaff
(a) Attach larger ball (50-mm) to rod.
Move 50-mm ball slowly toward v de Graaff
Draw sparks from vdeG with 50-mm ball.
Note distance between vdeG sphere and 50-mm ball when breakdown occurred
Projected meter shows that potential of vdeG drops suddenly at breakdown
(b) Repeat with smaller (25-mm) ball
Note that sparks are drawn from larger distance
Discuss physics: at same potential, surface charge is greater (more curvature),
hence field is greater, and breakdown more likely.
[Argument seems a bit glib; should explain in terms of charges that
are induced on sphere in field of VdeG. (How much
charge is required on each side to make sphere an equipotential,
as a function of radius?; I have not worked this out)]
(c) Repeat with pointed rod; never get breakdown because of corona
Meter shows potential of vdeG being dragged down.
(d) Hold point of tip just in front of nose and walk up to VdG; it is safe!
Lecturer can see blue corona in dark, but too faint for students to see
(Maybe those in front row can see it; still worth doing anyway.)
Breakdown voltages as function of electrode diameter and distance in air range from
0.5 - 3 kV/mm; see values given in Expt. X18 notes.
[I did not use Braun electroscope in 1987 because I forgot about it. (It might have given
difficulty
in previous years - but a good idea.]
Ref: wl video V16, tape 1, 01:48:42
REF FIGURE X18
SEE FIGURE NEXT PAGE
47
H. Bradt, 37-581, M.I.T.
Sh arp po in t
r = 25 mm
X21
r = 50 mm
ZOT!
B rau n?
Electrosco pe
B lu e coron a
B rau n?
Electrosco pe
48
H. Bradt, 37-581, M.I.T.
49
H. Bradt, 37-581, M.I.T.
X22. Faraday cage near Van de Graaff with Lecturer and Radio Inside - 3W
CAUTION: VVH VOLTAGES
Purpose: Illustrate the Faraday cage and shielding of electric fields.
Equipment: Large v de G generator, large metal cage with door, rod and bunches of tinsel attached;
small portable radio; additional bunch of tinsel; helper to run van de Graaff.
Procedure:
Cage need not be grounded; it sits on floor (or on insulators?)
(a) Turn on VdeG.
Show sparks jumping across to cage (extend rod on cage toward VdeG)
Tinsel outside flares up
Turn off VdeG
(b) Turn portable radio onto AM station
Lecturer gets in with tinsel in hand and playing radio on AM, and with room mike on.
Radio stops working, but microphone xmitter works! (higher freq.)
(c) Turn on VdG, get sparks, etc. Tinsel inside sees no field. Lecturer is safe!
Ask assistant to turn off VdeG so lecturer can step out;
he refuses (for awhile) to the amusement of the students.
Ref: wl video V14; Tape 1, 01:42:30
ZO T!
Rod
X22
Tinsel
inside
Tinsel
outside
Radio
Tinsel
in hand
50
H. Bradt, 37-581, M.I.T.
CAPACITORS
51
H. Bradt, 37-581, M.I.T.
X23.Wimshurst Machine - 3F
Purpose: Demonstrate the generation of high voltages witth the Wimhsrst Machine because we use it
to charge capacitors in the next demonstrations. Also, it contains two capacitors, the role of which can
be explained.
Equipment: Wimhurst Machine (WM); transparency showing principle and operation of WM
Procedure:
Point out parts that were shown schematically on the transparency
Disconnect the jar capacitors in the WM
Crank handle to get sparks across the gap
Connect the jar capacitors and crank again to bet BIG sparks.
Discuss role of jar capacitors (can anticipate role of dielectrics in capacitors)
Ref: hb transparency taken from old book (Black).
NO FIGURE (YET)
52
H. Bradt, 37-581, M.I.T.
X24. Electric Field Lines Inside and Outside a Parallel-plate Capacitor - 3F
Purpose: Demonstrate shape of field lines of a parallel -plate capacitor (sort of) with conducting ping
ball bouncing back and forth inside and outside a large capacitor.
Equipment: Large parallel-plate capacitor (r ~ 0.12 m); conducting ping-pong ball hanging on thread
between plates (a pendulum).
Procedure:
Separate plates about ~10 cm (or less?)
Charge capacitor with Wimshurst Machine
(a) Ball bounces very rapidly back and forth between plates with much noise
Ball picks up + and - charges alternately at the 2 plates
(b) Start ball bouncing on outside edge of cap.
Ball appears to follow fringing field lines (sort of) outside of cap.
[This is a nice dramatic demonstration, but how closely it follows
field lines is a good question; inertia still lives!]
Ref: wl video V10; tape 1, 01:29:06
Experiment X10; balloon following field lines of a dipole.
