Download Archie`s Chapter 5 - Electricity and Magnetism (ASmith)

Electricity and
Magnetism
Chapter 5
1. What is electricity?
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Many natural phenomena are electrical in
nature.
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Nerve impulses
Bolts of lightning
Chemical reaction
Electricity is one of the many forms of energy
used in today’s society
Electricity describes all the phenomena caused
by positive and negative electric charges
1.1 Electrical Charges
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Electrical charge is a property of protons and
electrons. A proton carries a positive charge,
while an electron carries a negative charge.
A negatively charged body contains more
electrons than protons.
A positively charged body contains fewer
electrons than protons.
Remember, only the electrons will move to
create charges.
Electrical Forces of Attraction and Repulsion

Like charges repel
 Opposite charges attract
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The elementary charge is the charge
carried by a single electron or proton.
 The coulomb (C) is the unit of
measurement for the quantity of electrical
charge. One coulomb is equal to the
charge of 6.25 x 1018 electrons or protons.
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1.2 Conductors and Insulators
Most objects are electrically neutral – they
have the same number of protons and
electrons.
 However, by transferring electrons from
one atom to another, some objects can
acquire a charge.
 Charging an object consists of creating an
imbalance in the electrical charge of that
object
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A conductor is a substance that permits
the free flow of electrical charges.
 At
the atomic level, the attraction between the
nucleus and the valence electrons is relatively
weak. Therefore the valence electrons can
easily pass from one atom to another.
 Metals and electrolytic solutions are usually
conductors
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An insulator is a substance that impedes
the free flow of electrical charges
 The
nuclei of insulators hold tight to their
valence electrons.
 Nonmetals are usually insulators.
 E.g. wood, plastic, glass, ceramics, rubber,
silk, paper, air
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Semiconductors exhibit a variable
conductivity, depending on different
factors.
 Metalloids
and carbon are semiconductors
 They are widely used in electronics.
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2. Static Electricity
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Static electricity describes all the phenomena
related to electrical charges at rest.
The static charge on an object will gradually
diminish due to the slow transfer of electrons to
the water molecules in the air.
Or, the transfer can be rapid if two objects come
close to each other or touch. This is called an
electrostatic discharge and is often
accompanied by a spark as the electrons pass
through the air and heat it up making it light up.
2.1 Charging an Object
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A object can be charged in various ways:
 Friction
 Conduction’
 Induction
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Charging by Friction
 When
two neutral object are rubbed together, the
atoms of one of the bodies may pull electrons away
from the atoms of the other body.
 The Triboelectric Series can be used to determine
which way the electrons will flow …
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Plastic is rubbed with silk
Glass is rubbed with wool
What will happen if the glass is then
brought close to the plastic?
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Charging by Conduction
 An
object is put in contact with another object
that is already charged
 The two objects will share the charge
between them resulting in both having the
same charge, but weaker than that of the
original object.
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Charging by Induction
 The
charge is created without direct contact of
the two objects.
 The charged object will acquire the opposite
charge to the original one
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3. Dynamic Electricity
Dynamic electricity describes all of the
phenomena related to electrical charges in
motion.
 The electrical charges part of a circuit,
meaning they can flow in a loop.
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3.1 Electric Current
As soon as current is generated at one
point in a circuit, all of the electrons in the
loop are set in motion. This is due to the
force of repulsion between the negative
electrons. The effect is almost
instantaneous.
 Electric current is the orderly flow of
negative charges carried by electrons.
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The conventional current direction is
the direction in which a positive particle
would flow in an electrical circuit. For this
reason, the direction goes from the
positive terminal of the power supply to its
negative terminal.
 BUT in reality, the electrons flow the
opposite way (from negative to positive).
This is called “electron flow”.
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Current Intensity
 Current
intensity is the number of charges
that flow past a given point in an electrical
circuit every second.
 The formula for current intensity is:
q
I
t
 An
Where: I is the current intensity, in amperes (A)
q is the quantity of charge, in coulombs (C)
Δt is the time, in seconds (s)
ammeter is an instrument that is used to
measure the current intensity
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Sample Problem
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21.875 X 1018 electrons flow through a resistor in 0.50
seconds
Determine the current flowing through the resistor.
# e- = 21.875 X 1018
Δt = 0.5 s
I=?
1. Find the quantity of charge (q):
21.875 1018
q
 3.5 C
18
6.25 10
2. Find the current (I):
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q
I
Δt
3.5 C

