Transcript Electric Current

Electric Current
AP Physics C
Montwood High School
R.Casao
The Battery
Volta’s original battery
• The electric battery,
invented in 1800 by Volta,
Ag
represented the basis for a
wet
Zn
wide range of developments
cloth
in electrical technology.
• The wet cloth separating
the plates is soaked in a
salt solution, producing a
potential difference
between the two end
+
plates.
electrical converter...
converts chemical energy to electrical energy
Electrical Description of a Battery
• A battery does work on positive charges
in moving them to higher potential
(inside the battery).
• The EMF (electromotive force) E is the
work per unit charge exerted to move the
charges “uphill” (to the + terminal,
inside), but you can just think of this as
an “applied voltage.”
• Current will flow, in the external circuit
outside the battery from the + terminal,
to the – terminal of the battery.
Electromotive Force (EMF)
• Batteries, generators, and solar cells,
transform chemical, mechanical, and
radiant energy, respectively, into electric
energy. These are examples of sources of
EMF.
• EMF is measured in Volts V;
1J
1V 
1C
• The source of EMF provides the energy
the charge carriers will conduct through
the electric circuit to the resistor.
Potential Difference or Voltage V
• Current in a circuit moves from an area
of high electric potential energy to an
area of low potential energy. This
difference in electric potential energy is
necessary for current to move through a
conductor.
• The positive terminal of a battery is the
high electric potential energy terminal
and the negative terminal is the low
electric potential energy terminal.
• Potential difference V is also measured in
volts.
Potential Difference or Voltage V
• Within the battery, a
chemical reaction
occurs that transfers
electrons from one
terminal to another.
• Because of the
positive and negative
charges existing on
the battery terminals,
a potential difference
(voltage) exists
between them.
Potential Difference or Voltage V
• The battery creates an
electric field within and
parallel to the wire,
directed from the positive
toward the negative
terminal.
• This field exerts a force
on the free electrons,
causing them to move.
This movement of charge
is known as an electric
current.
• The current in the circuit
is shown to flow from the
positive terminal to the
negative terminal.
Potential Difference or Voltage V
• EMF is the maximum amount of energy
per charge the battery can provide to the
charge carriers.
• Voltage is the energy per charge the
charge carriers have after moving
through the internal resistance r of the
battery.
– Some of the energy added to the charge
carriers has to be used to travel through the
battery.
– The remaining energy is carried to the
resistors outside the battery.
Electric Circuits
• A simple electric
circuit will consist of:
– A source of energy (in
this case a battery).
– Conducting wires.
– A resistor R that uses
the energy.
– A switch to open/close
circuit.
• The source of energy
has an internal
resistance r.
Two Types of Current
• DC current (direct current) is a steady
flow of current in one direction.
• AC current (alternating current) direction of current flow changes many
times a second. In the US, the frequency
of change is 60 Hz. Therefore, the
current changes direction 60 times per
second.
Electric Current
• When charges of like sign move, a
current exists.
• When the charges move
perpendicularly to a surface of area
A, the current is the rate at which
charge flows through this surface.
Electric Current
• Current I:
Q
I
t
• If the current varies in time,
instantaneous current, i:
dq
i
dt
C
• Unit: Ampere A 
s
• Charges flowing through a surface
can be positive, negative, or both.
Electric Current
• The direction of flow of positive
charge is used as the direction of
the current.
• In a metallic conductor, the current
is due to the motion of electrons, so
the direction of the current will the
opposite to the direction of flow of
the electrons.
I
E
Drift Velocity
• The volume of a conductor of length l is V
= A·l.
• Let n be the number of mobile charge
carriers per unit volume, then the
number of charge carriers in the volume
of the conductor is n·A·l.
Drift Velocity
• The total charge in the volume of the
conductor of length l is:
dq  n  A  l  q
• If the charge carriers move with
speed vd, the distance they move in
time t is d = vd· t. Let d = l.
• Current:
n  A  v d  Δt  q
dq
I

 n  A  vd  q
dt
Δt
Drift Velocity
• If an electric field is present in the conductor,
the electrons will start moving in a direction
opposite to the field.
• The motion of the electrons will be disrupted by
frequent collisions with the ions.
• The net result is that the electrons acquire a
slow average speed, or drift velocity.
Electron Motion in a Conductor With
and Without an Electric Field
Analogy of Electron Motion in a Conductor
12 Volts
0 Volts
Conductor with Current Moving from High
Electrical Potential (Volts) to Low Potential
Charges Drifting in a Conductor
• In a conductor, the
electric field that
drives the free
electrons travels
through the
conductor with a
speed close to that
of light. So when
you flip a light
switch, the electric
field reaches the
electrons instantly.
Helpful Websites
• DC Circuit Water Analogy
• Air Flow Analogy