The Parallel Circuit
A parallel circuit has more than one resistor (anything
that uses electricity to do work) and gets its name from having multiple
(parallel) paths to move along . Charges can move through any of
several paths. If one of the items in the circuit is broken then no charge
will move through that path, but other paths will continue to have charges
flow through them. Parallel circuits are found in most household electrical
wiring. This is done so that lights don't stop working just because you
turned your TV off.
Below is an animation of a parallel circuit where electrical
energy is shown as gravitational potential energy (GPE). The greater the
change in height, the more energy is used or the more work is done.
In this animation you should notice the following things:
- More current flows through the smaller resistance. (More
charges take the easiest path.)
- The battery or source is represented by an escalator
which raises charges to a higher level of energy.
- As the charges move through the resistors (represented
by the paddle wheels) they do work on the resistor and as a result, they
lose electrical energy.
- By the time each charge makes it back to the battery,
it has lost all the electrical energy given to it by the battery.
- The total of the potential drops ( - potential difference)
of each "branch" or path is the same as the potential rise (
+ potential difference) across the battery. This demonstrates that
a charge can only do as much work as was done on it by the battery.
- The charges are positive so this is a representation
of conventional current (the apparent flow of positive charges)
- The charges are only flowing in one direction so this
would be considered direct current ( D.C. ).
The following rules apply to a parallel circuit:
Ohm's Law may be used
in a parallel circuit as long as you remember that you can use the formula
with either partial values or with total values but you can not mix
parts and totals.
- The potential drops of each branch equals the potential
rise of the source.
- The total current is equal to the sum of the currents
in the branches.
- 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.
One important thing to notice from this last equation is
that the more branches you add to a parallel circuit (the more things you
plug in) the lower the total resistance becomes. Remember that as the total
resistance decreases, the total current increases. So, the more things
you plug in, the more current has to flow through the wiring in the wall.
That's why plugging too many things in to one electrical outlet can create
a real fire hazard.
©1998 Science Joy Wagon