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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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