Exercise
4
Voltage, Current, and Measuring Instruments
EXERCISE OBJECTIVE
When you have completed this exercise, you will be familiar with the notions of
voltage and current, and know how to measure these parameters using a
voltmeter and an ammeter. You will also know what a multimeter and a
clampmeter are, and how to use them.
DISCUSSION OUTLINE
The Discussion of this exercise covers the following points:
DISCUSSION
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The notion of current
The notion of voltage
Voltage and current: an analogy for better comprehension
Voltage measurement using a voltmeter
Current measurement using an ammeter
Introduction to the multimeter
Introduction to the clampmeter
Introduction to alternating current
AC voltage and current sine waves
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Circuit parameter measurements in ac circuits
Frequency and period of a sine wave. Peak value and RMS value of a
sine wave.
Measuring voltage. Measuring current.
The notion of current
Electrical current is defined as the electrical charge passing through a conductor
per unit of time; it is a flow of charge carriers. In the case of a typical conductor,
such as a copper wire, the charge carriers are electrons. The higher the current
in the conductor, the higher the number of electrons passing through it per
second.
Current is measured in coulombs per second (C/s)1 which is abbreviated as
amperes (A) after French mathematician and physicist André-Marie Ampère, who
is considered the father of electrodynamics2. Current is usually denoted using the
letter ‫ܫ‬.
There are two ways to represent the direction of current in a circuit:
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© Festo Didactic 89688-00
The first way is to consider the direction of electrical current to be the
same as the direction of the electrons flowing in the circuit. In this
representation, current flows from the negative terminal of the dc power
source to the positive terminal because electrons flow from the negative
terminal to the positive terminal. This is shown by the red arrow in the
An electron has an electrical charge of -1.602×10-19 coulombs
1 A = 1 C/s
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Exercise 4 – Voltage, Current, and Measuring Instruments  Discussion
circuit of Figure 43a. This representation of the direction of current flow is
very rarely used and will not be used in this manual.
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The second way is to consider the direction of electrical current to be the
same as the direction of the positive charges flowing in the circuit. In this
representation, current flows from the positive terminal of the dc power
source to the negative terminal. This is shown by the blue arrow in the
circuit of Figure 43b. This representation of the direction of current flow is
called the conventional direction of current and is the one that will be
used in this manual.
Direction of
electron flow
DC power
source
Conventional direction
of current
DC power
source
Indicator light
(a) Direction of electron flow
Indicator light
(b) Conventional direction of current
Figure 43. Current direction in a dc circuit.
The notion of voltage
Voltage is the electric potential energy per unit of charge. When using the term
voltage, we usually refer to the voltage difference between two points, such as
the terminals of a battery. A voltage difference is defined as the negative work
done when a charge moves between the two reference points.
A voltage difference across a conductor creates an electric field which exerts a
force on the electrons in the conductor and makes them flow. The higher the
voltage difference, the higher the force exerted on the electrons. Voltage is
measured in joules per coulomb (J/C), which is abbreviated as volts (V) after
Italian physicist Alessandro Volta who invented what is considered the first
chemical battery. In this manual, voltage difference is denoted using the letter ‫ܧ‬.
Note that voltage is also denoted using the letter ܸ.
Voltage is measured as a potential difference between two points in a circuit.
When two points are connected by a conducting material, electrical current flows
from the point with the highest voltage to the point with the lowest voltage. This
property is the basic principle of power sources. For example, in a dc power
source such as the 1.5 V battery shown in Figure 44, the voltage difference
between the positive terminal and the negative terminal is 1.5 V. This voltage
difference is applied to any load (e.g., flashlight, music player, photo camera,
telephone) connected to the battery terminals. When a load is connected to the
battery, an electrical current flows from the positive terminal, through the load,
and then to the negative terminal of the battery.
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