Electric Current and Circuits
HEAT
will flow if
there is a difference
in temperature
WATER
will flow if there is a
difference in
pressure
.
Is the pressure
inside
the jug (due to
the weight of the water) the same as
the pressure
outside
the jug (due to
the weight of the atmosphere)?
If the pressure outside was the same
as the pressure inside, would the
water flow out?
At which location is there a larger
difference between the pressure
inside and outside, near the top of the
water level or near the bottom of the
water level?
What evidence can you
see
that
demonstrates a larger difference in
pressure at the two locations?
Electrons will flow if there is a difference in electric
pressure.
Electric pressure is called
“
Potential
”
, and is
measured in Volts.
If there is no
difference
in pressure from one location to
another, the electrons will not flow.
In other words, if there is no
“
Potential Difference,
D
V
”
from one location to another, there will be no
electric
“
current
”
.
Current
CURRENT: a flow of charged
particles (electrons) through
a conductor
Current,
I
, is measured in
amperes, A, or
“
慭灳
”
.
time
charge
I
Andre Ampere
e

e

e

e

e

e

e

e

Example:
What is the current,
I
, if
8 C
of charge
passes through a wire in
2 seconds
?
I
= q / t
I
= 4 amps
The Damage Caused by Electric
Shock
1 mA = 0.001 A
Mild shock can be felt
5 mA = 0.005 A
Shock is painful
15 mA = 0.015 A
Muscle control is lost
100 mA = 0.1 A
Death can occur
60W light bulb

0.5 A
Starter motor
–
210 A
Clothes dryer
–
18 A
Iron

3 A
Circuits
ELECTRIC CIRCUIT: Charges
moving in a
closed
loop
•
A circuit requires a both a
conductor, usually metal wires,
and a
“
捨慲来g灵浰
”
.
•
CHARGE PUMP
: a device that
provides a
potential difference
so that charges keep moving.
Alessandro Volta
The
Potential Difference,
D
V
, provided by
the charge pump is called its
VOLTAGE
.
If the voltage of a battery is 9 V,
this means there is a
difference
of 9
V of potential (pressure)
between the positive terminal
and the negative terminal.
If the voltage of a battery is
1.5 V, this means there is a
difference
of 1.5 V of
potential (pressure) between
the positive terminal and the
negative terminal.
The pressure DIFFERENCE, the VOLTAGE, is required for charges to flow!
So, charge pumps such as batteries, are called
voltage sources
.
Circuits
•
This potential difference is
sometimes called the
emf,
e
⡥汥l瑲潭o瑩t攠景牣攩
•
Examples of charge pumps:
batteries, solar cells,
generators, power supplies
e
The source of the electrons moving in the circuit is NOT the battery
or the wall outlet! The free electrons are contained within the wire
itself.
An individual electron does not actually travel all the way around a
circuit.
One electron bumps into the next that bumps into the next that
bumps into the next …..
It is the ENERGY that gets transferred all the way around the circuit.
You are not buying electrons from your electric company

you
already have them! You are buying energy!
Resistance
all
conductors offer some
resistance to the flow of
charges, even metal wires.
RESISTANCE =
The unit for resistance is the
OHM,
W
.
This equation is often called
OHM
’
匠䱁L
I
V
current
voltage
R
George Ohm

