Electricity and Magnetism

awfulhihatUrban and Civil

Nov 15, 2013 (3 years and 9 months ago)

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Electricity
and Magnetism

10
.
Ohm's Law

11
. Electric Power

12
. Magnets


13
. Magnetic Field


14
.
Oersted's

Experiment


15
. Electromagnets


16
. Magnetic Force on a

Current


17
. Electric Motors


18
. Electromagnetic Induction


19
. Transformers


1
.
Positive and Negative


Charge


2
.
What is Charge?


3
.
Coulomb’s Law

4
.
Force on an Uncharged


Object


5
.
Matter in Bulk


6
.
Conductors and Insulators


7
.
Superconductivity

8
.
The Ampere

9
. Potential Difference


1
. Positive and
Negative Charge

Opposites attract
-
Same charges
repel

Let’s try it!

1
. Positive and Negative Charge

Electrons are stripped
from one component
and transferred to the
other to cause both to
be oppositely charged.

Ben Franklin discovered
electric charge.

2
. What is Charge?

Atoms are composed of protons (+), electrons (
-
)
and neutrons. The nucleus contains the protons
and neutrons and the electrons surround the
nucleus.

2
. What is Charge?

Protons are much larger than electrons but have
an equal and opposite charge.

The
coulomb (C)
is the unit of electric charge.

The basic quantity of electric charge (
e
) is 1.6 x 10
-
19

C.


3
. Coulomb’s Law


Q
1
Q
2



F = K
-------


R
2

Charles Coulomb
(1736
-
1806)

4
. Force on an Uncharged
Particle

Initially the paper is uncharged, but the
comb polarizes the charges in the paper.

5
. Matter in Bulk

Coulomb's law resembles the law of gravity; however,
gravitational forces are always attractive, whereas electric
forces may be attractive or repulsive.



Coulomb’s Law


Q
1
Q
2



F = K
-------


R
2


Law of Gravity


M
1
M
2



F = G
-------


R
2

Gravitational forces dominate on a cosmic
scale; electric forces dominate on an atomic
scale.



Conductors and Insulators

A

conductor
is a substance through which electric charge flows
readily. An
insulator
is a substance that strongly resists the flow of
electric charge.
Semiconductors
are substances whose electrical
conductivity is between that of conductors and insulators.

Semiconductors

Transistors
are switches that conduct
electricity only when a second source of
electricity is energized.

A

B

C

What a
Transistor
looks like.

A current will not flow from A to B unless C is energized.

John Bardeen


Nobel Prizes in 1956
and 1972 for developing
the transistor and
superconductivity.

Enlargement
of 5 mm
square
computer
chip.

7
. Superconductivity

Superconductivity
refers to the loss of all
electrical resistance by
certain materials at very
low temperatures.

Substances that are
superconducting at
150K are now known
which is warmer than
liquid nitrogen (77K).

Maglev Train in Japan

8
. The Ampere

The
Ampere

is a measure of how
much electrical current is flowing
and is measured in units of amps.


Q

I =
----


t

8
. The Ampere

The current varies depending on the force behind
the current and the resistance to flow.

9
. Potential Difference

Potential difference
, or
voltage
, is
the electrical potential energy per
coulomb of charge.


J

V =
----


C

Alessandro Volta
(1745
-
1827)

10
. Ohm’s Law

Resistance

is a
measure of
opposition to the
flow of charge
and is measured
in ohms (

)


V

I =
----


R

Georg Ohm (1787
-
1854)

Andr
é

Marie Ampére
(1775
-
1836)

11
. Electric Power

The
power
of an electric current is the rate at which
it does work and is equal to the product of the
current and the voltage of a circuit:


P= IV

The unit of electric power is the

watt.
The
commercial unit of electric energy is the

kilowatthour
(kWh).