X24
Capacitor
Ping Pong ball
Wim shurst
Machine
53
H. Bradt, 37-581, M.I.T.
X25. Potential Increase Across Charged Isolated Capacitor as Plates Separated - 3F
Purpose: Show and explain the increase of potential across isolated parallel-plate capacitor as plates
are separated.
Equipment: Large parallel-plate capacitor (r ~ 0.12 m); leaf electroscope (LE).
Procedure:
Charge plates to ~1000 V with spacing d ~ 2 mm(?); one side at ground
(a) Separate plates gradually to d ~ 8? mm.
Note that potential (measured by LE) does not change (seems reasonable)
(b) Remove charging leads (plates isolated)
Separate plates gradually to d ~ 8? mm.
See potential increase during separation with electroscope
(Q is same, thus E same; V = Ed gives increase in V)
Must explain why LE measures 'potential'; charge on leaf is a function of potential.
Stored Energy increases (1/2 CV2) - from work done in separating plates.
Work to increase energy was done by lecturer who created more volume of electric field.
Recall energy in field is (1/2 o E2)V where E is constant during movement of plates.
Ref: wl video V18, Tape 2; 02:00:00
X25
Capacitor
v
H.V.
1000V
Electroscope
54
H. Bradt, 37-581, M.I.T.
X26. Energy in Capacitor; Flash a Light Bulb - 3F CAUTION: CAPACITOR
CONTAINS
A LOT OF CHARGE AT AN INTERESTING VOLTAGE!
Purpose: Demonstrate the large stored energy in a capacitor by discharging it into a light bulb
Equipment: 1000-µF capacitor consisting of ten 100-µF capacitors mounted on a board for easy
visibility; 120V, 100W? light bulb; adjustable 300V DC power supply; calibrated voltmeter (visible to
students); two switches; circuit put together as shown.
Procedure:
Open switch A
(a) Charge 1000 µF - 500V Capacitor to 100V by closing switch B
Students see voltage climb on digital meter
When charged, open switch B
Dump energy from charged Cap. into light bulb by closing switch A
Light flashes brightly
At 100V, U = 1/2CV2 = 1/2 10-3 104 = 5 Joules
(b) Charge capacitor to 300 V, repeating procedure.
Light flashes very bright and burns out (maybe)
At 300 V, U = 45 Joules
[ It would be nice to arrange varibles so light burns out for sure at 300 V (try lower wattage
bulb?); as it was, it usually burned out on the 2nd time it was pulsed with 300 V. For this
reason, it is important to use a fresh bulb for part (a).]
Ref: wl video V19, Tape 2; 02:02:27
X26
Bulb (120V, 100 ? W)
A
1000 µF - 500 V
B
V
Po wer
Su pply
100V or 300 V
Open switch B before clos
ing switch A
55
H. Bradt, 37-581, M.I.T.
X27. Strobe Light Demonstrated - 3F
Purpose: Demonstrate use of capacitors for strobe light (and camera flash?)
Equipment: Commercial strobe light; camera with flash (if desired)
Procedure:
Run strobe light at different rates with lights down
Lecturer can move in flash; gets laughs
Explain that flash comes from a charged capacitor (mention value?)
Charging takes much longer than flash; power supply provides much less instantaneous power.
Can point flash at students and claim that correct frequency matching brain waves
will drive a human berserk (joke?)
Can also discuss and illustrate camera flash which uses battery
Refs: wl video V21 (strobe) and V20 (camera),tape 2; 02:06 and 02:05.
X27
56
H. Bradt, 37-581, M.I.T.
X28. Exploding Wire; Energy Stored in Capacitor -3F DANGER 4000V!
DANGER! EXPLOSION
Purpose: Illustrate energy contained in capacitor very dramatically by dumping 800 Joules into an iron
wire which explodes with a loud bang and many sparks.
Equipment: Huge oil-filled capacitor (100 µF, 4000V?); 10,000 V DC power supply (or at least
4000V); Projected voltmeter; iron wire of length about 100 mm; diameter ??; high voltage switches
(long plastic handles); safety plastic shield. [Switches A and B were one double-pole double-throw
switch with long plastic handle; this insures that supply is disconnected when wire is shorted.]
Procedure:
Use safety plastic shield between wire and students
Leave top of wire unshielded so bits can fly up - dramatic.
Clear students away from 1st couple rows.
Charge cap. to 4000V with 10 KV supply; takes ~5? min.; students watch meter climb.
Stop charging at 4000 V (open switch A); important to not overcharge
(Can suggest that students watch meter to prevent (!) overcharging.)
Dim lights
Close switch B which puts Iron wire across Cap.
SEE BIG FLASH and BANG; Iron wire goes bye bye.
