Experiment 4: The Potentiometer/ Capacitor Combinations
How to use meters:
Analog meters (the kind with a needle): Red connectors are for + and black for -, or they are just
labeled + or - . There is one of one sign, which you always connect to, and several of the other,
which you use just one of. Which of these you use determines which scale you read. For example,
if you put the wire on a connector labeled 30 V, read the scale which goes from 0 to 30 V. Also,
check that the meter is correctly zeroed, and have the instructor adjust it if it isn't.
Digital meters: Turn the dial to the function you want. For example, 20 in the area marked DCV
turns it into a DC voltmeter which reads from 0 to 20 V. DCA stands for DC amps. If the meter
has more than two connectors, the negative wire always goes to COM. Where the other wire goes
depends on whether you want to measure volts, amps, or milliamps; the sockets are labeled. On the
some meters, the dial goes in the same place for the 10 A scale and for the 20 mA scale and which
you are using depends on which socket the + wire is in.
PART ONE: The potentiometer
We wish to measure a battery’s emf, E; meaning the “voltage” created by the chemicals in it.
(Don’t confuse E with E, the electric field strength. E is in volts, E in volts per meter.) It might
seem that you could just measure E with a voltmeter. But, the materials
the battery is made of have resistance, and Ohm’s law says there is a
voltage drop across this “internal resistance” if a current flows. Since a
voltmeter draws current from the battery, the potential difference between
the battery’s terminals is E – Ir, which is less than E.
A potentiometer measures the voltage without drawing any current, so E – Ir just equals E , and you
get the true emf. The voltage from a power supply is put across a
long, bare, high resistance wire, CE, as shown. The connection at
point D is moved until VCD is equal to the emf of the battery. You
can tell the right spot for D because when the voltages balance, the
galvanometer (a sensitive ammeter) reads zero. You can then get VCD
(which equals E ) from a proportion: Voltage CD is to the wire’s total
voltage as length CD is to the wire’s total length (100 cm):

V


100cm
so,
 V

100cm
PROCEDURE:
Record the value for its emf printed on the battery.
Connect the battery directly to an analog voltmeter, the kind with a needle, and record the reading.
Ask for a different battery if it isn’t between .1 V and 1.0 V. (The effect you are supposed to see is
more pronounced with an old, dead battery because of its large internal resistance and with an
analog meter because it puts a greater load on the battery.)
Set up the potentiometer circuit. Each line on the diagram is a wire. Follow the wires like roads on
a map. Current leaves the + (red) terminal of the power supply, and travels to point C at one end of
the resistance wire. So run a wire between those points. The current goes through that wire to point
E. A wire from E to – (black) on the power supply completes the circuit. There is also a voltmeter
with one wire going to each terminal of the power supply. When the tap key at D is down, a small
current also comes out of the battery at A. It goes to C at the + end of the resistance wire then along
the resistance wire as far as D. From there it goes through the tap key, through the galvanometer
and finally back into the negative end of the battery.
Don't put the battery backwards: The wire from + on the battery should lead to + on the power
supply. Set the power supply at between 2 and 4 volts. Keep the power off when not taking
readings so the wire doesn't get hot.
Tap the key at point D on different parts of the wire to find where the galvanometer reads zero. Do
not hold the key down with the galvanometer at full scale, or it may suffer. Read the length l, from
the positive end of the wire. (The same section of wire the battery is connected across.)
Find the electromotive force of the battery.
In your conclusion, comment on which of your three values represents the true emf of the battery
and why.
Part Two: Capacitor Combinations
A group of capacitors is assembled, as shown. You will predict what the voltage across each
capacitor in the group should be, then measure them with a digital meter to see if you were right.
Some capacitors are labeled without units: "2-25DC" means 2 μF,
and a maximum voltage of 25 V DC. Select four different
capacitors of at least 1 μF each (the higher the better). The largest
capacitance should be no more than ten times the smallest.
Don’t hook capacitors up backward: Some have arrows pointing to
the negative end; with this kind of capacitor, that's important.
CALCULATIONS :
1. Choose some potential difference, V, to put across the group.
(How much doesn't matter.)
2. Calculate the equivalent capacitance of the group of four capacitors.
3. Calculate V1: Since C1 and the rest of the circuit are in series, QTot = Q1. So, Ceq V = C1 V1.
4. Find V2 from V and V1.
5. Calculate V3: Since C3 and C4 are in series, Q3 = Q4, so C3V3 = C4V4. Putting that together
with V4 = V2 - V3 gives C3V3 = C4(V2 - V3). You know everything in that except V3.
6. Calculate V4.
MEASUREMENTS. Use a digital voltmeter: (The capacitors would almost immediately discharge
through an analog meter.)
1. Put the voltmeter across the power supply and set it to the voltage you chose.
2. Connect the meter to a capacitor with the power supply off. If it doesn't read zero, short out
the capacitor to discharge it. (Temporarily run a wire from one side of the capacitor to the other.)
3. Turn on the power supply. Record what the meter said before it started decreasing. (The
capacitors will slowly discharge even through a digital meter.) Promptly turn the power supply
back off.
4. Move the meter to a different capacitor and repeat 2 and 3.
5. If you need to double check anything, turn off the power supply and short out each capacitor
first. Otherwise, some charge which flowed through the meter might affect your results.
Compare the measured voltages to what you calculated. The main source of experimental error is
the accuracy of the capacitances. Based on this, the difference between your calculated and
observed values should be no more than 10%.
A suggestion for your write-up: Sketching a circuit is easier and clearer than trying to describe it in
words. You still need words to explain how it works, but for the part where you tell the reader what
you were working with, a picture works a lot better than “We ran a wire from the power supply to
the battery. We ran another wire from …”
PHY 132
Report on Experiment 4: The Potentiometer/Capacitor Combinations
Part I:
E printed on battery = _____________
Analog voltmeter reading = _____________
Potentiometer:
V = _____________ l = _____________ Determine E :
Part II:
C1 = _____________ C2 = _____________ C3 = _____________ C4 = _____________
V = _____________
Calculation of Ceq:
Calculation of V1, V2, V3& V4 (continue on back if necessary):
Measured V's:
V1 = ____________ V2 = _____________V3 = _____________ V4 = _____________