# [noob] Resistance in paralel = increase in current?

Discussion in 'Electronic Basics' started by Simon, Apr 13, 2007.

1. ### SimonGuest

Hi,
I'm following a book here and it was talking about resistance and
how to calculate the total resistance of a circuit. I understand
everything pretty well except the part that said something like: now
having another resistance connected in paralel to the initial
resistance, the current will double and go in both resistance... it
also says something like the battery is producing more current because
of this.

I'm wondering, is this true? I don't have a multimeter (yet) to
verify... would an ammeter show an increase of current in this
circuit? And as I understand it, having a circuit in paralel is just
a great way of depleting a battery's power faster! (Oh i'm sure there
are good applications for this, i'm not there yet)

But also, can you explain what happens with the current and why it
increases when a paralel resistance is added?

Thanks a lot!
Simon

2. ### Charles SchulerGuest

A resistance in parallel with another resistance is an additional path for
current to flow. Given a fixed voltage, more paths (less resistance) means
more current flow. You might want to search on "conductance," which is
another way of understanding this phenomenon.

3. ### Peter BennettGuest

Yes - this is true.
The battery voltage will stay the same when you add a second resistor
(or close enough for this discussion), so you can consider each
resistor (each parallel branch of the circuit) independently.

If you have, for example, a 10 volt battery, and two 10 ohm resistors
connected in parallel across it, each resistor will draw 1 amp
(10volts/10 ohms), so the total current delivered by the battery will
be 2 amps.

In house wiring, all the lights and outlets are connected in parallel
across the 120V (in North America) or 240V (elsewhere) AC supply.
Likewise, in a car, all the lights, radio, starter, etc. are connected
in parallel across the 12 volt battery. In either case, each light or
other load draws what it wants from the power source, and the power
source therefore delivers the total of all currents demanded by the

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
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4. ### John PopelishGuest

That statement makes the assumption that the voltage across
the two resistors in parallel is the same as the voltage
across the original resistor and the second resistor is
similar to the first one.
You open the door of a car and the dome light comes on (is
connected across the battery. The battery generates enough
current to light that bulb. You leave the door open, and
turn the headlights on. The battery now also generates
enough current to light those bulbs (that are connected in
parallel with the dome light). Those bulbs are resistors.
Actually, you may notice the dome light dims, slightly, when
you turn the headlights on, because the battery will not
hold a perfectly steady voltage as it generates the higher
current. It has a little effective internal resistance that
is in series with those two sets of lights that are in
parallel with each other. The real world is complicated.
Sorry.
The second resistor forms a second circuit (path for
current) that is sort of beside the first path. The current
through the first resistor is little changed by this second
path, if the battery has a low internal resistance compared
to each of these two resistors.
Exactly so, but sometimes you need a power source to do two
things for you, at the same time.
Have you ever seen a Y connector to put on a water faucet so
you can connect two hoses? Running water through one hose
lets less water out of the faucet than running water from
two of them at, the same time.

5. ### PuckdropperGuest

*snip*
Christmas lights. When one bulb burns out, the strand stays light.
(Hm... 300 lightbulbs come on at once when the OP understands this. ;-))

Puckdropper

6. ### ehsjrGuest

Yes, as others have said.
Sure you are, you just don't know it. You've seen it
hundreds of times. For example a car's headlights are
in parallel, the 120 volt electrical devices in your
house are in parallel, the clock in a clock radio is
in parallel with the radio, and so forth.
way of thinking about it. Say you have a light bulb
lighting an area. You want to make it brighter, so you
add another bulb, and now it's twice as bright. That
means twice as much electrical energy needs to be consumed
That's why the current increases - you are using twice
as much energy.

In mathematical terms, E = I * R where E is voltage, I
is current and R is resistance. When E is a constant
voltage, say a 12 volt battery, and you decrease R,
I must rise. Say you have a 12 volt battery connected
to a 12 ohm resistor. E=I*R, so I must equal 1 amp.
Now, you add a second 12 ohm resistor in parallel
with the first. The paralle resistance equals 6 ohms,
so 12 = I * 6 which means I is 2. Each resistor will
have 1 amp through it, making 2 amps total.

Ed

7. ### RandyGuest

if you parallel two equal resistances the total resistance is half,
current equals voltage devided by the resistance, so when the
resistance is cut in half the current doubles. Also in a series
circuit you have the same current everywhere but different voltage
drops relative to the resistance of each component, while in a
parallel circuit you have the same voltage in each branch but
different amounts of current according to the resistance of the branch.