Download Chapter 7

Chapter 7
Parallel Circuits
Parallel Circuit
Has two or more paths for electron
flow. The electrons have choices to
make as to where they go.
Load
Load
Load
Voltage Source
Voltage
The voltage drop across each component is
equal to the source voltage.
Voltage Drop # 1
ET = E1 = E2 = E3 = E…= EN
Total Voltage
Voltage Drop # 2
Current
The total current in a parallel circuit is
equal to the sum of the individual
branch currents.
Current at branch # 1
IT = I1 + I2 + I3 + I…+ IN
Total Current
Current at branch # 2
Resistance
Total resistance is always less than the
smallest branch resistance.
Methods of determining resistance in a
parallel circuit:
Product over the sum
Reciprocal
Equal resistances
Graph method
Product Over The Sum
• Can only be used when there are two
resistances of unequal values
RT =
Total
Resistance
R1 x R2
R1 + R2
Reciprocal Method
Can be used with three or more
resistance values
1
1
1
1
1
=
+
+
…+
RT
R1
R2
R3
RN
Total
Resistance
Equal Resistances Method
Used when there are two or more resistances of
equal value.
The total resistance is equal to the value of one
resistor divided by the total number of resistors in
the parallel circuit
R
RT =
N
Graph Method
Use a bar chart; connect the top of the
bars to the bottom of the other bar and
estimate the value by where the lines
cross.
R1 = 20
20
15
10
5
0
R2 = 15
RT ~ 8
Power
Total Power consumed in a series circuit
is equal to the source voltage multiplied
by the circuit current.
PT = P1 + P2 + P3 …+ PN
Ohm’s Law
Can be applied to any individual component
of a series circuit.
If you know two values you can find the
third.
I =
V
R
R=
V
I
V = I x R
Troubleshooting
Use totals to find missing branch
values.
Use branch totals to find missing totals.
The voltage is the same everywhere.
Current should flow through each
branch, if it doesn’t something is wrong.
(compare with calculations)