Download Ch 19 Current and Potential Difference

Ch 19 Current
and Potential
Difference
Current is rate of
charge movement:
I = Δq/Δt.
The unit of current is
the ampere, or amp.
1A = 1C/s
The current in a light
bulb is 0.835 A. How
long does it take for a
total charge of 1.67 C
to pass a point in the
wire?
Although electrons are
the charged particles
moving in most cases,
conventional current is
described for most
flows of electricity.
Conventional current is
the current of positive
charge that would have
the same effect as the
actual charge that is
flowing.
When a circuit is
completed, an electric
field is produced in the
conductor.
This happens at near
the speed of light.
The electrons
themselves move
much slower. Collisions
with material in the
conductor cause
the electrons to
follow a zig-zag path.
The individual electrons
move slowly along the
conductor due to this zigzag path in the opposite
direction of the electric
field. Their velocity is
known as the drift velocity.
The magnitude of this drift
velocity is usually very small.
In a typical copper wire with
a current of 10 Amps, the
electrons take about an hour
to move 1 meter.The electric
field; however, moves close
to the speed of light.
A battery (cell) converts
chemical energy to electrical
energy. A potential difference
is produced between the two
terminals. This produces an
electric field in a conductor
that connects the two
terminals.
Generators convert
mechanical energy to
electrical energy. They
produce the average
potential difference of
120 volts in a wall
outlet.
The two types
of current are
alternating current
(AC) and direct
current (DC) .
In DC the current always
flows the same direction.
In AC the direction of
current flows changes
rapidly. If the changes
were too slow, you would
notice lights flickering, etc.
To prevent this, AC
oscillates 60 times per
second (60 Hz) in the
U.S. Batteries produce
DC; generators can
produce AC or DC.
Current in a circuit is
determined by the
potential difference. It is
also determined by the
resistance. Resistance is
the opposition to the
motion of charge.
The SI unit of
resistance is the
ohm (Ω).
One ohm =
1 volt/1 ampere.
Ohm’s law is V = IR.
It holds true for a
wide range of
materials and
voltages, but is not
true for all materials.
In a non-ohmic
material, the slope of
a graph of current vs.
potential difference
will not be a straight
line.
A diode is a
semiconducting
device that is
non-ohmic.
We will assume
that all resistors
follow Ohm’s
law.
Some factors
that affect the
resistance of
conductors are:
• length - longer conductors have
greater resistance,
• cross-sectional area - greater
area produces less resistance,
• material - better conductors have
less resistance,
• temperature - higher
temperatures increase
resistance.
By V = IR, changing the
resistance of a circuit
changes the current.
Changing the voltage also
changes the current, but
this is not practical in
household circuits.
The resistance of a
steam iron is 19.0 Ω.
What is the current in
the iron when it is
connected across a
potential difference
of 120 V?
Superconductors have no
resistance below a critical
temperature. By V = IR, if R
is zero, a current can exist
without a potential difference.
These have been observed
to exist for years!
Unfortunately, these
critical temperatures are
typically very low, many
below 10 Kelvins.
Work is proceeding to
find room temperature
superconductors.