Series and Parallel Wiring
GET A CALCULATOR!!!!!
SERIES CIRCUITS
The same current through each
resistor.
EQUIVALENT RESISTANCE:
What is the net resistance?
What one resistor could
replace a group of resistors?
For resistors wired in series, the
equivalent resistance is given
by:
Req = R1 + R2 + R3 + …
PARALLEL CIRCUIT
Same voltage across each
resistor.
For resistors wired in parallel, the
equivalent resistance is given
by:
1
1
1
1



 ...
R eq
R1 R 2 R 3
Example: What is the equivalent resistance
of a 10 W, 20 W, and 30 W resistor wired in
series? In parallel?
Series: Req = 10 W + 20 W + 30 W = 60 W
Parallel 1 / Req = 1/10 W + 1/20 W + 1/30 W
Req = 5.45 W
There’s much less resistance if resistors are
wired in parallel than if they’re wired in
series.
With less resistance, the charge pump will
able to push much more current around the
circuit.
Circuits and Ohm’s Law: V = IR
Series: Req = 10 W + 20 W + 30 W = 60 W
Parallel 1 / Req = 1/10 W + 1/20 W + 1/30 W
Req = 5.45 W
If the circuits were supplied 12 V by a
battery, what is the current in the series
circuit?
I = V/R
I = 12 V / 60 W  0.2 A
In the parallel circuit?
I = V / R = 12 V / 5.45 W  2.2 A
In a circuit with 3 light bulbs wired in
series, if the current in the first bulb is
0.12 A, what is the current in the 2nd
bulb? In the 3rd bulb?
The current is the same for all
resistors wired in series to each other!
If the circuit is supplied 9 V from a battery
and the voltage across the 1st bulb is 1 V
and across the 2nd bulb is 6 V, what is
the voltage across the 3rd bulb?
In a series circuit, the sum of voltages
across each resistor is equal to the
voltage across the whole circuit!
Ohm’s Law V = IR
A 6 W and a 4 W resistor are wired in parallel across
a 9 V battery. Determine the current in the circuit.
- First find the equivalent resistance, then the current.
R = 2.4 W
I=V/R
I = 3.75 A
How much current goes through the 6 W resistor?
- Resistors in parallel have the same voltage
(pressure) across them, even if they have different
resistance.
I=V/R
I = 9 V / 6 W = 1.5 A
How much current goes through the 4 W resistor?
I = 9 V / 4 W = 2.25 A
Characteristics of Series and
Parallel Wiring
Series: If one component goes out,
They all go out!
As more resistors are added, the equivalent resistance
Increases!
which means that the current in that part of the circuit
Decreases!
Parallel: If one component goes out,
The rest still work!
As more resistors are added, the equivalent resistance
Decreases!
which means that the current in that part of the circuit
Increases!
For maximum resistance- use series
wiring.
For minimum resistance- use parallel
wiring.
The flow of water is a very good analogy
to the flow of charges in both series
and parallel circuits.
Schematic diagrams
symbols to represent circuit components.
wires:
Charge pumps:
Resistors:
Switches:
Ground:
All devices connected to a circuit (light bulbs, TV’s,
toasters, etc.) resist the flow of charges and are
sometimes drawn as a resistor in the circuit (if you’re
considering the unit as a whole).
For resistors wired “in series” the
same current flows through each
one, however the potential
difference, voltage, is additive
Voltage gain through battery
=
Sum of Voltage drop through resistors
For resistors wired “in parallel”, the
potential difference, voltage, is
the same for each of them,
however the current is additive.
Total Current pushed by battery
=
Sum of Current going through all the resistors