Instruction manual

2
R-C Network 012-04221C
Five binding posts allow a power supply, such as the
PASCO Model ES-9049A Power Supply, and/or the elec-
trometer to be connected at every possible component
connection in the circuit.
➤IMPORTANT: Never place more than 50 Volts
DC across any component in the R-C network.
When using the ES-9040A Power Supply, use the
30 VDC range only.
The experiment using the R-C network will give only
suggested schematics, such as in Figure 1 (B)—and not
specific switch settings.
Experiment:
Capacitors - Charging and Discharging
It takes a definite amount of time to charge or discharge a
capacitor through a resistance. In this experiment we dis-
cover the relationships between capacitance, resistance,
and the time required to charge the capacitor.
Equipment Needed:
RC Network, DC Power Supply, and Electrometer.
The experimental circuits can be easily duplicated by
tracing through the schematic on the front of the network
and setting the switches accordingly. The battery in the
experimental schematics is replaced with the DC Power
Supply. Use the electrometer to measure voltages. DO
NOT GROUND THE ELECTROMETER. (We want
relative voltages.)
Procedure:
A. Charging Capacitors:
➀ Set up the circuit in Figure 2. At time t = 0, the switch
is closed and capacitor voltage is recorded at regular
time intervals. (A simple way to collect data is to open
the switch at regular intervals and record the steady
state reading. Why is this possible?)
R
C
Figure 2
Plot time versus capacitor voltage for several values of
R and C. What is the shape of the curve?
Let us define T as the time required for the capacitor
to charge from 10% to 90% of its final value. What is
the relation between R,C, and T? (Hint: Convert R to
ohms, C to farads, and T to seconds.)
➁ Repeat the same experiment as in (1), but measure the
voltage across R as a function of time. (This time
opening the switch stops the current flow, hence
steady-state readings cannot be made.)
According to Ohm’s law, potential is proportional to
resistance multiplied by current. Hence, the potential
across the resistor is proportional to the charging cur-
rent. What relation is there between the charging cur-
rent and the charge on the capacitor? (Hint: Part (1)
gives the relation between charge and time, whereas
part (2) gives the relation between time and charging
current.)
B. Discharging Capacitors:
➀ Set up the circuit in Figure 3. Charge the capacitor to
an initial potential of 30 VDC. At time t = 0, close the
switch and use the electrometer to measure the voltage
across the capacitor. (Use the switch for steady state
readings as in part A (1).
Repeat the experiment with several other values of R and
C. Plot time versus capacitor voltage. Does the time con-
stant (T) remain the same for discharging capacitors as it
does for charging capacitors?
➁ Using the same circuit as in B (1), charge the capacitor
to 30 VDC, and close the switch and measure the volt-
age across the resistor. Determine the relation be-
tween discharging current and capacitor potential.
Figure 3
R
C