Ver.1.3
Electromotive Forces (emf)
and
Basic Lab Measurements
Various phenomena in Nature result in the separation of charge. Among these are chemical
systems called batteries, by which chemical reactions produce an excess of electrons at one terminal (-)
and a deficit of electrons at the other (+). Two other commonly encountered devices are the thermocouple
which uses temperature differences to separate charge, and the photocell which uses light. Energy is
required to separate electric charges, and whenever a device performs this function, there has been some
energy transformation.
One of our primary goals in this experiment is to gain familiarity with the use of digital
multimeters (usually called DVM’s or DMM’s) for electrical measurements. We will frequently use
these devices to obtain data about electrical potentials and currents. In today’s lab we will be measuring
potentials (emf’s), or voltages produced primarily by batteries, but we will also examine a
thermocouples and photocells. In each case, some kind of work (chemical, thermal, light) is required to
separate the charges and produce the potential difference. The amount of work required per unit charge is
called the electromotive force (emf) or potential. The use of the word “force” is a misnomer because the
units are not those of force, but rather those of work (energy) per unit charge. In the SI system:
work/charge =
Joules/coulomb
= volts
The drawing below represents a gravitational analogy to a battery or other emf source and a
simple electrical circuit. The person at the left is lifting bowling balls to a height h, investing an amount
of work mgh per bowling ball. The balls then roll along the shelf, down the shaft and back to the person.
The amount of mechanical work per ball he does is measured in joules per ball and is analogous to the
emf in the battery.
In the battery, chemical work is done to separate charge and the potential (emf) is measured in
terms of work per unit charge or volts. If an external path is provided for the charge to return to the other
terminal of the battery the flow of charge passing through any point in the circuit is called current and is
measured in terms of the amount of charge passing through the circuit per unit time. In the SI system, this
is measured in coulombs per second and called amperes or amps for short. In the battery, once the
charge has returned to the opposite terminal by means of the external circuit, it recombines and further
chemistry separates the charges. This process continues until the chemicals in the battery have been
depleted. If the chemical reaction is not reversible the battery is now useless, but if it can be reversed we
have a rechargeable (usually NiCd) battery! A similar analogy can be applied to the thermocouple and
photocell where the phenomenon is thermal and light energy respectively.
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In this experiment we will be using three different digital multimeters to make our voltage or
potential measurements. Attached to this handout are views of each of the instruments:
1. BK Test Bench Model 388A or Tektronix. A hand held, portable meter.
2. Simpson Digital Multimeter Model 460. Located on the top of the bench station.
3. Tektronix Digital Multimeter Model DM501A. Located in the “bin” on the table top.
All of these devices can be used for a variety of measurements, but today we are interested in their use as
a voltage measuring device, or the voltmeter mode. Each of the meters share some basic design
elements. First all measurements must be made by connecting a set of probes to the meter inputs. All the
meters have 3 or 4 inputs that are clearly labeled. We are interested in using the inputs that are marked
VΩ (voltage and resistance) and COM (common). One of the probes will always be the COM input
and it is usually taken as the negative (-) polarity. If the meter face indicates a (-) it means what you
thought was the (-) probe isn’t! Each meter is capable of measuring either AC or DC voltages, BUT not
on the same scale, so it is necessary to set the meter to the type of measurement we will be making. The
next setting you must choose is the function of the multimeter. For almost all our measurements today
we will be interested in DC voltages, so begin with the meter function set there. Notice the way this is
done is different for each of the meters so get help from the instructor if it isn’t obvious to you. In our
case we want to make DC voltage measurements so make sure you are set for DC voltage. Generally the
clue is the symbol V. Lastly before making a measurement the range must be chosen. In the interest of
not overloading the meter the highest scale possible should be chosen even if you believe the voltage
input will be a small value! You are now ready to make voltage measurements.
Procedure: In this experiment we will measure a variety of emf’s and voltages in order to learn how to
use the multimeters. Along the way we will observe how potentials add in series, and discover some
properties of the photocell and the thermocouple.
Batteries as emf’s
Begin by measuring the voltage across the +/- terminal of each battery you have been supplied.
Be sure to record the type the battery and if you are doing these measurements for more than one type of
battery make sure that information is in your data table. For the individual batteries, make the
measurements using different range scale settings, i.e. use 2, 20, and 200 or 4, 40 and 400V scales. The
Tektronix hand held meter has an automatic range selection feature. Use the hand-held meter and either
the Simpson or Tektronix bench meters for two sets of data for each battery. The use of more than one
meter and more than one range will allow you to gain experience in what can be expected when you
choose a different device.
After you have determined the potential across each battery connect two of the same type in series
as shown below leaving the circuit open so that no current is supplied by the batteries and measure the
potential difference as shown for connections A, B and C. You need not use more then one meter,
preferably the handheld meter.
Now using the set of two batteries connected + to -, complete a circuit with a resistance of ~1000
and ~100 ohms. Now measure the voltage across the batteries (or the resistor) for the circuit. In this set of
measurements for circuit D we are investigating whether or not a current flow from the battery will effect
the emf. Note any changes.
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+
-
+
+
-
DVM
+
-
DVM
DVM
+
-
A
+
-
+
B
DVM
R
+
-
C
D Circuit with
load
Thermocouple emf’s
For the thermocouple part we need to have some sources of different temperatures. A suitable
range is ice, room temperature, you, and boiling water. Use the hand-held multimeter for these
measurements. Note the lowest range scale will be needed (mv). If a stable signal can’t be maintained,
the thermocouple needs to be repaired. Carefully observe the polarity and maintain the same
connections for all the measurements. If the ice water reading is NOT -, reverse the leads connecting
the meter.
Photocell emf’s
Use the microscope lamp or the setup indicated below to illuminate the photocell and measure the
voltage output under the following conditions:.
1. Photocell covered by an opaque object.
2. Photocell in ambient room light.
3. Photocell exposed to the bright microscope lamp with maximum intensity.
4. Photocell blocked by 1,2,3 and 4 layers of paper.
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Presentation: In this experiment you made a variety of electrical potential measurements using a
variety of multimeters. For the battery circuits summarize how closely the meters agreed with each other
and what happened when you changed range scales. Next evaluate how closely the theoretical sum or
difference of the battery potentials matched the series measurement. Lastly summarize any differences in
potential observed in the cases with and without a load resistor that produced a current flow in the
circuit. Make sure you address the question of ±’s.
For the thermocouple measurements make a graph of emf vs. temperature. Draw the best straight
line through this graph and determine the slope. Use this value to predict the emf if the thermocouple
were heated to 200ºC.
For the photocell make a plot of emf vs. the number of sheets of paper (0,1,2,3,4) blocking the
lamp. Is it the plot linear? Estimate the emf from 6 sheets of paper.
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DATA TABLES
Individual Battery EMFs
Tektronix Bin DMM
Simpson DMM
Range 1
Range 2
Range 3
Range 1
Range 2
Hand-held DMM
Range 3
Range 1
1
2
3
4
±‘s
Battery type 1:
Battery type 3:
Battery type 2:
Battery type 4:
Battery Combinations
Measured emf
Theoretical emf
A
B
C
±
±
Batteries With Load (D)
Load resistor
emf
~100 Ω
~1000 Ω
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Range 2
Range 3
Thermocouple Data
Temperature
Point
ice
emf
°C
room
temperature
body
temperature
boiling water
±
±
Photocell Measurements
Conditions
emf
Opaque object
Full room light
No papers (microscope light)
1 sheet of paper
2 sheets of paper
3 sheets of paper
4 sheets of paper
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