Van de Graaff generator or charged rods (glass + ebonite)
Silk and wool cloths
Suspend two metallised balls near each other on nylon threads.
Charge both balls the same type (both + via Van de Graaff).
Observe: balls repel, hanging apart.
Now give one ball + and one โ charge (opposite charges).
Observe: balls attract.
Same charges: balls repel, hanging apart.
Opposite charges: balls attract, moving towards each other.
Confirms: like charges repel; unlike charges attract (Coulomb's Law).
Like magnets โ two north poles push apart, north and south snap together.
F โ qโqโ/rยฒ (Coulomb's Law). Like charges repel; unlike attract. State this exactly.
2Charging by Induction
Metal conductor on insulating stand
Charged plastic rod
Earth connection (wire)
Gold-leaf electroscope
Bring positively charged rod near (NOT touching) the conductor.
While rod is near, briefly earth the far end of the conductor.
Remove earth first, then remove charged rod.
Test conductor with electroscope.
Conductor is negatively charged (opposite to rod) without contact.
Gold-leaf opens on testing.
Inducing rod retains its charge โ induction not conduction.
Like a magnet near a nail โ the nail becomes magnetised without touching.
Sequence matters: (1) bring rod near, (2) earth conductor, (3) remove earth, (4) remove rod. Charge = opposite sign to inducing charge.
3Demonstration of Point Discharge
Pointed conductor connected to Van de Graaff generator
Lighted candle (~10 cm from point)
Smoke strips as alternative indicators
Charge the pointed conductor to high potential.
Hold lit candle approximately 5โ10 cm from the point.
Switch on generator.
Observe the candle flame.
Candle flame is blown away from the point โ 'electric wind'.
Strong field at the point ionises air; ions repelled create an ion wind.
Rounded conductor shows no effect โ sharpness is key.
Like a water hose nozzle โ sharpest point concentrates the electric field just as narrow nozzle concentrates flow.
Field is greatest at sharp points (high charge density). Ionised air ions repelled = electric wind. This is why lightning conductors have sharp points.
4Charge on Outside of Hollow Conductor
Hollow metal can (Faraday's ice pail) on insulating stand
Van de Graaff generator
Proof plane (metal disc on insulating handle)
Gold-leaf electroscope
Charge the hollow can via Van de Graaff. Disconnect generator.
Sample charge from INSIDE with proof plane โ test on electroscope.
Discharge electroscope. Sample from OUTSIDE with proof plane โ test.
Compare leaf deflections.
Inside: negligible leaf deflection โ no charge inside.
All excess charge resides on the outer surface โ Faraday cage effect.
A car during lightning: charge stays outside โ the metal body shields the interior. Same principle.
State: 'For a hollow conductor in electrostatic equilibrium, the electric field inside is zero. All excess charge resides on the outer surface.' This is the Faraday cage principle.
5Electric Field Pattern
Two metal plate electrodes or point electrodes
Castor oil in a shallow dish
Semolina grains scattered in oil
EHT supply (~5โ10 kV)
Place dish of oil on non-conducting surface. Scatter semolina evenly.
Insert electrodes. Connect to EHT. Switch on.
Observe alignment of semolina grains.
Parallel plates: semolina aligns perpendicular to plates โ uniform field.
Point charges: semolina radiates outward โ radial field.
At sharp points: grains closer together โ stronger field.
Like iron filings in a magnetic field โ semolina rotates to align with the electric field.
Parallel plates = uniform field (equal spacing). Point charge = radial field (lines outward from +). Field lines go from + to โ, never cross.
6Capacitance of Parallel Plate Capacitor
Large parallel plate capacitor on insulating supports
Electrometer or high-impedance voltmeter
EHT to charge capacitor
Dielectric sheet (glass/perspex)
Ruler
Charge capacitor to fixed voltage. Disconnect supply (constant Q).
Measure voltage V.
Increase plate separation d โ observe V change.
