DC Fundamentals
Electronic Quantities
Exercise 2: Circuit Current
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to describe and measure current by using a
simple circuit. You will verify your results with a multimeter.
DISCUSSION
through a conductor from one point to another.
With a negative charge at terminal 1 and a positive charge at terminal 2, electrons are repelled from
terminal 1 and attracted to terminal 2.
Electrons enter the conductor from terminal 1 of the battery. Electrons leave the conductor and enter the
battery at terminal 2.
The total amount of current in an electrical circuit is determined by the voltage applied to the circuit and
the total resistance (R) of the circuit.
If the circuit resistance remains the same while voltage is varied, current is altered.
If the circuit voltage remains the same while resistance is varied, current is altered.
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Electronic Quantities
DC Fundamentals
In this circuit, voltage and / or resistance change.
Circuit current
a. remains constant.
b. is not affected.
c. must change.
To measure circuit current, place an ammeter in series with the circuit.
Place the COM meter terminal at the most negative side of the circuit entry point.
The unit of measure for current is the ampere (A).
One ampere of current can be expressed as 1000 milliamperes (mA or milliamps).
One milliampere of current can be expressed as 1000 microamperes (PA or microamps).
Examples of ampere/milliampere/microampere comparisons are:
1000 mA = 1A
100 mA = 0.1A
10 mA = 0.01A
1 mA = 0.001A = 1000 PA
PROCEDURE
฀
Locate the OHM’S LAW circuit block. Connect a voltmeter across the variable voltage
source.
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DC Fundamentals
Electronic Quantities
฀
Adjust the positive variable supply controls (on the base unit) for a reading of about 0V.
The positive variable supply has FINE and COARSE adjustments. The controls allow you to
set exact voltage values between 0V and +10V.
฀
Remove the voltmeter from your circuit. Select dc milliamps and a low value mA full scale
range on your multimeter. Connect the ammeter as shown in the circuit. Observe proper
polarity for your meter connection.
Connect R1 and R2 with a two-post connector.
฀
Your ammeter indicates a near zero current because
a.
b. circuit source voltage is near 0.
c.
฀
Adjust the positive supply controls until the ammeter reads 1.0. Does this reading indicate
that circuit current is 1A or 1 mA?
a. 1A because current can be expressed only in amperes.
b. 1 mA because an ammeter set to read milliamps can only indicate 1 mA.
c. 1 mA because the ammeter is set to indicate milliamperes and the circuit current is 1 mA.
฀
Monitor the current in your circuit as you adjust the positive supply voltage controls from
minimum to maximum.
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Electronic Quantities
฀
DC Fundamentals
What is the relationship between the current in your circuit and the applied voltage?
a. Current is directly related to voltage.
b. Current is indirectly related to voltage.
c. Current and voltage are not related.
฀
a. the negative battery terminal, through the load and
meter, and into the positive battery terminal.
b. both battery terminals and are averaged within the
meter.
c. the positive battery terminal, through the load and
meter, and into the negative battery terminal.
CONCLUSION
•
•
load, and back to the positive terminal of the power source.
•
The ampere, measured by an ammeter, is the unit of measure for current.
•
In a circuit, current increases when voltage increases; current decreases when voltage decreases.
Therefore, current and voltage are directly related.
REVIEW QUESTIONS
1. An ampere is the unit of measurement for
a. resistance.
b.
c. voltage.
d. potential difference.
2.
a.
b.
c.
d.
negative, negative
negative, positive
positive, positive
positive, negative
a.
b.
c.
d.
10 x 103A.
1 mA.
1.0 x 10–2A.
10 x 10A.
3.
4. A coulomb is the unit of measurement of
a. an electron.
b. a proton.
c. circuit current.
d. an electrical charge.
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Electronic Quantities
5. A reading of 8.55 mA is expressed in amperes as
a. 0.855A.
b. 0.0855A.
c. 0.00855A.
d. 0.000855A.
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