0.5 s
7A
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Current intensity is measured using an
ammeter.
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Potential Difference
 The
potential difference is the amount of
energy transferred between two points in an
electrical circuit.
 The formula for potential difference is:
E
V
q
Where: V is the potential difference in volts )V)
E is the energy in joules (J)
q is the quantity of charge in coulombs (C)
 A voltmeter
is used to measure the potential
difference in various parts of the circuit
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Practice problem
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25 coulombs of electricity carry 12.5 joules of
energy.
What is the potential difference in the circuit?
q = 25 C
E = 12.5 J
V=?
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E
V
q
25 J

12.5 C
2V
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In an electrical circuit, the energy comes
from a battery, a power supply, or from a
generator. A battery converts chemical
energy into electrical energy. The power
supply is a transformer converting 120 V
to a lower voltage. A generator converts
mechanical energy into electrical energy.
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A voltmeter is used to measure the
potential difference in a circuit.
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Resistance
 Any
part of an electrical circuit that uses
energy is called a resistor. Resistors transfer
electrical energy into another form.
 In a circuit, the resistance can be described
as a force that hinders the flow of the current.
The higher the resistance, the more energy it
takes for the current to flow.
 Electrical resistance is the ability of a material
to hinder the flow of electric current.
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There are four factors that determine the resistance of a
substance:
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Conductance is the ability of a material to allow the flow
of electric current. Good conductors are poor insulators.
The best conductors are good conducting material
and …
 Short
 Fat
 Cold
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Resistance is measured in ohms (Ω)
An ohm is equal to a potential difference of one
volt per ampere:
1V
1Ω 
1A
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Ohm’s Law
 Ohm’s
Law states that, for a given resistance, the
potential difference in an electrical circuit is directly
proportional to the current intensity.
V  IR
 Or
…
V
R
I
 Ohm’s
Where: U is the potential difference, in volts (V)
I is the current intensity, in amperes (A)
R is the resistance, in ohms (Ω)
And …
V
I
R
Law applies only to conductors, and not to
insulators or semiconductors.
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Sample Problem
A toaster
has 3 A of current flowing through
it while plugged in to a 120 V outlet.
Calculate the resistance of the toaster.
I=3A
V
R
V = 120 V
I
120 V
R=?

3A
 40 Ω
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3.2 Electrical Power
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Electrical power is a measure of the rate of
transformation of electrical energy. The more
powerful a device, the faster it works.
Electrical power is the amount of work an
electrical device can perform per second.
W
Pe 
t
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Where: Pe is electrical power, in watts (W)
W is work, in joules (J)
Δt is time, in seconds (s)
Sample Question
A 500 W electric motor runs for 2 minutes.
 How much work does it use?
 How much energy does it use?
W
EW
Pe=500 W
Pe 
Δt
 60 kJ
Δt = 2 min
W  Pe Δt
= 120 s
 500 W  120 s
W=?
 60000 J or 60 kJ
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Another equation for power …
Pe  VI
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where: Pe is electrical power, in watts (W)
V is potential difference, in volts (V)
I is current intensity, in coulombs (C)
Sample problem
A 100 W light bulb is plugged into a 120 V outlet.
Determine the current passing through the bulb.
Pe = 100 W
V = 120 V
I=?
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Pe  VI
Pe
I
V
100 W