Return to original. Insert dielectric โ observe V change.
Decrease plate area โ observe V change.
Increasing d: voltage rises (C decreases). C โ 1/d.
Inserting dielectric: voltage falls (C increases).
Decreasing area: voltage rises (C decreases). C โ A.
Verifies C = ฮตโฮตแตฃA/d.
A capacitor is like a sponge โ larger area, smaller gap, better material = more charge stored.
C = ฮตโฮตแตฃA/d. Three factors: plate area (โA โ โC), separation (โd โ โC), dielectric (โฮตแตฃ โ โC).
7Capacitor Stores Energy
Large capacitor (1000โ4700 ฮผF electrolytic)
9V battery
LED + 470 ฮฉ resistor
Switch
Connecting wires
Connect capacitor to 9V battery for 5โ10 seconds (charge it).
Disconnect battery.
Connect LED + resistor across capacitor.
Close switch โ observe LED.
LED flashes briefly and brightly โ capacitor releases stored energy.
Flash fades quickly as capacitor discharges.
Demonstrates energy stored = ยฝCVยฒ.
A capacitor is like a bucket of water โ the battery fills it, remove the tap, connect a pipe and the water empties quickly.
Energy = ยฝQV = ยฝCVยฒ = Qยฒ/2C. Show calculation if C and V given.
8Capacitors Conduct AC but Not DC
Capacitor (10โ100 ฮผF)
LED
Signal generator (AC) and battery (DC)
Connecting wires
DC test: connect capacitor in series with LED and DC battery. Observe.
AC test: replace battery with signal generator at 50โ100 Hz. Observe.
DC: LED flickers briefly (charging flash) then goes out โ capacitor blocks steady DC.
AC: LED glows continuously โ capacitor repeatedly charges/discharges each cycle.
Capacitor passes AC, blocks DC (in steady state).
Like a revolving door โ AC keeps spinning through; DC stands still and gets blocked once charged.
State: 'Capacitor blocks steady DC (current stops when fully charged) but allows AC (charges/discharges each cycle).' Basis of capacitive coupling in electronics.
9Conductor/Insulator Test with Ammeter
Battery (6โ9V)
Ammeter (milliammeter)
Connecting wires with crocodile clips
Test materials: copper, aluminium, graphite, steel, rubber, plastic, wood, glass
Build series circuit: battery โ ammeter โ two crocodile clip leads.
Bridge gap with each test material. Record ammeter deflection.
Group materials as conductors (deflection) or insulators (none).
Metals (copper, aluminium, steel): large deflection โ good conductors.
Graphite: small deflection โ conductor but not as good.
Rubber, plastic, glass, dry wood: no deflection โ insulators.
A conductor is a motorway for charge; an insulator is a wall; a semiconductor is a country road.
Conductor: many free electrons. Insulator: tightly bound electrons, no free carriers. Examples: conductor = Cu, Al; insulator = rubber, glass.
10Electroplating by Electrolysis
Two copper electrodes
Copper sulfate (CuSOโ) solution
Beaker
Battery (6โ9V)
Ammeter
Balance
Clean and weigh both electrodes. Label anode (+) and cathode (โ).
Connect anode to positive terminal, cathode to negative.
Run current for 10โ15 minutes. Record ammeter.
Remove, dry carefully, re-weigh both electrodes.
Cathode gains mass โ copper deposited on it.
Anode loses mass โ copper dissolves into solution.
Mass gained at cathode โ mass lost at anode.
Verifies Faraday's First Law of Electrolysis.
Like a 3D printer using electricity โ copper atoms are 'printed' one by one onto the cathode from the solution.
Faraday's First Law: 'Mass deposited โ charge passed.' m = ZIt, Z = electrochemical equivalent. Cathode always gains; anode always loses (if copper anode in CuSOโ).
11Magnetic Effect of Current
Straight wire through horizontal card
Iron filings or compass needles
Power supply (DC, 3โ6V)
Switch
Pass wire vertically through card with iron filings. Connect to power.
