Transcript Electricity and Magnetism

Electricity
A source of energy that can be easily
converted into light, heat, or power.
Atoms and Electricity
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All matter is made of atoms
An atom is the smallest particle of any element that still retains
the characteristics of that element.
An atom has a nucleus, or center, which is surrounded by an
electron cloud.
The nucleus contains protons and neutrons
The electron cloud contains electrons
Electrons can move from atom to atom. When an electron moves
to a different atom, it causes another electron to have to move.
 When electrons move quickly from one atom to another
it is called an Electrical Current - the flow of electrons!
Charge and Force
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Protons have a positive charge (+)
Neutrons have no charge
Electrons have a negative charge (-)
An atom in its normal state has the same number or
electrons as protons so it is neutrally charged.
When an atom loses one or more electrons (Positive
Ion), it has a positive net charge.
When an atom gains one or more electrons (Negative
Ion), it has a negative net charge.
A force is a pull or push on an object
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Static Electricity
Static electricity is the
build up of electric
charges that stays on an
object.
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Charges do not flow they remain
at rest.
Electric Discharge - the loss
of Static electricity as
electric charges move off an
object.
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Lightning
What is static electricity?
When two objects rub against each other electrons transfer and
build up on an object causing it to have a different charge from
its surroundings.
Like the shoes rubbing against the carpet. Electrons are
transferred from the carpet to the shoes.
 When charged particles come
near one another, they give rise to
two different forces
 Forces of attraction
 Unlike charges attract one
another
 - and +
A charge is a measure of
the extra positive or
 Forces of repulsion
negative particles that
 Like charges repel one another
an object has.
 + and + or - and –
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The most important difference between gravitational
and electrical forces is that electrical forces may be
either attractive or repulsive, whereas gravitational
forces are only attractive.
As electrons collect on an object, it becomes negatively charged.
As electrons leave an object it attains a positive charges. Charges
interact with each other:
Often when you remove clothes
from the clothes dryer, they seem
to stick together. This is because
some of the clothes have gained
electrons by rubbing against other
clothes. The clothes losing
electrons become positive. The
negative clothes are attracted to the
positive clothes.
What causes you to be shocked when you rub your feet
across carpet?
An electrical discharge is the passing of an electric current
through the air from a negatively charged object to a
positively charge object. This is what causes
lightning!
Methods of Charging
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Friction –
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rubbing two objects together
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Conduction –
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direct contact of objects in which electrons flow
through one object to another object –(conductors)
Induction –
involves a rearrangement of electric charges in which a
neutral object need only come close to a charged
object, no contact is necessary
Law of Conservation of Charge
When we charge something, no electrons are
created or destroyed.
 They are simply transferred from one material to
another
 Charge is conserved
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Coulomb’s Law and Charge Polarization
The Force Between The Two Charged Particles Varies
Directly As The Product Of Their Charges And
Inversely As The Square Of The Separation Distance
 Coulomb (C) – is the unit of charge
 A charge of 1 C is the charge on 6.25 billion billion
electrons (6,250,000,000,000,000,000)
 the amount of charge that passes through a common
100-watt light bulb in little more than a second
 A polarized object has no net charge. Only the
distribution of charge in the material is altered.
(example – balloon sticking to wall - p.174)
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Electrical Terms
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Electrical Circuit – a path along which electrons can flow
Electric Current - the flow of electrons
Two types of Electric Current:
Direct Current (DC) – electrons flow in one direction only (the
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electrons flow from the negative terminal of a battery to the positive terminal)
Battery – electrons flow from repelling negative terminal toward the attracting
positive terminal
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Alternating Current (AC) – electrons in the circuit flow initially
in one direction and then reverses and flows in the opposite
direction (1 cycle)
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Generator
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Hertz – represents one cycle per second
In US electrical energy is delivered at 60 Hertz (Hz) – 60
cycles per second (changes direction of flow 120 times every second)
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Electrical Terms (cont.)
