Geoscience Reference
In-Depth Information
Note: In studies of electricity and electronics, many circuits are analyzed that consist mainly of
specially designed resistive components. As previously stated, these components are called
resistors . Throughout our remaining analysis of the basic circuit, the resistive component
will be a physical resistor; however, the resistive component could be any one of several
electrical devices.
Keep in mind that the simple circuits shown in the figures to this point illustrate only a few of
the many symbols used in schematics to represent circuit components. Other symbols will be intro-
duced, as we need them. It is also important to keep in mind that a closed loop of wire (conductor)
is not necessarily a circuit. A source of voltage must be included to make it an electric circuit. In
any electric circuit where electrons move around a closed loop, current, voltage, and resistance are
present. The physical pathway for current flow is actually the circuit. By knowing any two of the
three quantities, such as voltage and current, the third (resistance) may be determined. This is done
mathematically using Ohm's law , which is the foundation on which electrical theory is based.
11.7.3 o hm ' s l aW
Simply put, Ohm's law defines the relationship between current, voltage, and resistance in electric
circuits. Ohm's law can be expressed mathematically in three ways:
1. The current ( I ) in a circuit is equal to the voltage applied to the circuit divided by the resis-
tance of the circuit. Stated another way, the current in a circuit is directly proportional to
the applied voltage and inversely proportional to the circuit resistance. Ohm's law may be
expressed as
E
R
I
=
(11.19)
where
I = Current in amps.
E = Voltage in volts.
R = Resistance in ohms.
2. The resistance ( R ) of a circuit is equal to the voltage applied to the circuit divided by the
current in the circuit:
E
I
R
=
(11.20)
3. The applied voltage ( E ) to a circuit is equal to the product of the current and the resistance
of the circuit:
E = I × R = IR
(11.21)
If any two of the quantities in Equations 11.19 through 11.21 are known, the third may be easily
found. Let us look at an example.
EXAMPLE 11.18
Problem: Figure 11.21 shows a circuit containing a resistance ( R ) of 6 ohms and a source of voltage
( E ) of 3 volts. How much current ( I ) flows in the circuit?
Solution:
E
R
3
6
I
===
0.5 amp
 
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