Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: voltages etc.



I agree there will be a small chemical reaction, depending on the
capacitance of the object it is connected to, but since this
capacitances are quite small, the reaction will be minimal.
You can use a bridge circuit...that is what a potentiometer circuit
does. You adjust the voltage of one part until no current flows from
the battery. The battery voltage is then equal to the potentiometer
voltage.
Alternatively you can build a capacitor whose spacing varies quickly.
There will be an induced ac current if there is a potential difference
across the plates of the vibrating capacitor. Attach one side to the
unknown battery and the other side to a variable voltage. When the ac
current goes to zero, the variable voltage equals the battery voltage.
Its called a Kelvin probe.
Those 18 and 19th century guys were clever.

joe

On Mon, 3 Mar 2003, Bernard
Cleyet wrote:

A have a different take on the question of no chem. reaction until the
connection is made. That is, no additional reaction. When the cell is
created a reaction occurs until the cells PD is obtained, and then the
reaction stops. It recommences when the PD is reduced by making the
connection.

BTW, how may one, pray tell, measure a PD without a current flow?

bc

Seth T Miller wrote:

Hello all,

I have a question, actually two, that I am discussing with another
faculty member.

Part One:
Take a 6 volt dry cell battery. My colleague opines that one cannot
speak rightly of a voltage existing between the two sides of the battery
until the terminals are connected, thus completing the circuit. Her
reasoning is that the chemical reaction is the source of the voltage and
does not occur unless the terminals are connected in a circuit (except
perhaps in very small amounts as batteries run down when left to stand).

Is this a correct analysis or can we rightfully speak of a voltage
difference between the two sides of a battery, and if so is it 6 volts
in this case?

Part Two:
My colleague connected the positive side of a 6 volt dry cell battery to
a 1.5 volt light bulb, then connected the other side of the light bulb
to the ground. The bulb does not light. Is there a voltage difference
between the positive side of the battery and the ground and if so is it
6 volts? Why doesn't the bulb light?

Her latest theory is that as soon as one touches the positive side of
the battery, any excess charges are immediately drained off and that
side of the battery becomes equalized with the ground. The chemical
reaction is not occurring, so there is no way for a voltage difference
between the positive terminal and the ground to be maintained, thus the
bulb does not light.

Is this a correct analysis? How could it be analyzed in more detail?

Thank you-
-Seth Miller


Joseph J. Bellina, Jr. 574-284-4662
Associate Professor of Physics
Saint Mary's College
Notre Dame, IN 46556