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Ampere (A) or (I) – measures the rate of electrical
flow
An ampere is the rate of flow of 1 coulomb of charge per second (In a wire that
carries 5 amperes, 5 coulombs of charge pass any cross section in the wire each
second)
Voltage (V) or (E) – measures the electrical
pressure
Electrons flow in a wire only when a difference in electrical pressure exists
across the ends of the wire.
A steady current needs an “electrical pump” (batteries, generators) to provide a
difference in voltage
Voltage produces current (if there is a complete circuit)
 Voltage
result
is the pressure – Current is the
Electrical Terms (cont)
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Ohms (Ω) or (R) – measures the electrical
resistance to current flow
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Resistance in a Circuit depends on:
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Width
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Length - long conductors offer more resistance than short conductors
Temperature – the higher the temp. the greater resistance a conductor has
Kind of Material – elements that have a higher number of valence
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– narrow conductors resist electrical current more than wider
conductors
electrons have greater resistance than elements with a lower number of valence
electrons.
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Conductor – material that provides a path for electrons to move
Good Conductors – Silver, Copper, Gold, Platinum, Aluminum
Poor Conductors (Insulators) – Rubber, Plastic, Wood, Glass
Semi Conductors – neither good for conductors or insulators – elements with
4 valence electrons – silicon, germanium
OHMS LAW
Named after Georg Simon Ohm –discovered a
simple relationship among voltage, current, and
resistance
 The amount of current in a circuit is directly
proportional to the voltage across the circuit and
inversely proportional to the resistance of the
circuit
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So for a given circuit of constant resistance, current and voltage
are proportional to each other (the greater the voltage, the greater
the current)
But if the resistance is doubled for a circuit, the current is reduced
to half (the higher the resistance, the lower the current)
OHMS LAW
 Current = Voltage / Resistance
 Amperes = Volts / Ohms
I = E / R
 Volts = Amps x Ohms
E = I x R
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It takes one volt to push one amp through one ohm
 Ohms
= Volts / Amps
R = E / I
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E=IxR
I=E/R
R=E/I
Electrical Power
Power is the rate at which work is done or energy is
used.
Electrical power is a measure of the rate at which
electricity does work or provides energy that is
converted to another form of energy
Unit of electrical power is the Watt, symbol is W or P
Electrical Power = Voltage x Current
Watts = Volts x Amperes W = E x I
One horsepower (hp) is equal to 746 Watts W
E x I
Electrical Energy
Electrical energy is a measure of the amount of power used
and the time of use.
Electrical energy is the product of the power and the time.
The unit of measure of electrical energy or work is the
watthour (Wh)
Kilowatt-hour (kWh) = Kilowatts x hours of use
1 kWh = 1000 W per hour
Electrical energy may be transformed to :
Mechanical energy – motor
Light energy – lamp
Thermal energy – heater
W
E x I
Problems
Volts = 12 Amps = ? Ohms = 6 Watts = ?
 Amps = (E/R) or (12/6) 2 Watts = (ExI) or (12x2) 24
 Volts = 6
Amps = ? Ohms = ? Watts = 30
 Amps = (W/E) or (30/6) 5 Ohms = (E/I) or (6/5) 6/5
 Volts = ?
Amps = 2
Ohms = 14 Watts = ?
 Volts = (IxR) or (2x14) 28 Watts = (ExI) or (28x2) 56
 Volts = ?
Amps = ?
Ohms = 9 Watts = 144
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Watts = ExI and Volts = IxR
so Watts = Ix (IxR)
so 144 W = IxIx9 or I2 x9
so I2 = 144/9 or 16
so I=square root of 16 or 4 A
so Volts = (IxR) or (4x9) 36 V
Example problem:
Electrical Energy is measured in Kilowatt-hours so we need to determine the
amount of Electrical Power first which is measured in Watts
Watts = Volts x Amps
Electrical Power = (120 V) (2A) = 240 W
Next we will determine the Electrical energy which is measured in Kilowatt-hours
kWh = (W x hrs.) / 1000
Electrical Energy = (240 W) (4 h) = 960Wh / 1000 =
0.96 kWh