Transcript Current #2 - Southgate Schools

Electric
Current
Physics A
Current #2
Electric Potential Review
 Electric
potential is the amount of electric
potential energy per unit of charge that would
be possessed by a charged object if placed
within an electric field at a given location.
 The concept of potential is a locationdependent quantity - it expresses the quantity
of potential energy on a per charge basis such
that it is independent on the amount of charge
actually present on the object possessing the
electric potential.
 Electric potential difference is simply the
difference in electric potential between two
different locations within an electric field.
Electric Potential Review
 To
illustrate the concept of electric
potential difference and the nature of
an electric circuit, consider the following
situation.
What is an Electric Circuit?
 Now
suppose that the two oppositely
charged plates are connected by a
metal wire.
 What would happen?
What is an Electric Circuit?
 The
wire serves as a path through which charge can
flow.
 One could think of positive charges moving from the
positive plate through the charge pipe (wire) to the
negative plate.
 Do positive charges in a material move?
 As a + charge leaves the upper plate, the plate
would become less positively charged.
 As a + charge reaches the
- plate, that plate would
become less negatively
charged.
What is an Electric Circuit?
 Since
the electric field depends upon the amount
of charge present on the object creating the
electric field, the electric field created by the two
plates would gradually diminish in strength over the
course of time.
 Eventually,
the electric field would become so small
that there would
be no observable
movement of charge
between the plates.
What is an Electric Circuit?
 The
plates would ultimately lose their
charge and reach the same electric
potential.
 In the absence of an electric potential
difference, there will be no charge flow.
What is an Electric Circuit?
 The
illustration comes close to
demonstrating the meaning of an
electric circuit.
 To be a true circuit, charges must
continually flow through a complete
loop, returning to their original position
and cycling through again.
What is an Electric Circuit?
 An
activity that illustrates the necessity of a
complete loop utilizes a battery pack, a light
bulb, & some connecting wires.
 The objective is to observe the effect of
connecting and disconnecting a wire in a
simple arrangement of the battery pack, light
bulbs and wires.
 What has to happen in
order for this light to turn on?
What is an Electric Circuit?
 The
fact that the light bulb lights and
remains lit is evidence that charge is flowing
through the light bulb filament and that an
electric circuit has been established.
 A circuit is a closed
loop through which
charges can
continuously move.
What is an Electric Circuit?
 To
demonstrate that charges are not only moving
through the light bulb but also through the wires
connecting the battery pack and the light bulb, a
compass is placed beneath the wire at any location
such that its needle is placed in alignment with the
wire.
 Current is a flow of
charge within the circuit.
What is wrong?
Why is did this one light?
Coloring Circuits
 Different
voltages can be represented by
using different colors on a circuit diagram.
 Use the following system to color code the
circuits.
Red
Orange
Yellow
Green
Blue
Highest
Voltage
Higher
than
Normal
Normal
Voltage
Lower
than
Normal
Lowest
Voltage
Rules for Coloring Circuits

The positive side of the battery is the highest voltage.
(Color it red.)

The negative side of the battery is the lowest voltage.
(Color it blue.)

A conducting wire should have the same voltage (color)
throughout.

A resistor (light) can cause a voltage drop (color change).

