Circuit Connections
Circuit Symbols and Circuit Diagrams
Thus far, this unit of The Physics Classroom tutorial has
focused on the key ingredients of an electric circuit and upon the concepts of
electric potential difference, current and resistance. Conceptual meaning of
terms have been introduced and applied to simple circuits. Mathematical
relationships between electrical quantities have been discussed and their use
in solving problems has been modeled. Lesson 4 will focus on the means by which two
or more electrical devices can be connected to form an electric circuit. Our
discussion will progress from simple circuits to mildly complex circuits.
Former principles of electric potential difference, current and resistance will
be applied to these complex circuits and the same mathematical formulas will be
used to analyze them.
Electric circuits, whether simple or complex, can be
described in a variety of ways. An electric circuit is commonly described with
mere words. Saying something like "A light bulb is connected to a
D-cell" is a sufficient amount of words to describe a simple circuit. On
many occasions in Lessons 1 through 3, words have been used to describe simple
circuits. Upon hearing (or reading) the words, a person grows accustomed to
quickly picturing the circuit in their mind. But another means of describing a
circuit is to simply draw it. Such drawings provide a quicker mental picture of
the actual circuit. Circuit drawings like the one below have been used many
times in Lessons 1 through 3.
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Describing Circuits with Words "A circuit contains a light bulb and a 1.5-Volt
D-cell." |
Describing Circuits with
Drawings
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A final means of describing an electric circuit is by use of
conventional circuit symbols to provide a schematic diagram of the circuit and
its components. Some circuit symbols used in schematic diagrams are shown below.
A single cell or other power source is represented by a long
and a short parallel line. A collection of cells or battery 
is represented by a collection of long and short parallel lines. In both
cases, the long line is representative of the positive terminal of the energy
source and the short line represents the negative terminal. A straight line is
used to represent a connecting wire between any two components of the circuit.
An electrical device that offers resistance to the flow of charge is
generically referred to as a resistor and is represented by a zigzag line. An
open switch is generally represented by providing a break in a straight line by lifting a portion of the line upward at a
diagonal. These circuit symbols will be frequently used throughout the
remainder of Lesson 4 as electric circuits are represented by schematic
diagrams. It will be important to either memorize these symbols or to refer to
this short listing frequently until you become accustomed to their use.
As an illustration of the use of electrical symbols in
schematic diagrams, consider the following two examples.
Example
1:
Description
with Words: Three D-cells are placed in a battery pack to power a circuit
containing three light bulbs.
Using
the verbal description, one can acquire a mental picture of the circuit being
described. This verbal description can then be represented by a drawing of
three cells and three light bulbs connected by wires. Finally, the circuit
symbols presented above can be used to represent the same circuit. Note that
three sets of long and short parallel lines have been used to represent the
battery pack with its three D-cells. And note that each light bulb is
represented by its own individual resistor symbol. Straight lines have been
used to connect the two terminals of the battery to the resistors and the
resistors to each other.
The above circuits presumed that the three light bulbs were
connected in such a way that the charge flowing through the circuit would pass
through each one of the three light bulbs in consecutive fashion. The path of a
positive test charge leaving the positive terminal of the battery and
traversing the external circuit would involve a passage through each one of the
three connected light bulbs before returning to the negative terminal of the
battery. But is this the only way that three light bulbs can be connected? Do
they have to be connected in consecutive fashion as shown above? Absolutely
not! In fact, example 2 below contains the same verbal description with the
drawing and the schematic diagrams being drawn differently.
Example
2:
Description
with Words: Three D-cells are placed in a battery pack to power a circuit
containing three light bulbs.
Using
the verbal description, one can acquire a mental picture of the circuit being
described. But this time, the connections of light bulbs is done in a manner
such that there is a point on the circuit where the wires branch off from each
other. The branching location is referred to as anode. Each
light bulb is placed in its own separate branch. These branch wires eventually
connect to each other to form a second node. A single wire is used to connect
this second node to the negative terminal of the battery.
These two examples illustrate the two common types of
connections made in electric circuits. When two or more resistors are present
in a circuit, they can be connected in
series or in parallel. The remainder of
Lesson 4 will be devoted to a study of these two types of connections and the
effect that they have upon electrical quantities such as current, resistance
and electric potential. The next part of Lesson 4 will introduce the distinction between series
and parallel connections.
Check Your Understanding
1. Use circuit symbols to construct schematic diagrams for
the following circuits:
a. A single cell, light bulb and switch are placed together
in a circuit such that the switch can be opened and closed to turn the light
bulb on.
b. A three-pack of D-cells is placed in a circuit to power a
flashlight bulb.
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c.
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d.
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2. Use the concept of conventional current to draw an unbroken line on the schematic
diagram at the right that indicates the direction of the conventional current.
Place an arrowhead on your unbroken line.
The conventional current in
the external circuit is the direction that a positive charge would move. It is
directed from the positive terminal to the negative terminal. The positive
terminal of a cell is the long line and the negative terminal is the short
line. If there is a two-cell battery, then the long line on the end is the
positive terminal of the battery. The short line on the opposite end is the
negative terminal of the battery.