9.h_parallel_circuits.pdf

Parallel Circuits
by
Nada Saab-Ismail, PhD, MAT, MEd, IB
nhsaab.weebly.com
nhsaab2014@gmail.com
P4.10g Compare the currents, voltages, and power in parallel and series circuits.
P4.10B Identify common household devices that transform electrical energy to other forms of energy, and describe the
type of energy transformation.
P4.10C Given diagrams of many different possible connections of electric circuit elements, identify complete circuits, open
circuits, and short circuits and explain the reasons for the classification.
1

Items;
1- Parallel Circuits
2- Equivalent Voltage
3- Equivalent Resistor
2

Parallel Circuits
In a parallel circuit, the electrons have a choice of several paths through the
circuit, as a result, may pass through any one of the several loads in the circuit.
Parallel wiring means that the devices are connected in such a way that the
same voltage is applied across each device.
Since R1 and R2 are connected in parallel, they have the same voltage (V) across
them.
3

When two resistors are connected in parallel, each receives current from the
battery as if the other were not present. Therefore, the two resistors connected in
parallel draw more current than does either resistor alone.
4

Example 1: Household Outlets;
Household outlets are connected in parallel instead of series so that each
appliance sees the same voltage; also, if one appliance blows out, power is not
cut to all appliances.
5

Equivalent Current and Resistance in Parallel Circuit
Ohmʼs Law: V = I R
I = V/ R
6

According to Kirchhoffʼs junction rule:
sum of into currents = sum of currents out
I = I1 + I2
V/ Rp = V/ R1 + V/R2 = V ( 1/ R1 + 1/R2)
So,
Rp =1/ R1 + 1/R2
Rp is the equivalent resistance of the parallel circuit (parallel resistors).
Equivalent Resistance in Parallel Circuit
Two parallel resistors R1 and R2 have a single equivalent resistor (Rp) that is
smaller than either R1 or R2.
7

Analogy, Additional Insight to Parallel Wiring
The two parallel pipe sections in figure (a) are equivalent to a single pipe of the
same length and same total cross sectional area in figure (b).
The pipe in part (b) offers less resistance to the water flow than either of the
narrower pipes offers individually in (a). Therefore, the pipe in (b) can push
through more water per second. That can explain both concepts: “A larger
current arises from a smaller resistance” and “Two parallel resistors R1 and R2
have a single equivalent resistor (Rp ) that is smaller than either R1 or R2.”
8

Example 2: Equivalent Resistance in Parallel Circuit;
The figure shows a circuit C1 composed of a 24-V battery and four resistors: 110,
180 and 470 Ω . Its equivalent circuit C2 has two resistors: 110 and 130 Ω .
C1 C2
The two parallel resistors 180 and 470 Ω in C1 can be replaced by an equivalent
resistor of 130 Ω in C2.
1/Rp = 1/R1 + 1/R2 = 1/180 + 1/470
Rp = 130 Ω
9

Example 3: Main and Remote Stereo Speakers;
Most receivers allow the user to connect to “remote” speakers in addition to the
main speakers. At the instant represented in the picture, the voltage across the
speakers is 6.00 V in the circuit schematic representation C1.
C1 C1 C2
10

Determine:
(a) the equivalent resistance of the two speakers in the circuit representation
C2,
(b) the total current supplied by the receiver,
(c) the current in each speaker, and
(d) the power dissipated in each speaker.
Data Table
Data Table
Data Table
Data Table
Data Table
Data Table
Data Table
Data Table
R1 R2 V Rp Itotal I for R1 I for R2 Ptotal
4.00 Ω 8.00 Ω 6.00 V ? ? ? ? ?
(a) The two parallel resistors 4.00 and 8.00 Ω in C1 can be replaced by an
equivalent resistor of 2.67 Ω.
11

Example 4: Three-Way Light Bulb and Parallel Wiring;
Within the bulb there are two separate filaments. When one burns out, the bulb
can produce only one level of illumination, but not the highest. The filaments
connected in parallel.
13

The two filaments are used to produce three different illumination levels (1, 2 and
3) with output power of 50, 100 and 150 W.
Level 1: A on, B off --> 50 W output
Level 2: A off, B on --> 100 W output
Level 3: A on, B on --> 50 W + 100 W = 150 W output
14

Example 5: Why do your headlights dim when you start up your car?
Before turning on the starter switch, I1 = 0 A.
After turning on the switch, a large current flow through I1. The headlights has a
reduced potential, so they dim.
15

Example 6: Circuits Wired Partially in Series and Partially in Parallel.
16

References:
1) Humanic. (2013). www.physics.ohio-state.edu/~humanic/. In Thomas Humanic
Brochure Page.
Physics 1200 Lecture Slides: Dr. Thomas Humanic, Professor of Physics, Ohio State
University, 2013-2014 and Current. www.physics.ohio-state.edu/~humanic/
2) Cutnell, J. D. & Johnson, K. W. (1998). Cutnell & Johnson Physics, Fourth Edition.
New York: John Wiley & Sons, Inc.
The edition was dedicated to the memory of Stella Kupferberg, Director of the Photo
Department:“We miss you, Stella, and shall always remember that a well-chosen
photograph should speak for itself, without the need for a lengthy explanation”
17

3) Martindale, D. G. & Heath, R. W. & Konrad, W. W. & Macnaughton, R. R. & Carle,
M. A. (1992). Heath Physics. Lexington: D.C. Heath and Company
4) Zitzewitz, P. W. (1999). Glencoe Physics Principles and Problems. New York:
McGraw-Hill Companies, Inc.
5) Schnick, W.J. (n.d.). Calculus-based physics, A Free Physics Textbook. Retrieved from
http://www.anselm.edu/internet/physics/cbphysics/index.html
6) Nada H. Saab (Saab-Ismail), (2010-2013) Westwood Cyber High School, Physics,
Detroit, Michigan, U.S.A.
7) Nada H. Saab (Saab-Ismail), (2009- 2014) Wayne RESA, Bilingual Department,
Detroit, Michigan, U.S.A.
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