120 V
 0.83 A
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The Relationship Between Power and Electrical
Energy
 Electrical
energy can be measured in joules or
kilowatt-hours
 The kilowatt-hour is the unit used to calculate
electrical consumption for electricity bills.
 1kWh = 1000 W X 3600 s = 3 600 000 J
 Presently, the residents of Quebec pay about 8 cents
per kilowatt-hour.
E  Pe t
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where: E is the electrical energy in joules (J)
or kilowatt-hours )kWh)
Pe is the electrical power in watts (W)
or kilowatts (kW)
Δt is time in seconds (s) or hours (h)
Sample Problem 1
A 5 W toy motor runs for 0.5 minutes.
 Calculate the energy used.
Pe = 5 W
E  Pe Δt
Δt = 0.5 minutes
 5 W  30 s
= 30 s
 150 J
E =?
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Sample Problem 2
A 150 W light bulb is on for 1800 minutes.
 Hydro Québec charges 8 cents per kWh.
 Calculate the cost of using this bulb.
1. Find the Energy used:
Pe = 150 W
E  Pe Δt
= 0.150 kW
 0.150 kW  30 h
Δt= 1800 minutes
 4.5 kWh
= 30 hours
2. Find the cost:
Cost = 8¢/kWh
Cost  cost/kWh  # of kWh
E =?
 8 ¢/kWh  4.5 kWh
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 36 ¢
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3.3 Electrical Circuits
An Electrical Circuit is a network in which
electrical charges can flow continuously in
a loop.
 Diagrams and symbols are often used to
represent electrical circuits. See the
“Toolbox” for information on how to draw a
circuit diagram.
 The current direction shown in a diagram
usually corresponds to conventional
current (+’ve to –’ve)
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There are two ways in which components
in an electrical circuit can be connected:
 In
Series
 In Parallel
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Series Circuits
 The
components are connected one after the other.
There is only one way for the current to flow. There
are no branches in the circuit.
 A series circuit is a circuit in which the elements are
connected end to end.
 If one of the components is defective, the entire circuit
stops working.
 The amount of energy used by the resistors adds up
so that with each new resistor, the amount of energy
available for each resistor decreases. If you add
more light bulbs in series, the brightness of each bulb
decreases.
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Parallel Circuits
 Parallel
circuits have branching. There is more than
one way for the current to go.
 A Parallel circuit is a circuit that has at least one
branch.
 The points where the circuits branch into two or more
paths or combine into one are called nodes.
 If one of the components is defective, the other
components will continue to operate.
 The amount of energy available to each component
does not change as new resistors are added. Adding
new resistors does not decrease the energy used by
the others.
 As resistors are added, the total resistance
decreases.
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4. What is Magnetism?
A magnet is an object that can attract
other objects containing iron, cobalt or
nickel.
 Magnetism describes all the phenomena
caused by magnets.
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4.1 Magnets
Some substances never acquire magnetic
properties. Others, like iron, can become
magnetic under certain circumstances.
 Iron is made up of domains that are like
tiny magnets. When the iron is not
magnetized, the domains are not aligned.
Their magnetic effects cancel out. When
the iron is magnetized, the domains line
up. The greater the number of aligned
domains, the stronger the magnet.
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Magnetic Forces of Attraction and
Repulsion
 All
magnets have a north-seeking pole and a
south seeking pole.
 The north pole of a magnet, by convention, is
the pole that turns towards the earth’s
magnetic pole located near the geographical
North Pole. The other end of the magnet is its
south pole.
 So, what we call the “North Pole” is really the
south pole of the earth’s magnetic field.
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The following is true of magnets:
 Opposite
magnetic poles attract each other
 Like magnetic poles repel each other
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Magnets will always have a north and a
south pole.
 If the magnet breaks, each piece will have
a north and a south.
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4.2 Magnetic Fields
The force of attraction or repulsion
between two magnets is called the
magnetic force. This force can act over a
distance through a magnetic field.
 A magnetic field is the area of space in
which the magnetic force of a magnet can
act on another magnet.
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The effect of the field can be observed by
sprinkling iron filings around a magnet …
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By convention, the magnetic field is said
to have the direction indicated by the
north pole of a compass if it was placed
in the field.
The magnetic field can be represented
by magnetic field lines.
Magnetic field lines:
1.
2.
3.
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Go from north to south
Never cross
Are closer together near the poles, where the
magnetic field is strongest
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4.3 Magnetizing Objects
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A nonmagnetic substance cannot be magnetized
and is not attracted to a magnet.
A ferromagnetic substance has the ability to
acquire magnetic properties. (Iron, nickel or cobalt)
A magnet is a substance with magnetic properties.
It can attract ferromagnetic objects. Although it is
usually made of iron, nickel or cobalt, some rare
earth element can make extremely strong magnets.
Magnetic remanence describes the ability of a
material to acquire and conserve magnetic
properties.
5. Electromagnetism
Electricity and magnetism are connected.
 Electric currents will always generate a
magnetic field.
 Magnetic fields can be used to generate
an electric current.
 Electromagnetism describes all the
phenomena resulting from the interaction
between electricity and magnetism.
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5.1 Magnetization by Electricity
An electric current produces a magnetic
field. The field is created by moving
electrons.
 Static electricity does not involve moving
electrons, therefore, a statically charged
object will not possess a magnetic field.
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The Magnetic Field of a Live Wire
 In
1819 Hans Christian Oersted observed that
a compass was deflected when it came close
to a live wire. He was the first of observe the
relationship between electrical and magnetic
phenomena.
 The direction of the magnetic field around a
live wire depends on the direction of the
current.
 The “Right-Hand Rule” can be used to
determine the direction of the magnetic field.
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Steps for the Right Hand Rule
1.
Find the positive and
2.
3.
4.
5.
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negative ends of the wire
The conventional current
flows from positive to
negative
Point thumb of your right
hand in the direction of the
conventional current
Wrap your fingers around
the wire
Your fingers will point in the
direction of the magnetic
field
Direction of the
magnetic field
Direction of the
Conventional Current
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