Download Power In Parallel Circuits

Circuits
Objectives
1. Identify a parallel circuit.
2. Determine the voltage across each parallel branch.
3. Determine the current across each parallel branch.
4. Apply Kirchoff’s current law
4. Determine total parallel resistance.
5. Apply Ohm’s law in a parallel circuit.
6. Use a parallel circuit as a current divider.
7. Determine power in a parallel circuit.
Twice as many cars can travel on a double road, three times as
many on a three-lane road and so on.
One could say that these two roads are parallel to each other in
that there is more than one path for the cars to follow.
This analogy can be applied to parallel circuits.
Parallel circuit – a circuit that provides separate
conducting paths for the current.
In a parallel circuit, each current path is called a branch.
The presence of branch lines means that there are multiple
pathways by which charge can traverse the external circuit.
In parallel circuits, each branch shares a direct connection
to a battery.
3 light bulbs connected to
a battery in a parallel circuit.
The same parallel circuit
as a circuit diagram.
More current flows through the smaller resistance. More
charges take the easiest path.
As charges move through the resistors (represented by the
paddle wheels) they do work on the resistor and as a result
lose electrical energy.
By the time the charges make it back to the battery, they have
lost all their energy.
The total current is equal to the sum of the currents in the
branches.
Itotal = I1 + I2 + I3 + ...
Rules for Parallel Circuits
1. The voltage is equal across all components in the circuit.
All components share the same voltage. The voltage drops
of each branch equals the voltage rise of the source.
The voltage across R1 is equal to the voltage across R2 which
is equal to the voltage across R3 which is equal to the voltage
across the battery.
As with series circuits, the sum of the potential differences
as you go around the loop is zero. This is true no matter
which branch you look at. (Kirchoff’s 2nd Law)
2. The current divides into separate branches such that the
current can be different in every branch.
The total current is equal to the sum of the individual
branch currents.
It is still the same
amount of current,
only split up into more
than one pathway.
3. When resistors are connected in parallel, the
total resistance of the circuit decreases.
The more branches you add to a parallel circuit, the lower
the total resistance becomes.
4. The total resistance of a parallel circuit is always
less than the value of the smallest resistor.
In a parallel circuit, the point where the current a
separates is called a junction.
Kirchoff’s Current Law
The sum of the currents entering a junction is
equal to the sum of the currents leaving the
junction.
In this example you will notice 8 Amps and 1 Amp entering
the junction while 7 Amps and 2 Amps leave. This makes a
total of 9 Amps entering and 9 Amps leaving.
A
B
The current going into
The junction equals
7 amps (1A + 2A + 4A).
The current entering the
junction is 6 amps ( 5A + 1A).
The current leaving the
Junction is 7 amps (7A)
The current leaving the
Junction is 6 amps (4A + 2A)
The diagram above represents current flowing in
branches of an electric circuit. What is the current
at point B? 13 A
Formula for Total Parallel Resistance
The inverse of the total resistance of the circuit (also called
effective resistance) is equal to the sum of the inverses of
the individual resistances.
Power In Parallel Circuits
Total power in a parallel circuit is found by adding up
the powers of all the individual resistors, the same
as for series circuits.
Fuse - a device used in electrical systems to protect against
excessive current.
Fuses are always connected in series with the component(s)
to be protected, so that when the fuse blows (opens) it will
open the entire circuit and stop current through the
component(s).
The fuse opens the circuit my melting a thin metal filament
inside the casing.
Once a fuse is blown, it must be replaced.
NEVER replace a fuse with a fuse rated for
higher current than the recommended fuse.
Glass Cartridge Fuses
Plug Fuses
Automotive Fuses
Automotive fuses are a class of fuses used to protect the
wiring and electrical equipment for vehicles.
They are generally rated for circuits no higher than 24 volts
direct current, but some types are rated for 42 volt electrical
systems.
The color of the
fuse is an indication
of its rating.
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Circuit Breakers
A circuit breaker
automatically shuts off the
power to the circuit in the event
of a dangerous electrical
overload or short circuit.
A circuit breaker can also be
used manually to disconnect a
circuit from incoming power so
that you can repair or upgrade
your receptacles, outlets, and
fixtures.
Circuit breakers are much easier to fix than fuses.
When the power to an area shuts down, the homeowner
can look in the electrical panel and see which breaker has
tripped to the "off" position.
The breaker can then be reset to the "on" position and
power will resume again.
If the breaker continues to trip after you flip it, you may
have a faulty breaker, a wiring problem, or there may be an
issue with an electrical device that relies on that breaker.
You should leave the breaker off and consult an electrician.
A main circuit breakers shuts off power to the whole house.
Individual circuit breakers connect to circuits throughout the
house.
GFCI breakers are designed to protect people from electrical
shock, rather than prevent damage to a building's wiring.
The GFCI constantly monitors the current in a circuit's
neutral wire and hot wire. When a surge in current is detected
on the how wire, the GFCI breaks the circuit, preventing
electrocution.
Since it doesn't have to wait for current to climb to unsafe
levels, the GFCI reacts much more quickly than a
conventional breaker.