Elementary Electrical Measurements_2004

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PHY 212Elementary Electrical Measurements Page 1 of 10

ELEMENTARY ELECTRICALMEASUREMENTS

PHYSICS 212LABORATORY 3

OBJ ECTIVES

The objectives of this laboratory are that you learn how to measure potential difference, resistance, andcurrent, and how to test Ohms Law.

EQUIPMENT

Digital multimeter Analog Devices power supply

6-V flashlight battery (D-cell) Assorted resistors

6-V transistor battery Resistor code chart

Resistor boards Alligator clips w/ probe (red & black)

22-, 20-, 18-gauge NiChrome wire Calipers

22-, 20-, 18-gauge copper wire Meter stick or metric tape

THE DIGITAL MULTIMETER

The meter shown in Figure 1 (next page) will measure potential difference(volts, V), electrical

resistance(ohms, ), orcurrent (amps, A), depending on the setting of the knob and the selection of

terminals at the bottom of the meter into which the test leads are inserted. Pay close attention toinstructions about these settings. Errors in settings will giveerroneous results and could damage themeter.

MEASURING DC POTENTIAL DIFFERENCE

The abbreviation DC is used fordirect current, meaning that the electric charge always tries to flow onedirection only. Set the selection knob to V -----, and connect the red and black test leads to the terminals

labeled V and COM as shown in Figure 2. The meter will read the potential difference V between the

red and black probe tips. The intent is that the red terminal will be in contact with a higher (more positive)


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potential than the black terminal. However, reversing the red and black terminals is not a serious error

the meter will simply indicate a negative value on its display.

Figure 1Sample digital multimeter. [Note: Yours may look a bit different.]

Figure 2Measuring potential difference of a battery.

Batteries are good examples of DC systems. For a flashlight battery, the positive and negative terminals

are at the ends of the battery. Measure the potential difference of a flashlight battery (D-cell, 6V).

To measure the potential difference of a transistor battery, recognize that the positive and negative

terminals are connectors on the top of the battery. Measure the potential difference of a transistor

battery.

Your third measurement will be the potential differences between the red and black terminals on the

Analog Devices power supply (Figure 3). The power supply is essentially a substitute for a battery which

takes its energy from the wall outlet and produces potentials at the colored terminals. Plug the power

supply in and turn the switch on, then make your measurements.


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Figure 3Sample power supply.

MEASURING AC POTENTIAL DIFFERENCE

The term AC means alternating current, a system in which the direction of the electric flow changesperiodically. The electricity present at a wall outlet is commonly called 110 volt 60 cycle, meaning that

the direction of the current changes (oscillates) 60 times per second. You will probably find that thepotential difference is not exactly 110 volts.

Set selection knob to V~, and leave the red and black test leads connected to the terminals labeled Vand COM as in Figure 4.This measurement could be dangerous if done incorrectly. Double check yourmeter settings, and grasp the test probes by the plastic handles. The plastic is designed to insulate you

from the potential of the outlet. Do not allow your hands to touch the metal tips of the test probes. Insert

the tips of the probes into the terminals of the outlet and move them into contact with the metal

connectors inside the outlet. The rounded terminal at the bottom of the outlet is called the grounding

terminal and has a potential of zero (when correctly wired).

Measure and record the potential difference between the three possible pairs of terminals in the outlet.From the fact that the potential of the grounding terminal is zero deduce the potentials of the longer and

shorter slot-shaped terminals.

Figure 4Measuring potential difference of a wall outlet.


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MEASURING RESISTANCE

The difficulty that electric charge has in flowing through an object is called the electrical resistance of the

object and is given in units of ohms, abbreviated . Thus, it is more difficult for electric charge to flow

through an object with resistance of 100 than through an object with resistance 10 .

Set the selection knob to 0, and leave the red and black test leads connected to the terminals labeled V 9

and COM as in Figure 5. The meter will display the resistance between tips of the red and black test

probes. You should not touch the tips of the probes while making a resistance measurement, because the

resistance of your body may affect the result of your measurement.

Figure 5Measuring resistance.

Electrical components called resistorscan be manufactured to have a wide range of electrical resistanceby varying the composition of the material from which the resistor is made.

Exercise 1:

Measure the resistanceR of a set of resistors designated by your laboratory instructor. Label them R1, R2,, Rn and record these resistances on the data sheet. Also record a brief description of each resistor (suchas color or size) so that you can identify it later in the experiment. Note that the color codes are

standardized for electrical resistors, and cue-cards can be obtained from most electrical-supply, specialty

electronic, Radio Shack stores, or electric-component supply catalogues.