Now use shorting probe to insure cap. is shorted
Discharge the rest of charge - BIG SPARK - a surprise.
U = 1/2CV2 = 800 Joules
Ref: wl video V22, tape 2 02:07:42
57
H. Bradt, 37-581, M.I.T.
BAM!
X28
Iron Wire
A
B
Power
Supply
10,000 V
100 µF
4000 ? V
Transparent
Screen
V
Meter jum ps to zero when
Caution: Stop charging at
Open A and then Close B
(Stand back and look away
A closed, then clim bs slo
4000V
or wear safety glasses)
58
wly.
H. Bradt, 37-581, M.I.T.
X29. Dielectric in a Capacitor - 4M
Purpose: Demonstrate the effects (current and potential difference) of placing a dielectric between the
plates of a parallel-plate capacitor. The effects of changing plate spacing are also shown.
Equipment: Large parallel-plate capacitor (r ~ 120 mm) with adjustable plate separation; glass plate ~
2 mm thick ??; DC power supply ~ 2000 V; projected ammeter and voltmeter. [We used a Keithley
for current and the E-field static meter for the voltmeter in 1987.]
Procedure:
Set up with care: proper grounds, thick isolating glass plate(s), etc.; it is tricky
Small charge on capacitor can leak off easily after power supply disconnected.
TURN OFF ROOM MIKE TRANSMITTER - KEITHLEY PICKS IT UP
REMOVE WOOL JACKET TOO (maybe not necessary).
Important to not rush this demo;
Quite a few phenomena to describe; each should be related to theory already presented
(a) Plates at 1mm at 1500V; disconnect supply, open plates slowly to 3 mm.
Note that V increases to ~4500 V (sort of) and that i = constant.
Voltage increase is repeat of demo X25 but it is quantitative this time.
[Charge tends to leak off, so do this part rather quickly to try to get 4500V]
(b) Connect power supply at 1500V and set plates back to 1 mm.
Move plates back and forth
Voltage constant, current flows
Note polarity of current on meter; when reducing spacing, charge flows onto plates
This calibrates meter for current direction; to be used in next steps.
(c) Insert glass while supply still hooked up (plate setting to accomodate plate - 3 mm?)
Note charge flow onto plates; capacitance has increased as expected; V = const.
(d) Remove glass and see charge flow off plates.
Glass in and out - current back and forth, V = const.
(e) Dielectric in and out with supply disconnected
No current flow, V down as dielectric goes in.
Ref: wl video V23, tape 2 02:10:50
59
H. Bradt, 37-581, M.I.T.
Glass plate
X29
Large Capacitor (~0.2 m d
one plate movable w. mic
iam.;
rom eter)
A
Set up is subtle.
Capacitance is small;
charge can leak off.
V
Power
Supply
60
H. Bradt, 37-581, M.I.T.
X30. Can/beaker/can Capacitor Puzzle - 4M
Purpose: Demonstrate interesting property of a glass dielectric in a capacitor, namely that the glass
can retain substantial charge even when removed from the surrounding electrodes.
Equipment: Capacitor consisting of Can/beaker/can; Wimshurst Machine WM; small shorting fork
(insulated handle); Al foil; thick glass plate, clip leads, etc. [The smaller of the can/beaker sets is the
best.]
Procedure:
Setup: Large 1" thick glass plate under Wimshurst and in front of it.
6" x 6" square Al foil on glass serves as connection to outer can
Beaker on foil; short leads from Wim. to foil and to curved handle of center can.
Lead to center can is clipped to small piece of foil which is folded over the
'handle' so the lead can be removed easily without touching it.
(a) Explain with sketch the configuration of this capacitor
Short it with the shorting rod to illustrate that it is not charged (could omit)
(b)Demonstrate that it is a capacitor
Hook up leads from WM (if not already done)
Charge the capacitor by cranking WM, can hear faint breakdown crackles when
charged.
Disconnect center lead by knocking it off with shorting rod (don't touch)
Short the capacitor with shorting rod; obtain quite a large noisy spark.
(c) Show effect of disassembly and reassembly when charged.
Charge as before.
Remove lead from center piece
Disassamble: pull center can out with shorting rod, lift out beaker (with rod? or
fingers?)
Short inner and outer cans with rod; draw weak spark (if any) as expected (low Cap.)
Reassemble and argue that there should now be no charge on capacitor (it was shorted)
Short it with shorting rod and draw big spark (SURPRISE)
Puzzle: why? Ans: glass holds charge on surface
Ref: wl videos V24 (tape 2; 02:18:54) and V37 (tape 2; 02:54:04).
B30
Cu
Wim shurst
Ma chine
Glass
Cu
Al foil
Shorting
Fork
Glass
61
H. Bradt, 37-581, M.I.T.
62