Users' Guide User guide

Xplorer GLX Users’ Guide 131
Activity 8: Capacitor Discharge
Background
In the circuit pictured above, the rate at which the voltage, V, decreases is directly
proportional to the voltage. Mathematically, this can be written:
(eq. 1)
where dV/dt is the rate of voltage change, and τ is a constant greater than 0
known as the Capacitive Time Constant. The negative sign indicates that the
voltage decreases over time.
Equation 1 can be rewritten:
(eq. 2)
where V is the voltage at any given time, t; and V
0
is the voltage at t =0.
In this experiment, you will create a circuit like the one in the diagram. The GLX
will measure and graph voltage over time as the capacitor discharges. From the
collected data, you will determine the capacitive time constant, τ, of your circuit.
Before You Begin
Start a new experiment on the GLX.
1. Press to go to the Home Screen.
2. Use the arrow keys to highlight the Data Files icon and press to open
the Data Files screen.
3. Press to open the Files menu and press to select New File.
Equipment
1
Quantity
Xplorer GLX
1
Voltage Probe
1
Capacitor (between 0.015
and 1 F)
1
Resistor (between 100 and
1000 Ω)
1
Battery and battery holder
(“AA,” “C,” “D,” or similar)
1
Alligator Clip Leads
4
1
Use separate electronic components or
the PASCO EM-8678 Charge/Discharge
board, which includes a capacitor, resis-
tors, and battery holder.
C
R
V
dV
dt
------- τV–=
ln
V
V
0
------
τt–=
F4
1pqrs