A difference in voltage (color) means charge will flow
(from high to low potential).
2 Requirements of a Circuit
 There
must be an energy supply capable
doing work on charge to move it from a low
energy location to a high energy location
and thus establish an electric potential
difference across the two ends of the
external circuit.
 There
must be a closed conducting loop in
the external circuit that stretches from the
high potential, positive terminal to the low
potential, negative terminal.
If an electric circuit could be compared to a water circuit at a
water park, then the
...... battery would be analogous to the ____
.... positive terminal of the battery would be analogous to the ____
.... current would be analogous to the ____
.... charge would be analogous to the ____
.... electric potential difference would be analogous to the ____.
A. Water pressure
B. Gallons of water flowing down slide per minute
C. Water
D. Bottom of the slide
E. Water pump
F. Top of the slide
Utilize your understanding of the requirements of an electric circuit
to state whether charge would flow through the following
arrangements of cells, bulbs, wires and switches. If there is no
charge flow, then explain why not.
A
C
B
D
The diagram at the right shows a light bulb
connected to a 12-V car battery. The + and terminals are shown.
As a + charge moves through the battery from D
to A, it ________ (gains, loses) potential energy and
________ (gains, loses) electric potential. The point
of highest energy within a battery is the ______ (+, ) terminal.
The diagram at the right shows a light bulb
connected to a 12-V car battery. The + and terminals are shown.
As a + charge moves through the external circuit
from A to D, it ________ (gains, loses) potential
energy and ________ (gains, loses) electric
potential. The point of highest energy within the
external circuit is closest to the ______ (+, -)
terminal.
The diagram at the right shows a light bulb
connected to a 12-V car battery. The + and terminals are shown.
Use >, <, and = signs to compare the electric
potential (V) at the four points of the circuit.
VA _________VB _________ VC
In the movie Tango and Cash, Kurt Russell and
Sylvester Stallone escape from a prison by jumping
off the top of a tall wall through the air and onto a
high-voltage power line. Before the jump, Stallone
objects to the idea, telling Russell "We're going to
fry." Russell responds with "You don’t know about
electricity, do you. As long as you're only touching
one wire and you're feet aren't touching the
ground, you don't get electrocuted."
Is this a correct statement?
Electric Current
 An
electrical current is a flow of charge.
 Current is the rate at which charge flows
past a point on a circuit.
 The current in a circuit can be determined if
the quantity of charge Q passing through a
cross section of a wire in a time t can be
measured.
 The current is simply the ratio of the quantity
of charge and time.
Electric Current
 Current
 It
is a rate quantity.
can be expressed mathematically as
 Note
that the equation above uses the
symbol I to represent the quantity current.
Electric Current
 Current

is measured in the unit of ampere.
Ampere is often shortened to Amp and is
abbreviated by the unit symbol A.
 A current
of 1 ampere means that there is 1
coulomb of charge passing through a cross
section of a wire every 1 second.
1 ampere = 1 coulomb / 1 second
Conventional Current Direction
 Which
subatomic particles carry charge
through wires in a circuit?
 How
is electric field direction defined?
 Thus,
these negatively charged electrons
move in the direction opposite the electric
field.
Ben Franklin is to blame!
 Ben
Franklin envisioned positive charges as
the carriers of charge.
 The direction of an electric current is by
convention the direction in which a positive
charge would move.
 Thus, the current in the external circuit is
directed away from the positive terminal and
toward the negative terminal of the battery.
 Electrons would actually move through the
wires in the opposite direction.
A current is said to exist whenever
_____.
a.a wire is charged
b.a battery is present
c.electric charges are unbalanced
d.electric charges move in a loop
Current has a direction. By convention,
current is in the direction that ___.
a. + charges move
b. - electrons move
c. + electrons move
If an electric circuit could be compared to a
water circuit at a water park, then the current
would be analogous to the ____.
Choices:
A. water pressure
B. gallons of water flowing down slide per
minute
C. water
D. bottom of the slide
E. water pump
F. top of the slide
Use the diagram to complete the
following statements:
a. A current of one ampere is a
flow of charge at the rate of _______
coulomb per second.
b. When a charge of 8 C flows past any point
along a circuit in 2 seconds, the current is
________ A.
Use the diagram to complete the
following statements:
c. If 5 C of charge flow past
point A (diagram at right) in
10 seconds, then the current is _________
A.
d. If the current at point D is 2.0 A, then
_______
C of charge flow past point D in 10
seconds.
Use the diagram to complete the
following statements:
e. If 12 C of charge flow past
point A in 3 seconds, then 8 C
of charge will flow past point E in ___
seconds.
f. True or False:
The current at point E is considerably less
than the current at point A since charge is
being used up in the light bulbs.