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Potential Difference and Battery Hookup
Name: August
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: N/A
Question:
What prevents an electric circuit from functioning if you
use the positive terminal from battery #1 and the negative terminal
from battery #2? It seems to me that the voltage difference required
to drive the circuit should still be present since a positive cell is
connected to a negative cell.
As an example, if I were to construct a simple circuit that contained
only one 9v battery and a flashlight bulb, the bulb would light. If I
were to use two 9 volt batteries, running my wiring from the positive
terminal of one 9 volt battery to the bulb, then continuing on to the
negative terminal of the second 9 volt battery (critical point = but
NOT connecting the two batteries together), the bulb will not light.
Why? Everything I read states that a voltage differential is required
to cause a current to flow and most texts state that the this required
voltage differential is created by the chemical reactions within a
battery that cause one side to have an excess of electrons while the
other side has a deficiency of electrons. So why does the positive
and negative sources have to be from the same object? Why can you not
use the positive terminal of one object, but the negative terminal of
another object? Isn't the voltage differential still present?
Illustration:
battery one (positive) <-------|
battery one (negative)-unused |
light bulb
battery two (positive)-unused |
battery two (negative) >-------|
Replies:
Hi August
Yes, you do need a voltage differential to cause a current to flow. In
your configuration, however, a voltage difference does not exist, and you
can verify this by putting a voltmeter across the terminals where the bulb
is connected. Voltage differences are all relative to some reference point.
Because your batteries are not connected together, they are independent from
each other and there is no common reference point. Another way to think of
his is that the current that results from a voltage difference must have a
path in which to flow. By not connecting the batteries together, the path
(or circuit) is incomplete and the current cannot flow.
Hope this helps!
Bob Froehlich
August,
There are two factors here of importance. One relates to what “voltage
difference” is. The other relates to “current flow”.
The voltage difference of a battery is the difference in electric potential
between the two terminals of the battery. It does not mean that one is
greater than zero and one is less than zero. It just means that the positive
terminal of a battery is 9V greater than the negative terminal of the same
battery. It is possible that, compared to some reference point, the two
terminals are at 100V and 109V. It could be that they are at -49V and -40V.
Any charge passing through the battery will experience a change of electric
potential by 9V. This does not mean that the positive terminal of one
battery is 9V higher than the negative terminal of another battery. A
non-electrical example of a similar concept is a pair of meter sticks.
The 100cm end of a meter stick is 100cm away from the 0cm end of the same
stick. It is NOT necessarily 100cm from the 0cm end of any other meter stick.
Electric current is the motion of electrons through a circuit. The electrons
move around and around the circuit, gaining energy as they pass through the
power source (perhaps a battery) and losing energy as they pass through other
devices, such as motors or light bulbs. If the two batteries are not
connected together, there is no way for electrons in one battery to get to
the other. A little bit of charge may build up at the battery terminals, but
there is no way for current to flow. Without current, there is no way to move
energy from the battery to the light bulb. A light bulb glows because of
electric current constantly moving through it, not because of electric charge
built up inside it.
Dr. Ken Mellendorf
Dear August,
A battery creates a difference in voltage between the two terminals of THAT
battery. The absolute potential of a terminal is not determined by that
battery. So if, using 9 volt batteries, you connect the positive terminal
of battery A to the negative terminal of battery B, those two terminals
will be at the same potential. If you then connect a bulb to those two
terminals, no current will flow because the two terminals are at the same
voltage. Note that the disconnected terminals will be at -9V for the
negative terminal of battery A and +9V for the positive terminal of
battery B (I recommend you draw a few sketches).
If you now hook the disconnected terminals together with a zero
resistance wire, those terminals will be at the same potential (call
it 0 volts). The positive terminal of A will then be at +9V and the
negative terminal of B at -9V. A bulb connected to those terminals
will then burn brightly and, in fact, probably burn out immediately.
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
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Update: June 2012
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