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Chapter 34-35 - Electric Current
ELECTRIC POTENTIAL
ENERGY
In order to bring two like charges near each other
work must be done.
If monkey-girl brought 2 or 3 charges instead of
one, then she would have had to do more work so
she would have created more electrical potential
energy.
ELECTRIC POTENTIAL
Since the electrical potential energy can
change depending on the amount of
charge you are moving, it is helpful to
describe the electrical potential energy
per unit of charge
VOLTAGE
VOLTAGE CALC. EXAMPLE:
The amount of work done by the
person is 30J, this is also the
amount of electrical potential
energy that is possessed by all
three charges together.
The electrical potential (not
energy) is the amount of energy
per unit of charge
VOLTAGE
Voltage is an “electric pressure” that
can produce a flow of charge, or
current, within a conductor.
The flow is restrained by the resistance
it encounters.
Voltage Sources
Charges flow only if there is a potential
difference. Something that provides a
potential difference is known as a
voltage source. If a positively, and a
negatively charged metals are next to
each other, there will be a large voltage
between them.
Voltage Sources
In dry cells and wet cells, energy
released in a chemical reaction
occurring inside the cell is converted to
electrical energy.
Generators convert mechanical energy
to electrical energy. The electrical
potential energy produced is available
at the terminals of the cell or generator.
Flow of Charge
Charge flows from one end to the
other. Charge flows when there is a
potential difference, or difference in
potential (voltage), between the ends
of a conductor. The flow continues
until it reaches a common potential.
When there is no potential difference,
there is no longer a flow of charge
through a conductor.
Flow of Charge
When there is no potential difference,
there is no longer a flow of charge
through a conductor. To attain the flow
of charge continuously, some
arrangement must be provided to
maintain a difference in potential.
CURRENT:
The flow of charge is called electric
current
When the flow takes place along one
direction, it is called direct current (DC).
When it flows to and fro, it is called
alternating current (AC).
CURRENT:
When a metal wire is connected across the two
terminals of a DC voltage source such as a
battery, the source places an electric field
across the conductor. The moment contact is
made, the free electrons of the conductor are
forced to drift toward the positive terminal
under the influence of this field.
The free electron is therefore the current carrier
in a typical solid conductor.
CURRENT:
For an electric current of 1 ampere, 1
coulomb of electric charge (which
consists of about 6.242 × 1018
electrons) drifts every second through
any imaginary plane through which the
conductor passes.
CURRENT:
where
Q is the electric charge in coulombs (ampere
seconds) , AND
t is the time in seconds
Electric Current
Electric current is the flow of electric
charge. In solid conductors the
electrons carry the charge through the
circuit because they are free to move
throughout the atomic network. These
electrons are called conduction
electrons.
Electric Current
Once again:
Electric current is measured in amperes.
An ampere is the is the flow of 1
coulomb (6.24 billion billion electrons)
of charge per second. If there are 5
amperes in a current, then there are 5
coulombs.
Electric Resistance
The current also depends on the
resistance that the conductor offers to
the flow of charge which is called
electric resistance. The resistance of
a wire depends on the conductivity of
the material used in the wire and also
on the thickness and length of the wire.
Electric Resistance
Thick wires have less resistance than
thin wires. Longer wires have more
resistance than short wires. The
greater the jostling about of atoms
within the conductor, the greater
resistance the conductor offers to the
flow of charge.
Electric Resistance:
This resistance depends on a few things:
• First, the conductivity of the material the wire
is made of. If electrons can move through
the material better, then there will be less
resistance.
• Second, it depends on the thickness of the
wire. The thicker the wire, the more paths it
offers for flow of charge; therefore, the less
resistance it gives to the movement of
charge.
Electric Resistance:
• Third, it depends on the length of the
wire. If the wire is longer it will have
greater resistance to the flow of charge.
• Finally, it can depend on temperature.
Generally as the temperature increases
so does the resistance, however there
are some exceptions to this rule.
Electric Resistance:
The unit for resistance is the
Ohm, which we abbreviate Ω.
The ohm is the electric resistance between two points of a
conductor when a constant potential difference of 1 volt,
applied to these points, produces in the conductor a
current of 1 ampere, the conductor not being the seat of
any electromotive force.
Ohm’s Law
- Ohm’s law is the relationship among
voltage, current, and resistance.
current = voltage
resistance
- Relationship among the units of
measurement
1 ampere = 1 volt
ohm
Ohm’s Law
A voltage source, V, drives an electric current, I ,
through resistor, R, the three quantities obeying
Ohm's law: V = IR.
Electric Power
Electric Power is
the rate at which
electrical energy is
converted into
another form such
as mechanical
energy, heat, or
light.
