RT =
In our circuit:
RT
= = = 33.33 Ω
The total current IT is then:
IT = = 6 A
To find how much current will flow in
branch A:
IA
= = 2 A
In branch B:
IB = = 4 A
The sum of the branch currents (IA + IB)
is 2 amps + 4 amps = 6 amps. This is the
same as the total current flowing in the
circuit. This again confirms Kirchhoff’s
current law.
Conductance
The ability to conduct electricity is oppo-
site the ability to resist the flow of electricity.
So we can consider the current carrying
ability of any wire or circuit either by stating
its resistance to the flow of electrons, or by its
ability to conduct electrons. Its ability to con-
duct is called conductance. The letter symbol
for conductance is G. Conductance is mea-
sured in siemens (S). Conductance is the
reciprocal of resistance. That is to say, conduc-
tance is one divided by the resistance value.
For example, if the resistance of a circuit is 4 Ω,
its conductance can be found using:
G =
so
= 0.25 siemens (S)
If the circuit resistance is 500 Ω, its
conductance is:
G = = = 0.002 S
Two or More Resistors
in Parallel
When two or more unequal resistors
are connected in parallel, the conductance
method of finding RT is:
RT =
This formula involves the use of frac-
tions, and is sometimes called the reciprocal
method. An example will show you how it
works. See Figure 7-6.
RT =
1
1
1
500 Ω
1
R
1
4 Ω
1
R
200 V
50 Ω
200 V
100 Ω
200 V
33.33 Ω
5000 Ω
150 Ω
100 Ω × 50 Ω
100 Ω + 50 Ω
R1 × R2
R1 + R2
58
Electricity
Ernst Werner von Siemens (1816–1892)
Siemens is honored with the unit of
conductance being named after him.
Educated as an electrical engineer in
Germany, he led the way for many
advances in the principles of electricity.
For example, he developed a method to
coat wire with an insulation that was
seamless—quite an advance for his time.
He founded a German company that still
carries his name and continues its
founder’s refinements in the field of elec-
trical devices and generating equipment.
History Hit!
1
R1
1
R2
1
R3
+ +
1
100 Ω
+
1
200 Ω
+
1
400 Ω