Download V and R in parallel circuits

Chapter 9: Circuits
Series circuits have only one path
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Parallel circuit – current can take more than one path at a time
Here a second 60 W bulb is added in
parallel with the first bulb
Current may now flow through the first
bulb or the second bulb.
Both bulbs glow at their intended brightness.
Each receives the full circuit voltage of 120 volts.
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Parallel circuit – Another 60 W bulb is added, in parallel to the others
Every load connected still
receives the full 120 V circuit
voltage.
How is this possible? Each additional bulb causes more current
(amperage) to be drawn from the wall outlet.
There is a lot of voltage (“pressure”) in electrical lines coming to your
house, just like there is a lot of water pressure in the water pipes
coming to you house.
If you open up more faucet taps, then the water pressure forces more
water current into your faucet taps.
If you open up more parallel paths for electricity, then the voltage
forces more electrical current into your light bulbs.
Danger! Current from the wall outlet increases each time another
load is added to the circuit in parallel – and it is too easy to keep
plugging in more items in parallel.
You can draw so much current through
your wires that they will melt the wire
insulation, heat up, and start a fire!
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Houses have circuit breakers or fuse-boxes to prevent this.
When they detect too much current being drawn, they break the
electric circuit to that part of your house.
Electrical circuits in homes are parallel circuits.
1. Each outlet has its own path. One can have something connected
and be on, while another has nothing connected, or have something
connected while turned off.
2 Every outlet sees the same voltage, because each outlet is
connected to the same wire.
In a series circuit, current ( I ) is the same at all points.
What goes in one end, must come out the other end !
Analyze this circuit using Ohm’s law. 2 batteries, 3 bulbs.
Every charge starts with 3
volts of electrical “pressure”.
(At the positive part of the
top battery)
As charge moves through
the circuit, some energy is
transformed into light.
Thus after every bulb, the
energy must be lower.
We see a drop in voltage
from 3 v, to 2 v, to 1 V,
to 0 v.
How much current is in this circuit?
V = IR
I=V/R
= 1.5 v / 3 
= 0. 5 amps
Every part of a circuit has resistance, even wires and batteries. However, light bulbs,
resistors, motors and heaters have much greater resistance than wires and batteries.
When doing problems, we usually can treat resistance of wires and batteries as
negligible.
Find voltage drops in a series circuit: Conservation of Energy
What is the voltage drop across each resistor?
V = IR = ( 0.5 amps )( 1  ) = 0.5 volts
Energy is not created or destroyed.
Energy may be transformed from one form to another.
As current flows along the circuit, each resistor uses some of the energy,
transforming it into waste heat.
Thus, the voltage gets lower after each resistor.
Water pressure law:
Around the path of any closed water system, water pressure changes
must add up to zero.
If water pressure changes didn’t add up to zero, then somewhere
we’d mysteriously be gaining water pressure, as the water loops around.
(Eventually the water pressure would rise to infinity?!)
Kirchhoff’s voltage law – Same as above, but for electricity
Around any electric circuit, voltage changes must add up to zero
If voltage changes didn’t add up to zero, then somewhere
we’d mysteriously be gaining voltage, as the charge loops around .
(Eventually the voltage would rise to infinity?!)
Kirchhoff’s voltage law example
Batteries raise the voltage
Resistors, light bulbs, diodes,
lower the voltage
Voltage changes = + 1.5 v – 0.5 v - 0.5 v – 0.5 v = 0 volts
Current ( I ) always goes
somewhere:
If I flows into a branching
point, then the same I must
flow out again.
Kirchhoff’s current law.
3 light bulbs in parallel
Each has a current of 1 amp.
Battery must supply 3 amps
since each bulb draws 1 amp,
and there are 3 bulbs.
At the first branch, 3 amps
flow in
1 amp flows down to the first
bulb, so 2 amps flow on to
the remaining 2 bulbs.
V and R in parallel circuits
Each branch sees the same voltage
Imagine several series circuits connected to the same battery.
Each branch has a path back to the battery, without any other
resistance in the way.
Branches may have different currents
Current in each branch depends on the
branch’s Resistance.
When you plug a desk lamp and a
power saw into an outlet, they each
use different amounts of current.
Open circuits, closed circuits, and short circuits
Open circuit
Closed circuit
Short circuit = path with zero, or very low, resistance.
Ex: Connect a wire directly between two ends of a battery.
Often accidentally
made
Creates a parallel
path with very low
resistance.
In parallel circuits,
the branch with the
lowest resistance
draws the most
current
Why short circuits are dangerous – Danger, Will Robinson!
In the above example, current
through your wire could be as high as
1,500 amps!
This could melt the wire in an instant
and probably burn you as well.
They should always be a concern
when working around electricity.
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