Transcript Chapter 11- ELECTRICITY

Chapter 11- ELECTRICITY
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Rule 1 and 2
Rule 1: Like charges repel one another.
Rule 2: Unlike charges attract one another.
Electric charge is a basic
characteristic of matter
Atomic Parts
• Every atom has a positively charge
nucleus, surrounded by negatively
charged electrons.
An atom in its normal state will
have the same number of protons
and electrons
A charged atom is called an ion
• When an atom loses one
or more electrons, it has
a positive net charge.
• When an atom gains
one or more electrons,
it has a negative net charge.
Electrons are transferred
from the fur to the rod. The
rod is then negatively
charged. Is the fur charged?
How much compared to the
rod? Positively or
negatively?
QUESTION: STATIC
Electric charge can be dangerous. Two
hundred years ago, young boys called
powder monkeys ran below the decks
of warships to bring sacks of black
gunpowder to the cannons above. It
was ship law that this task be done
barefoot. Why?
ANSWER
Because it was important that no
static charge build up on the
powder on their bodies as they ran
to and fro. Bare feet scuffed the
decks much less than shoes and
assured no charge buildup that
might produce an igniting spark
and an explosion.
Coulomb’s Law
The electrical force between two charged
bodies is directly proportional to the product
of the charges and inversely proportional to
the square of the distance between them:
Formula:
F= k q1q2
2
d
Charles Coulomb
18th century
Coulomb’s Law
• The unit of charge is the coulomb (C)
• Charge of 1 C is the charge on 6.25 billion
electrons.
• k – constant (similar to constant G in law
of gravity), but it is very
big number
9,000,000,000 N
2
2
m /C
Orbits of satellite, electron are similar
Gravity force
Electrical force
Electrically Polarized
• Center of charge
moves.
• Fig a – center of
negative cloud
coincides with
center of positive
nucleus.
• Fig b – electron
cloud is distorted,
“polarized”
Balloon
polarizes
atoms in
the wall
Electric Current
• Current is the flow of electrons.
• Loose electrons in metals are responsible for
good heat conduction. This is true for electrical
conduction.
• The rate of flow is measured in amperes
(A)
Voltage – Electric Pressure
Voltage =
Potential Energy
Charge
Current flows in a wire when there is a
difference in voltage across the ends of
the wire.
A steady current needs a pumping device
to provide a difference in voltage.
Batteries, generators
are “electrical pumps”
Electric Eel
• An unusual
source of voltage.
The electric
potential between
the head and tail of
the electric eel
(Electrophorus
electricus) can be
up to 600 V.
Electrical Resistance
• Measured in units called
ohms Ω
• Named after Georg Simon
Ohm, a German physicist in
1826.
Electrical Resistance
• More resistance
– Narrow wire
– Long wire
– Higher temperature
– Poor conductors
(rubber)
• Less Resistance
– Wide wire
– Short wire
– Low temperature
– Good conductors
(metals)
Van de Graaff Generator
OHM’S LAW
How much current?
• How much current flows through a
lamp with a resistance of 60 Ω when
the voltage across the lamp is 12 V?
• Using Ohms law,
• Current = Voltage/resistance
• So: Current =12
V/60 Ω
= 0.2 A
ELECTRIC SHOCK
Tasers
A Taser works by delivering high
voltage — but low amperage — to the
human body. A Taser delivers a
powerful but temporary shock rather
than a sustained and deadly charge.
• To receive a shock, there must be
a difference in voltage between
one part of your body and another
part. Electron flow will pass along
the path of least electrical
resistance connecting these two
points.
• More info on electric shock on
page 180
The bird can stand harmlessly on
one wire of high voltage, but it had
better not reach over and grab a
neighboring wire! Why not?
Direct Current and
Alternating Current
• Direct Current
• Alternating Current
• “dc”
• “ac”
• Electrons flow
one direction
• Electrons flow
initially one
direction, then
in the opposite
direction
Electric Power
Electric Power
• Power is expressed in watts (when current
is in amperes and voltage is in volts)
• Power is equal to current multiplied by
voltage.
POWER = Current x voltage
WATTS = AMPERES x VOLTS
Superconductors
• Zero electrical
resistance to the
flow of charge.
• Very low
temperatures (4K)
• No current is lost,
no heat generated
• Energy saving
potential!
Series and Parallel Circuits
Series Circuit
• The current doesn’t pile up in any lamp but
flows through each lamp.
• The current has only one pathway.
• The resistance of the current starts in the first
lamp in the circuit, then the second, then the
third, and so on.
• If one device fails, current in the entire circuit
stops.
Parallel Circuit
• The total current in the circuit divides
• More branches means overall resistance
of the circuit is lowered.
• When one bulb burns out, the other are
not affected.
Fuses and Circuit Breakers
If a fuse is rated
at 20 amperes, it
will pass 20
amperes and not
more. A current
above 20
amperes will melt
the fuse and
breaks the
circuit.