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Exercise 2:

Resistance also depends not only on the composition of the conductor but also on the cross-sectional areaof the conductorand its length. On the resistance board shown below (Figure 6) you will mount 3-mlengths of nickel-chromium (NiChrome) wire. Our setup is not like this picture below but you should be

able to ascertain the specific lengths of the wire.

Figure 6Three lengths of wire are mounted on the resistance board, the holes of which are 1 m apart.To get a 2-m length, use two holes; for 3-m, use three holes.

The holes in the wooden resistance board are placed one meter apart, making it easy to measure the

resistance of 1-, 2-, or 3-m lengths of a wire, as shown in Figure 7.

Figure 7Measuring resistance of a 3-m length of wire. (Indicated reading on Multimeter are fordemonstration purposes only and are probably not correct for your materials.)

Measure the resistance of 1-, 2-, and 3-m lengths of each gauge of NiChrome wire, and record in the data

sheet. The resistance of such short lengths of wire is low, so it is important to get a good contact between

the probe tip and the wire. Poor contact can introduce additional resistance, giving an erroneous result. It

is better to touch the probe to the wire itself rather than to a screw about which the wire is wound,

because the electrical contact between the wire and the screw may not be good.

With the calipers, measure the diameter (D) of the wire and calculate the cross-sectional area (A) of the

wire from the formula:

22

,4

DA r= =

where r is the radius of the wire and D is its diameter. Record the cross sectional areas on the data sheet.


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MEASURING CURRENT

Connect resistorRl to the red and black terminals of the power supply as shown in Figure 8 and turn thesupply on. Current is forced from the higher-potential terminal, through the resistor into the lower-

potential terminal.

Figure 8Simple circuit.

The setting and connection of the meter to measure the current flowing through the resistor is critical.Note in Figure 9 that the red test lead is connected to the 300-mA (milliamperes) terminal of the meter.

This means that the meter will read in units of milliamperes. The knob is set to the direct current position,

A -----. Note carefully how the meter is connected to the circuit. First, the circuit is broken open, which

can conveniently be done by removing the alligator clip from the resistor. Then, the meter is connected so

that it bridges the gap created in the circuit. Thus, all of the current that was flowing through the resistor

in Figure 8 must now flow through the meter and then through the resistor. When the red test lead isconnected to the 300-mA terminal of the meter, it is very easy for charge to flow through the meter-the

resistance of the meter itself is very low. Thus the meter has little effect on the amount of charge thatflows, and the current will be almost exactly the same as it was in Figure 8.

Figure 9Correct connection. The charge must flow through both the meter and the resistor.

Figure 9 shows the meter connected correctly, and Figure 10 shows the meter connected incorrectly.With the incorrect connection, the charge does not have to flow through the resistor to get from the red to

the black terminal of the power supply, because the meter provides an alternate path. Because the charge

does not have to flow through the resistor, much more current flows from the power supply with this


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connection than in Figure 9, and, because most of the excess current goes through the meter, the meter

could be damaged.

Figure 10Incorrect connection. The charge does not have to flow through the resistor to complete thecircuit. The meter may be damaged.

Exercise 4:

Measure the current flowing through R1 as described in Figure 9 and record your result on the data sheet.

Ohm's Law states that the current, i, is given in terms of the potential differenceV and the resistance R

by the equation

.V

iR

=

Calculate the current from Ohms Law using the Vbetween the red and black terminals, which you

measured in the potential-difference section, and the resistance R1., which you measured in the resistancesection.

Repeat this for resistorR2 and the other resistors.


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DATA SHEETSELEMENTARY ELECTRICAL MEASUREMENTS

Name ______________________________________________________________________

DC Electric Potential Measurements

Flashlight Battery ___________________

Transistor Battery _________________

Power Supply ______________________

AC Electric Potential Measurements

Short Slot to Round Slot ___________________

Long Slot to Round Slot ___________________

Short Slot to Long Slot ____________________

Resistance Exercise 1MeasuredResistance

Color 1 Color 2 Color 3 Color 4Coded

Resistance

Resistance Exercise 2

MeasuredResistance

Length of WireDiameter of

Wire

CrossSectional Area

of WireResistivity Error

Current Exercise 3 and 4Resistance

DC ElectricPotential

MeasuredCurrent

CalculatedCurrent

Error


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Elementary Electrical Measurements_2004