Ch 19 Current and Potential Difference

1
Ch 19 Current and Potential Difference
2
Current is rate of charge movementI
?q/?t.The unit of current is the ampere, or
amp. 1A 1C/s
3
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?
4
Although electrons are the charged particles
moving in most cases, conventional current is
described for most flows of electricity.
5
Conventional current is the current of positive
charge that would have the same effect as the
actual charge that is flowing.
6
When a circuit is completed, an electric field is
produced in the conductor. This happens at near
the speed of light.
7
The electrons themselves move much slower.
Collisions with material in the conductor cause
the electrons to follow a zig-zag path.
8
The individual electrons move slowly along the
conductor due to this zig-zag path in the
opposite direction of the electric field. Their
velocity is known as the drift velocity.
9
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.
10
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.
11
Generators convert mechanical energy to
electrical energy. They produce the average
potential difference of 120 volts in a wall
outlet.
12
The two types of current are alternating current
(AC) and direct current (DC) .
13
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.
14
To prevent this, AC oscillates 60 times per
second (60 Hz) in the U.S. Batteries produce DC
generators can produce AC or DC.
15
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.
16
The SI unit of resistance is the ohm (?). One
ohm 1 volt/1 ampere.
17
Ohms law is V IR. It holds true for a wide
range of materials and voltages, but is not true
for all materials.
18
In a non-ohmic material, the slope of a graph of
current vs. potential difference will not be a
straight line.
19
A diode is a semiconducting device that is
non-ohmic.
20
We will assume that all resistors follow Ohms
law.
21
Some factors that affect the resistance of
conductors are
22

• 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.

23
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.
24
The resistance of a steam iron is 19.0 ?. What is
the current in the iron when it is connected
across a potential difference of 120 V?
25
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!
26
Unfortunately, these critical temperatures are
typically very low, many below 10 Kelvins. Work
is proceeding to find room temperature
superconductors.
27
(No Transcript)