first
determined the
math relationship
now called Ohm
’
猠
Law
I
V
current
voltage
R
Henry Cavendish, who found a value
for
“
G
”
, also experimented with
electricity. His studies laid the
groundwork for George Ohm to write
Ohm
’
s Law.
Example
What is the
resistance
of an appliance if
2 amps
of current run through it when
supplied with
120 V
?
R = V / I
R = 120 V / 2 A
R = 60
W
Ohm
’
s Law
As voltage (pressure) changes, the current
flowing through a conductor will change.
If
the ratio of
remains constant, that conductor is said to
“
obey Ohm
’
猠䱡w
”
.
Example: For one conductor, when
Voltage = 12 V, Current = 6 A and when
Voltage = 8 V, Current = 4 A.
Does it obey Ohm
’
猠䱡眿
I
,
current
V
,
voltage
WATER ANALOGY
Water
Electricity
Flow of water
current
flow of charge
Water pump
keeps flow going
charge pump
psi.
pressure
voltage
Pipes of different
resistance
wires of
diameter
different
diameter
Small electrical
components called
“
resistors
”
慲a
inserted into circuits
to control the
amount of current
flowing.
Certain metals offer less resistance to the
flow of charges than others.
Example: Copper is a better conductor than
iron
The resistance of a wire of length L and
cross sectional area A is given by
RESISTANCE, R =
where
r
楳i瑨攠
resistivity
of that particular
metal.
A
L
r
Schematic diagrams
symbols to represent circuit components.
wires:
Charge pumps:
Resistors:
Switches:
Ground:
All devices connected to a circuit (light bulbs, TV
’
猬s
toasters, etc.)
resist
the flow of charges and are
sometimes drawn as a
resistor
in the circuit (if
you
’
牥r捯c獩摥物湧⁴桥⁵湩n 慳aw桯汥⤮
Series and Parallel Wiring
SERIES CIRCUITS
The same current through each
device.
EQUIVALENT RESISTANCE:
What is the
net
resistance?
What
one
resistor could
replace a group of resistors?
For resistors wired in series, the
equivalent resistance
is given
by:
R
eq
= R
1
+ R
2
+ R
3
+ …
PARALLEL CIRCUIT
Same voltage across each
device.
For resistors wired in parallel, the
equivalent resistance
is given
by:
...
R
1
R
1
R
1
R
1
3
2
1
eq
Example: What is the
equivalent resistance
of a
10
W
, 20
W
,
and
30
W
resistor wired in
series? In parallel?
Series: R
eq
= 10 + 20 + 30 =
60
W
Parallel 1 / R
eq
= 1/10 + 1/20 + 1/30
R
eq
=
5.45
W
There
’
s
much
less resistance if resistors are
wired in parallel than if they
’
re wired in
series.
With less resistance, the charge pump will
able to push
much
more current around the
circuit.
For maximum resistance

use series
wiring.
For minimum resistance

use parallel
wiring.
The flow of water is a very good analogy
to the flow of charges in both series
and parallel circuits.
For resistors wired
“
in series
”
the
same
current
flows through each
one, however the
potential
difference,
voltage
, is additive
Voltage gain through battery
=
Sum
of Voltage drop through resistors
For resistors wired
“
in parallel
”
, the
potential difference,
voltage
, is
the same for each of them,
however the
current
is additive.
Total Current pushed by battery
=
Sum
of Current going through all the resistors
Characteristics of Series and
Parallel Wiring
Series: If one component goes out,
They all go out!
As more resistors are added, the equivalent resistance
Increases!
which means that the current in that part of the circuit
Decreases!
Parallel: If one component goes out,
They rest still work!
As more resistors are added, the equivalent resistance
Decreases!
which means that the current in that part of the circuit
Increases!
Electric Power
Power is the
rate that work is done
or
energy is transferred
, that is
Power =
Power is measured in
Watts, W
)
onds
(sec
Time
)
Joules
(
Energy
Electric power
delivered to a circuit by a
power supply is given by
Power = Current x Voltage
P = IV
This equation can be combined with Ohm
’
s
Law, R = V / I in its different forms:
V = IR, I = V / R
Energy
leaves a circuit
through the different
“
牥獩s瑯牳
”
楮i瑨攠景牭f
of
light
,
heat
, and any
kind of
work
done by
the appliance the
current is running
through.
The rate that the energy
leaves the circuit is the
power output
.
Examples
How much
energy
does a 75 W light bulb give off in
five minutes?
Power = Energy / time
Energy = Power x time
Energy = 75 W x 5 x 60 seconds
Energy = 22500 J
What is the
power
output of a 3 A motor running on
regular house voltage?
P = IV
P = 3 A x 120 V
P = 360 W
“
Power
”
lines
•
The higher the
current
, the more
the wires in the circuit heat up,
thereby
“
睡獴w湧
”
敮敲杹⸠⁔ 楳i
is a big problem when electric
companies must provide
electricity at great distances
away from the power plants.
•
The solution: Electric lines that
carry current great distances
are at very
high
voltage
, so the
current
can be relatively small.
P =
I
V
P =
I
V
High Voltage / Low Voltage
TRANSFORMERS
:
devices that
“
獴数

