This document provides information about parallel circuits. It defines parallel circuits as those where components are connected such that current can divide and flow through multiple components simultaneously. Some key points made include: - In parallel circuits, the current through individual branches can vary but the total current increases as total resistance decreases. - The total resistance of a parallel circuit decreases as more resistors are added in parallel, as these provide additional paths for current to flow. - The voltage across each component/resistor in a parallel branch is the same. - Examples of parallel circuits include household electrical wiring and multi-core computer processors.

1. ONACD - Editable curriculum designed for teachers by teachers
Parallel Circuits
Branched Paths
ONACD –Editable Curriculum designed for teachers by teachers
http://www.allaboutcircuits.com/vol_1/chpt_5/3.html

2. ONACD - Editable curriculum designed for teachers by teachers
Parallel
For electrical circuits the term “parallel” indicates that
components are connected in a way such that current is
divided to flow through multiple components at the same
time.
Household plumbing is like this. That way you can take a
shower while someone else does the dishes.
Arrows indicate flow direction of positive
charge.
Note: negative charge flow is in opposite direction.
If voltage is supplied at a constant value, as it typically is, the
following observations for current and total resistance can
be made:

3. ONACD - Editable curriculum designed for teachers by teachers
Current
The value of current can be different in different branches
of a parallel circuit.
Since there are multiple paths for the current to flow, a
break in one path does not prevent the other paths from
working.
Household circuits are like this. Each device can be turned
on or off without affecting other devices.

4. ONACD - Editable curriculum designed for teachers by teachers
Total Resistance
The addition of more resistors in parallel reduces the total
resistance.
Parallel resistors provide multiple ways for the current to flow.
Thus the total current increases as the total resistance
decreases.
Notice that since all the components in a parallel branch has
the same start point and end point: the voltage across all
the resistors is the same.
Suppose you had 5 hoses and needed to drain a pool. If you
connect the hoses together in series the water would only
flow slower. Run the hoses in parallel you drain the pool 5
times faster!

5. ONACD - Editable curriculum designed for teachers by teachers
Total Resistance
In a parallel circuit total resistance is calculated by
adding the reciprocal of the resistors together:
Where RT is resistance total and R1, R2, R3,… are
the resistance of each resistor.
Extension:
Investigate Duel Core computer processors and
explain why this makes a computer some much
faster.

6. ONACD - Editable curriculum designed for teachers by teachers
Sample Calculations
Consider the following parallel circuit with two identical resistors.
1.What is the:
a)total voltage?
12V
b)total resistance?
12V R1=2Ω R2=2Ω
A B
C
1 2 3
1 1 1 1
...
TR R R R
= + + +
1 1 1
2 2TR
= +
1 2
2TR
=
1TR = Ω

7. ONACD - Editable curriculum designed for teachers by teachers
c) total current?
2. What is the current
 at A?
12A (same as total current)
 at B? Only need current through R2.
V IR=
12 (1)I=
12I A=
V IR=
2 2 2V I R=
212 (2)I=
2
12
2
I=
2 6I A=

8. ONACD - Editable curriculum designed for teachers by teachers
c. at C?
12A (same as total current)
3. What is the Voltage across:
a. A to C?
Notice voltage across resistors is the same as the battery: 12V
b. A to B?
There is no resistance across these points so there is no voltage drop:
0V
c. B to C?
You can do a calculation with V=IR or V=(6)(2) for each resistor but
notice that there is no difference between AC and BC (this is just the
voltage across the battery : 12V