Instruction manual

Basic Electrostatics System Model No. ES-9080
32
®
1. Calculate 63% of the voltage of the source. Locate the position in the
graph where the voltage has reached this value. How long a time has
passed to reach 63% of the voltage of the source? This time is RC.
(Using the Smart Tool in Data Studio or the Smart Cursor Tool in
ScienceWorkshop

makes these measurements easy!)
2. Compare the measured time constant from the graph with the
calculated from the known values of and R. Now, when a fully
charged capacitor is discharged through a resistor, the voltage
across (and the charge on) the capacitor decreases with time
according to the equation . After a time t = RC (one
time constant), the voltage across the capacitor decreases to 37% its
maximum value.
3. Determine how much is 37% of the total voltage and locate where
in the discharging plot this value has been reached. How long a
time since the start of discharging did it take to reach this value?
(Use the smart cursor tools!)
4. Compare this measured RC constant with the known value.
Procedure 4B: Charging/Discharging Capacitors with Signal
Generator
When a positive square wave signal is applied to a capacitor in an RC
circuit, the capacitor periodically charges and discharges, as shown in
Figure 4.2. The period of a full charge-discharge equals the period of
the wave.
Note: The procedure listed here specifies values for R, C and the frequency of
the signal that work well together. If you decide to use any other R or C
value, you have to adjust the frequency of the wave. Notice that the voltage
has to remain constant for enough time to fully charge the capacitor before
the voltage goes to zero and the capacitor is discharged. A good estimate of
the time needed to fully charge a capacitor can be determined as t = RC[lnV
o
VV
0
e
lRC ⁄
=
V
o
V
Figure 4.2: Charging and Discharging with a Square Wave Signal