Electricity and Magnetism > Capacitance > Capacitors DCS# 5C10.20 v.2

CAPACITORS AND DIELECTRICS



APPARATUS
large aluminum capacitor plates with spacers
202-19-A
plexiglass sheets
202-19-A
DC power supply
202-08-E
cap charger
202-19-A
oscilloscope or multimeter and camera
202
grounded aluminum foil
202-02-E1


DESCRIPTION
The three plates form a guarded parallel plate capacitor.  The inner plate is 30.5 cm x 37.3 cm , so A = 0.119 m2 and the plate separation (on both sides) is 1 cm.  The plate separation is fixed by eight plexiglass spacers.

The net capacitance, using air-gap parallel plate formula, is 210 pf.

1.  Fixed voltage, variable charge:  
Charge the capacitor and leave it connected to the power supply.

Insert and remove a piece of dielectric material (plexiglass, bakelite and glass are available).  The current through the 1 Mohm scope impedance gives abut 3 mV deflection for about 1/2 s insertion or removal time.  Noise is mostly 60 Hz about 1 mV peak to peak.

2.  Fixed charge, variable voltage.
Flip the cap-charger switch to charge the capacitor.  Vary the charging voltage to show that the output follows the capacitor voltage.

Flip the switch to "read" to disconnect capacitor from power supply.  The charge on the capacitor remains fixed - on a cool dry day, the discharge time constant is greater than ten minutes.  On a hot, wet, June day the time constant canbe as low as 10 seconds.

Insert dielectric, voltage goes down.  (If a significant amount of charge is on the dielectric, the voltage may go up or down below zero.  Rub the dielectric surface with a grounded ball of steel wool or aluminum foil to remove the charge.)

Remove the dielectric.  The voltage returns to its original value.

Other possibilities:  demonstrate dependend of potential on plate separation by pushing on the top flexible plate.  If only the top and bottom plates are used as the capacitor, proximity effects are large. 

NOTES
Set power supply to 15 V.  Remove the shorting strap.

Connect top and bottom plates to ground, inner plate to input.

Cap charger contains op-amp, switch, second power supply - up to 7.5 VDC.

The Pasco variable parallel plate capacitor is more reliable and easier to use.