Electric Current and Circuits - Red Oak ISD Staff and Group Pages

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23 Νοε 2013 (πριν από 3 χρόνια και 11 μήνες)

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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


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.