PASCO ES-9080B, Instruction Manual

Page: 28 / 36

Basic Electrostatics System
ES-9080B
28 ®
5. After inserting the dielectric, return the plates to the original 3 mm separation and record
the new electrometer reading, V
f
.
6. Pull the plates apart again, and lift and carefully remove the dielectric sheet.
7. Return the plates to the original 3 mm separation and check that the electrometer reading
agrees with the original V
i
reading.
Analysis:
The calculations needed to determine the dielectric constant are long, but straight forward:
Before inserting the dielectric...
Let q
p
be the charge on the capacitor plates, and C
p
be the capacitance of the plates, without
the dielectric.
Let q
E
be the charge on
C
E
, the internal capacitance of the electrometer.
Let V
i
be the initial reading of the electrometer.
The total charge in this initial system is given by q
p
+ q
E
= ( C
p
+ C
E
) V
i
After inserting the dielectric…
Let q´
p
be the new charge on the capacitor plates; the capacitance is now C´
p
.
Let q´
E
be the new charge on
C
E
, the internal capacitance of the electrometer. Since there is
no dielectric in
C
E
, its value is still the same.
Let V
f
be the new reading of the electrometer.
The total charge in the system after inserting the dielectric is given by
Now, the total amount of charge in the system was never changed, so
and ( C
p
+ C
E
) V
i
= ( C´
p
+
C
E
) V
f
.
V
O
C
P
q
P
C
E ,
q
E
V
O
C’
P
q’
P
C
E ,
q’
E
before
after
Figure 3.6: Circuit diagrams with and without the dielectric
q

p
q

E
C

p
C
E
+
 
=
V
f
+
q
p
q
E
+ q

p
q

E
+ =