Data Sheet

-71-
113. Measure of Electric Charge - The Coulomb
Build the circuit shown, connect points A and B with the spring wire (9),
then turn on the switch (62). The heart LED (69) will light and then fade
out as the 470mF capacitor (74) is charging.
As discussed in the previous project, capacitors build up charge on their
plates when a voltage source is placed across their leads. The amount
of electric charge built up on the plates of a capacitor is measured in
Coulombs. 1 Coulomb is equal to the electricity conveyed in 1 second
by 1 Amp of current.
114. Measure of Capacitance - The Farad
Remove the spring wire (9) from points A and B in project #113
and place the spring wire (9) across points B and C. You will see the
star LED (70) light for a while and fade to be very dim as the 470mF
capacitor (74) discharges.
The capacitance of a capacitor is measured in Farads. A capacitor is
said to have 1 Farad of capacitance when a potential difference of 1 Volt
across its plates will charge it with 1 Coulomb of electricity.
115. Approximation for Capacitance of a Capacitor
Build the circuit shown on the left, connect points A and B with the
spring wire (9), then turn on the switch (62). The heart LED (69)
will light and then fade out as the 470mF capacitor (74) is charging.
Remove the spring wire (9) from points A and B and place the spring
wire (9) across points B and C. You will hear the alarm (78) for a while
and then fade out as the 470mF capacitor (74) discharges.
It was discussed in project #112 that the capacitance of a capacitor is
related to the size of the plates, the distance between the plates, and
the dielectric material between the plates. You can actually approximate
the capacitance of a capacitor through a formula based on Gauss’s
law to be C=eA/d where A is the area of the plates, d is the distance
between the plates and e is the permittivity of the dielectric between the
plates (permittivity is the ability of a substance to store electrical energy
in an electric eld).
+
–
+
–
+
–
+
–