11
. Electric Power


Typical Power Ratings


Appliance




Power (W)

Stove





12,000

Clothes Dryer





5,000

Heater






2,000

Dishwasher





1,600

Photocopier





1,400

Iron






1,000

Vacuum Cleaner





750

Coffee Maker





700

Refrigerator





400

Portable Sander





200

Fan






150

Personal Computer




150



TV Receiver





120

Fax Transmitter/Receiver




65

Charger for Electric Toothbrush



1

12
. Magnets

Every
magnet

has a
north

pole
and a
south

pole.


13
. Magnet Field

How to make a magnet:
Heat in magnetic field.


Magnetic force lines.

14
.
Oersted’s

Experiment

Hans Christian Oersted discovered in 1820 that an
electric current near a compass causes the compass
needle to be deflected.
Oersted's experiment showed
that every electric current has a magnetic field
surrounding it.

Hans
Christian
Oersted
(1777
-
1851)

14
.
Oersted’s

Experiment

According to the
right
-
hand rule
, the electron current
in a wire and the magnetic field it generates are
perpendicular to each other.


14
.
Oersted’s

Experiment

All magnetic fields originate from moving electric
charges. A magnetic field appears only when relative
motion is present between an electric charge and an
observer. Electric and magnetic fields are different
aspects of a single electromagnetic field.


15
. Electromagnets

An
electromagnet
consists of an iron core placed
inside a wire coil.

The magnetic field strength of a
wire coil carrying an electric current increases in
direct proportion to the number of turns of the coil.


15
. Electromagnets

An
electromagnet
can
be used to move large
quantities of metal.
When the current is on
the magnet will pick up
the metal. When you
want to drop it you turn
off the power and the
electromagnet is
disabled and the metal
drops.

16
. Magnetic Force on a Current

A magnetic field exerts a
sideways push on an
electric current with the
maximum push occurring
when the current is
perpendicular to the
magnetic field. Currents
exert magnetic forces on
each other. The forces are
attractive when parallel
currents are in the same
direction and are repulsive
when the parallel currents
are in opposite directions.

The experimental
Japanese Maglev train
uses magnetic forces
for both support and
propulsion.

How a TV works.







17
. Electric Motors

An electric motor uses
the sideways push of
a magnetic field to
turn a current
-
carrying
wire loop.

Electric
motors use a
commutator

to
change the direction
of the current in the
loop.

Alternating
current electric motors
do not use
commutators.

18
. Electromagnetic Induction

The effect of producing an
induced current is known as
electromagnetic induction
.
The direction of the induced
current can be reversed by
reversing the motion of the wire
or reversing the field direction.
The strength of the current
depends on the strength of the
magnetic field and the speed of
the wire's motion.


The stationary
windings of a
large electric
motor. magnetic
forces underlie
the operation of
such motors.

Michael Faraday (1791
-
1867) built the first electric
motor and discovered
magnetic induction.

Sharks navigate
with the help of
the earth’s
magnetic field.
They detect the
field using
electromagnetic
induction.

Alternating and Direct Current

Alternating current
(ac) is current that flows in a back
-
and
-
forth manner; household current changes direction 120 times
each second (60 Hz).
Direct current
(dc) flows in one
direction.


The ac generator (or alternator) produces an ac current and can be
modified to produce dc current by

1. Use of a
commutator
.

2. Use of a
rectifier

which permits current to pass through it in only
one direction.

19
. Transformers

A
transformer
is a device composed of two unconnected
coils, usually wrapped around a soft iron core, that can
increase or decrease the voltage of ac current.


19
. Transformers

A
transformer
is used to step the voltage
down and the current up (P=IV) so that
we can use it. Low power is desired for
the transport of electricity long distances
to avoid loss of energy to heat loss.


A moving coil
activated by voice
vibrations is used
as a microphone.
The coil induces a
current in the
magnet that can be
amplified or
recorded.

19
. Transformers

A

taperecorder

records signals from a microphone on
magnetic tape which then can be run across a magnet and
played back.