Transcript Document

Voltaic Cells: The Discovery of Current Electricity
•A conductor is any material that will allow charges to flow.
•An insulator is any material that will not allow charges to
flow easily.
•In the early 1800’s the study of static electricity was well
underway.
•Alessandro Volta discovered
that electric charges would
build up on two different
conductors, (metals) if
salt-water-soaked paper was
placed between them.
Zinc
Negative charge
build-up
Copper
Positive charge
build-up
Salt-watersoaked paper
•The metal plates are called electrodes.
•The water-soaked paper is called the electrolyte.
•Volta found that by adding more layers, he could increase
the amount of charge that would build up.
•This is called a Voltaic Pile; a very
early version of the battery.
•Volta named the positive end of the
pile the cathode, and the negative end
the anode.
•If the anode and cathode are
connected by another conductor, the
charges will flow from one to the
other.
•Volta, and many other scientists at the time, (Ben
Franklin for example) assumed that the positive charge
would flow out of the cell.
•Conventional current, (the
flow of positive charge)
was widely accepted until
the discovery of the electron
several decades later.
Charge
•Charge is measured in groups called coulombs.
•A coulomb is a group of 6.25x1018 charges.
•The amount of charge in an object can be calculated
as follows:
Q  Ne
where,
Q = amount of charge,
(coulombs, “C”)
N = number of charges
e = elementary charge
1.6 x 10-19 C
Voltage
•Voltage is a measure of the energy carried a coulomb
of charge.
E
V
Q
Where
V = voltage, (V)
E = energy, (J)
Q = amount of charge, (coulombs, “C”)
•Potential difference refers to the change in voltage of
charges in two different parts of a circuit.
Example #1
Find the potential difference of a dry cell if it does 0.35 J of
work moving 1.56 x 1016 charges.
Electric Current
Current is the rate at which charge flows in a current. It can
be found by measuring the amount of charge that passes a
given point in a circuit per second.
Q
I
t
where
I = current, (amps, “A”)
Q = amount of charge, (C )
t = time, (s)
Example #2
It takes a current of 1.0x101 A 30.0 s to boil a kettle of
water. If this kettle requires 360 000 J of thermal energy,
what is the potential difference across the kettle?
Homework
pg. 441 #1-8, 10
pg. 445 #1-6