up
”
瑨攠癯汴慧攠慴
瑨攠灯睥爠灬慮琠慮搠
瑨敮t
“
獴数

摯睮
”
the voltage at the
customers
’
汯捡瑩潮t
Even with very high voltage, there is still
some current running through those wires
and power (dissipated through heat) is
lost. If the entire length of wire has a total
resistance R, the power lost along the way
is given by
Power lost =
I
2
R
Open circuit
Closed circuit
Electrical Safety
•
Fuse
: a short piece of
metal that melts if
current exceeds a set
value (to protect device)
•
Circuit breaker
: an
automatic switch that
opens the circuit when
current exceeds a set
value. (uses a bimetallic
strip)
•
Ground

fault circuit
interrupter
: opens a
circuit if the current
going into a device is
not the same as the
current coming out of
the device.
•
gfci
•
Short Circuit

a circuit
that is formed when wires
touch. This effectively
shortens the path of the
circuit because the current
no longer passes through
the resistor. The
extremely low resistance
in the circuit produces very
high current, which could
cause melt down and fire.
NOT a good idea!!
•
Ground wire
: provides a path for high current
to go in case of a short circuit

•
Ammeter

device to measure amps
(current)
•
Voltmeter

device to measure voltage
•
Ohmmeter

device to measure resistance
•
Multimeter

does all three
•
Diode

allows only one direction of current
to pass through it.
•
Light emitting diode, LED
•
Transistors
are devices that are often used
to amplify small signals into a larger one.
Alternating Current, AC
The electricity in your houses is
alternating
current, AC
, not
direct current, DC
–
as in
batteries.
The main reason for this is that
transformers
can
only step up or step down the voltage if it is
alternating current.
Without
transformers
, there would be great loss of
electrical power between the power stations and
your houses. For electricity to be very efficient,
there would have to be a power generating plant
in EVERY community!
Alternating current
means that the
voltage pushes back
and forth, not just
one direction.
The electrons just
wiggle back and
forth.
In the US, they wiggle
back and forth 60
times every second.
In other words, the
frequency is
60 Hz
.
The first use of alternating
current in the US was at the
Chicago World
’
s Fair
, in
1893, where
Nicholas
Tesla
’
s
alternating current
circuits were utilized instead
of direct current, which was
backed by Thomas Edison
and his General Electric
Company.
•
Tesla Coil
•
Alternating current
•
Systems for wireless communication (radios, remote
controls)
•
robotics
•
electrotherapy
•
wireless transfer of electricity
•
x

ray tubes
•
arc lights
•
concepts for electric vehicles
•
devices for lightning protection
•
concepts for vertical takeoff aircraft
Ferris designed and built the first
264 foot (80 meter) wheel for the
World's Columbian Exposition
in
Chicago, Illinois
in 1893. This first
wheel was 26 stories tall and could
carry 2,160 persons. There were 36
cars accommodating 60 people
each (40 seated, 20 standing). It
took 20 minutes for the wheel to
make two revolutions

the first to
make six stops to allow passengers
to exit and enter; the 2nd a single
non

stop revolution

and for that,
the ticket holder paid 50 cents. The
wheel was moved twice after the
1893 Fair and was eventually
disassembled in 1906.
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