Switch on current. Gently tap card.
Observe iron filings. Place compass needles around wire.
Reverse current โ observe deflection reversal.
Iron filings form concentric circles around wire.
Compass needles deflect tangentially.
Reversing current reverses field direction.
Closer to wire = stronger field (tighter rings).
Like ripples in a pond โ except the 'ripples' (magnetic field) are circles around the wire rather than expanding outward.
Right-Hand Grip Rule: thumb = current direction, fingers = field direction. State: 'A current-carrying conductor produces a circular magnetic field.'
12Heating Effect of Current
Thin nichrome resistance wire (28 SWG, ~20 cm)
Thick connecting wires
Battery (6V) or power supply
Variable resistor (rheostat)
Ammeter
Switch
Connect thin wire in series with rheostat and ammeter.
Start at maximum resistance (minimum current). Switch on.
Gradually decrease resistance to increase current.
Observe thin wire at different currents.
At low current: wire feels warm.
At higher current: wire glows red then white.
Thin wire heats most (highest R = ฯl/A; smaller A โ higher R).
Thick wires stay cool โ low resistance.
Demonstrates Joule's Law: P โ Iยฒ.
Like a toaster โ thin high-resistance element gets very hot while thick wires to the plug stay cool.
Joule's Law: 'Rate of heat โ Iยฒ. P = IยฒR.' H = IยฒRt. Doubling I quadruples power output.
13Rheostat as Potentiometer
Rheostat (variable resistor, 50 ฮฉ)
Battery (6V)
Voltmeter (0โ6V)
Connecting wires
Switch
Connect both end terminals of rheostat to battery.
Connect voltmeter between sliding contact and one end.
Switch on. Move slider from one end to other.
Observe voltmeter reading.
Slider moved end to end: voltmeter varies continuously from 0V to full battery voltage.
Voltage output proportional to fraction of resistance used.
Demonstrates rheostat as potential divider.
Like a volume knob โ you can set any volume level between silent (0V) and maximum (battery V).
V_out = V_in ร Rโ/(Rโ+Rโ). A potentiometer gives variable voltage from 0 to Vmax. Different from simple variable resistor (which limits current but output stays connected to supply).
14Ohm's Law
Known resistor (10 ฮฉ wire-wound)
Rheostat
Battery (6V)
Ammeter (0โ1A)
Voltmeter (0โ6V)
Switch
Connecting wires
Connect R in series with rheostat, ammeter, battery, switch.
Connect voltmeter in parallel across R.
Start at max resistance (min current). Record V and I.
Decrease rheostat in steps โ take at least 5 readings.
Plot V (y-axis) vs I (x-axis).
V increases proportionally with I.
Graph: straight line through origin โ confirms V โ I.
Slope = R = ฮV/ฮI, agrees with marked value.
Verifies Ohm's Law: V = IR at constant temperature.
Like a garden hose โ harder pressure (higher V) โ faster flow (higher I), ratio always the same (constant R).
Must state 'at constant temperature.' V vs I graph: straight line through origin, slope = R. 'Current is directly proportional to potential difference.'
15Current Flow Across P-N Junction
P-N junction diode (or LED)
Resistor (470 ฮฉ)
Battery (9V)
Ammeter (milliammeter)
Switch
Connecting wires
Forward bias: connect + terminal to P-type (anode), โ to N-type (cathode). Include ammeter and resistor.
Forward bias: clear current flows โ junction conducts.
Reverse bias: ammeter reads zero โ junction does not conduct.
LED: emits light only in forward bias.
P-N junction is a one-way valve for current.
Like a one-way turnstile โ correct direction: sail through; wrong way: blocked.
Forward bias: + to P, โ to N. Depletion layer narrows, current flows. Reverse bias: depletion layer widens, no current. Applications: rectification (ACโDC), LED.