Capacitors Pre-lab Exercise
last updated Sept. 17, 2002
home | notes menu
capacitors in my office This exercise is designed to help you understand how to operate the equipment used in the Capacitors lab. Read the selection below and answer the questions at the end.


1. Circuit elements
Circuit Element
Photo of Element
Circuit Symbol
Description
Capacitor
an electrolytic capactor
circuit symbol for capacitors
Capacitors are circuit elements which store charge. Capacitance, the amount of charge that a capacitor can store per unit voltage, is measured in Farads (F). The capacitance of a capacitor is typically printed on the capacitor.

If you would like to see capacitors charging up in a circuit, see the applet at:
Potentiometer (pot.)
potentiometer
circuit symbol for potentiometer
A potentiometer is a variable resistor. The potentiometer to the left has a dial attached to it, but the capacitors lab has knobs instead.

Potentiometers have three leads. The first two leads are one resistor, the second lead and the third lead are another resistor. Usually the total resistance is fixed, but the ratio of the two resistances changes as you turn the knob.

2. The Capacitance Meter: part of the circuit we will be using for the capacitors lab is already created for you and contained in a box.
circuit diagram for the capacitors lab
This is the circuit that we will be analyzing in class. C1, R1 , and R2 are already connected for you. You need to connect the oscilloscope, the frequency generator, and several C2 values. (See the question.)
the front of the capacitors circuit
This is how the box looks like from the front. The knobs labeled R1 and R2 control the resistance of the resistors in the box. Capacitors labeled A - E will be used as C2.
back of capacitors circuit
The inside of the box. (Yours will not be cut open like this). The orange shapes are capacitors.
 
3. Capacitors in Series and Parallel
When you connect two or more capacitors together in a circuit, sometimes the capacitances add together to create an overall larger capacitance. Sometimes the overall capacitance is lowered. In either case, we can replace the capacitors in the circuit with a single capacitor  -- without changing the behavior of the circuit. The single capacitor needs to have capacitance equal to the overall capacitance of the old circuit in order to do this. We call this new capacitance C eff, or the effective capacitance.

The way you connect the capacitors tells you whether the capacitance of the circuit will increase or decrease. There are two simple ways to connect circuit elements together: in series and in parallel. Multiples of these can be made to make more complicated circuits.
Connection type
Description
Circuit diagram
Equation
Series
the capacitors are connected directly on one side only
capacitors in series
  1
+
 1
=
  1  
CA
CB
Ceff
Parallel
the capacitors are connected on each side to the other
capacitors in parallel
C A + CB = Ceff
Questions:
1. Use the box below to connect capacitor "A", the oscilloscope, and the frequency generator according to the circuit diagram given. (Note: C2 = capacitor "A", and the frequency generator is the circle with the sine wave in it.) Draw lines to indicate connections.
circuit
oscilloscope
the box
freq gen
2. Now connect capacitors "A" and "B" in series; we will call this "A series B". The capacitor "A series B" is now going to be C2 in the circuit diagram above. Connect "A series B" to the circuit on the box accordingly. (Ignore the frequency generator and the oscilloscope in this exercise.)
caps circuit

3. Now connect capacitors "A" and "B" in parallel; we will call this "A || B". The capacitor "A || B" is now going to be C2 in the circuit diagram. Connect "A || B" to the circuit accordingly. (Ignore the frequency generator and the oscilloscope in this exercise.)

caps circuit