Electric Power (Continued)
Electric power is equal to the
product of current and voltage.
Ex: electric power = current x
voltage
Unit form: 1 watt = 1 amp x 1 volt
WATT
Power in Electric Circuits
Power = W = E
t
t
E=VIt
Power = V I t = V I
t
J/sec
Power in Electric Circuits
P=VI
V=IR
P = (I R) I
P = I2 R
I = V/R
P = V (V/R)
P = V2 / R
Ohm’s Law and Electric Shock
The damaging effects of electric shock
are the result of current passing
through the body.
From Ohm’s law we can see that this
current depends on the voltage applied,
and also on the electric resistance of
the human body
Converting AC to DC
An AC-DC converter
consists of a
transformer to lower
the voltage and a
diode, a tiny
electronic device
that acts as a one
way valve to allow
electron flow in only
one direction
Speed of Electrons in a Circuit
When you make a
telephone call, the
signal is transmitted
through the
conductors at nearly
the speed of light
It is not the
electrons that move
at this speed but the
signal
Speed of Electrons in a Circuit
(Continued)
The electric field lines between the
terminals of a battery are directed
through a conductor, which joins
the terminals.
In an AC circuit, the conduction
electrons don’t make any net
progress in any direction.
The Source of Electrons in a
Circuit
The source of
electrons in a circuit
is the conducting
circuit itself.
When you plug a
lamp into an AC
outlet, energy flows
from the outlet into
the lamp, not
electrons.
Source of Electrons in a Circuit
(Continued)
When you are jolted by an AC electric
shock, the energy simply causes free
electrons in your body to vibrate in
unison.
Small vibrations tingle
Large vibrations can be fatal.
Direct Current and Alternating
Current
Direct current – a flow
of charge that always
flows in one direction
Alternating current –
electrons in the circuit
move first in one direction
and then in the opposite
direction, alternating back
and forth through
relatively fixed positions
Light Bulbs in Series
Light Bulbs in Parallel
Analysis of Series Circuit
Req = R1 + R2 + R3 = 17 + 12  + 11  = 40 
Voltage Drop for Series Circuits
Vbattery = V1 + V2 + V3 + ...
Electric Potential Diagram
Analysis of Series Circuit
Analysis of Series Circuit
Ohm's law equation ( V = I • R)
Itot = Vbattery / Req = (60 V) / (40 ) = 1.5 amp
Analysis of Series Circuit
The 1.5 amp value for
current is the current at
the battery location. For
a series circuit with no
branching locations, the
current is everywhere
the same.
Ibattery = I1 = I2 = I3 = 1.5 amp
Analysis of Series Circuit
V1 = I1 • R1
V3 = I3 • R3
V1 = (1.5 A) • (17 )
V3 = (1.5 A) • (11 )
V1 = 25.5 V
V3 = 16.5 V
V2 = I2 • R2
V2 = (1.5 A) • (12 )
V2 = 18 V
Parallel Circuit Current
Itotal = I1 + I2 + I3 + ...
Parallel Circuit Current
Series and Parallel Circuits
Parallel Circuit Current
Itotal = I1 + I2
Parallel Circuit Resistors
1 / Req = 1 / R1 + 1 / R2 + 1 / R3 + ...
Parallel Circuit Resistors
Parallel Circuit Resistors
Parallel Circuit Resistors
Parallel Circuit Resistors
Voltage Drop for Parallel Circuits
Analysis of Parallel Circuit
1 / Req = 1 / R1 + 1 / R2 + 1 / R3 =
(1 / 17 ) + (1 / 12 ) + (1 / 11 )
1 / Req = 0.23306  -1
Req = 1 / (0.23306 -1)
Req = 4.29 
Analysis of Parallel Circuit
Req = 4.29 
Itot = Vbattery / Req = (60 V) / (4.29)
Itot = 14.0 amp
Analysis of Parallel Circuit
The voltage drop across each
one of the three resistors is
the same as the voltage
gained in the battery:
V battery = V1 = V2 =  V3
= 60 V
Analysis of Parallel Circuit
I1 =  V1 / R1
I3 = V 3 / R3
I1 = (60 V) / (17 )
I3 = (60 V) / (11 )
I1 = 3.53 amp
I3 = 5.45 amp
I2 = V 2 / R2
I2 = (60 V) / (12 )
I2 = 5.00 amp
Series and Parallel Circuits
•As the number of resistors (light bulbs) increases, what happens to
the overall current within the circuit?
•As the number of resistors (light bulbs) increases, what happens to
the overall resistance within the circuit?
•If one of the resistors is turned off (i.e., a light bulb goes out), what
happens to the other resistors (light bulbs) in the circuit? Do they
remain on (i.e., lit)?
Practice Problem
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