Transcript Batteries don`t store charge. They store energy. A chemical battery

Batteries don’t store charge. They store energy.
A chemical battery works because electrons like to
leave some materials to go to others.
The change in energy when the electron goes “downhill”
becomes available as electric POTENTIAL between
the battery terminals
Lead Acid (Car) Battery
The rxn on the right will not go in
the direction indicated unless the
electrolyte sol’n potential is closer
than 1.685 V to the + electrode
potential.
i.e. the + electrode can be no more
than 1.685 V higher in potential
than the electrolyte sol’n.
Pb+2 is in the form of solid lead sulfate on
the electrodes. When “discharged”, both
electrodes turn into lead sulfate.
Lead Acid (Car) Battery
The rxn on the left will not go in the
direction indicated unless the
electrolyte potential is closer than
0.356 V to the - electrode potential.
i.e. the - electrode can be no more
than 0.356 V lower in potential than
the electrolyte soln.
Lead Acid (Car) Battery
Overall, then, the reactions will
STOP when the potential between
the +/- electrodes is 2.041V.
Only a tiny tiny tiny amount of
charge needs to build up on the
electrodes for the reaction to stop.
How much?
But if you connect the electrodes
with a resistive wire, the reaction
will start to go as the potential
drops a hair below 2.041V.
Lead Acid (Car) Battery
What happens if you force the
potential difference to be higher
than 2.041 V?
The reactions run backwards!
Lead sulfate turns into lead oxide
and lead (metallic).
This is charging the battery.
Lead Acid (Car) Battery
Note that the rxn doesn’t make a
big “reservoir” of electrons.
The battery doesn’t die because it
runs out of stored electrons.
It doesn’t store electrons.
It “pumps” electrons “on demand”,
i.e. when the potential falls below
2.041 V.
The two electrodes of an ideal V volt battery are shown. The field lines
for the electric field between the electrodes are shown when nothing is
attached to the electrodes. The electrodes have a separation = d.
A resistive wire of length L is then attached to the battery.
What is the electric field in the wire, when steady state is reached?
A] 0
B] it varies, but averages to V/d
C] it varies, but averages to V/L
D] it is V/L everywhere in the wire
E] no way to determine
The two electrodes of an ideal V volt battery are shown. The field lines for
the electric field between the electrodes are shown when nothing is
attached to the electrodes. The electrodes have a separation = d.
A resistive wire of length L is then instantaneously attached to the battery.
What is the electric field in the wire, immediately after attaching it?
A] 0
B] it varies, but averages to V/d
C] it varies, but averages to V/L
D] it is V/L everywhere in the wire
E] no way to determine
Where on this wire will negative charges “build up” (a tiny amount)
B
A
Assume wires have zero resistance
Assume wires have zero resistance