Download Chapter 8 - Texas Southern University Department of Physics

Chapter 8
Electricity and
Magnetism
Sections 8.1-8.5
Electric Charge & Force
• Electric charge is a fundamental quantity –
we don’t really know what it is
– But we can describe it, use it, control it
– Electricity runs motors, lights, heaters, A/C,
stereos, TV’s, computers, etc.
• Electric Forces – at the microscopic level they
hold atoms and molecules together
– Electric Forces hold matter together
– Gravitational Forces hold the universe together
• Magnetism is also closely associated with
electricity
Copyright © Houghton Mifflin Company. All rights reserved.
Intro
8|2
Electric Charge and Current
• Experimental evidence leads us to conclude
that there are two types charges
– Positive (+)
– Negative (-)
• All matter is composed of atoms, which in
turn are composed of subatomic particles
– Electrons (-)
– Protons (+)
– Neutron (neutral)
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.1
8|3
Properties of Electrons, Protons,
and Neutrons
• Note that the Proton and Neutron each have
about 1000x more mass than the Electron
• If the atom has the same number of protons
and electrons it is electrically neutral
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.1
8|4
Coulomb = Unit of Electric Charge
• Named after a French scientist – Charles
Coulomb (1736-1806)
• Note that the charge on a single electron
(-) or proton (+) is 1.60 x 10-19 C (very
small!)
• When charge is in motion  electric
current
• Current = time rate of flow of electric
charge
• Current = charge/time = I = q/t
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.1
8|5
Current
• I = q/t
– I = electric current (amperes)
– q = electric charge flowing past a point (coulombs)
– t = time for the charge to pass point (seconds)
• 1 ampere (A) = flow of 1 Coulomb per second
• Rearrange equation above:
– q = It
or 1 coulomb = 1 ampere x 1 second
• Therefore, 1 coulomb is the amount of charge
that flows past a given point in 1 second
when the current is 1 ampere
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.1
8|6
Conductors/Insulators
• Electrical conductor – materials in which
an electric charge flows readily (most
metals, due to the outer, loosely bound
electrons)
• Electrical insulator – materials that do
not conduct electricity very well due to
very tight electron bonding (wood,
plastic, glass)
• Semiconductor – not good as a
conductor or insulator (graphite)
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.1
8|7
Finding the Amount of
Electric Charge - Example
•
A wire carries a current of 0.50 A for 2
minutes.
a) how much (net) charge goes past a point in the
wire in this time?
b) how many electrons make up this amount of
charge?
•
•
•
GIVEN: I = 0.50 A, t = 2 minutes (120
seconds)
WANTED: q (charge) & n (number of
electrons)
(a) q = It = (0.50 A)(120 s) = 60 C
(coulombs)
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.1
8|8
Finding the Amount of
Electric Charge - Example
•
A wire carries a current of 0.50 A for 2
minutes.
a) how much (net) charge goes past a point in the
wire in this time?
b) how many electrons make up this amount of
charge?
• To solve for (b), we know that each electron
has a charge of 1.6 x 10-19 C and we know
the total charge from part (a) = 60 Coulombs
• n = 60/(1.6 x 10-19 C / electron) = 3.8 x 1020
electrons
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.1
8|9
Another Example
• If 5.0 x 1019 electrons go by a given
point in a wire in 2/3 minute, what is the
current in the wire?
• Given: n = 5.0 x 1019 electrons, t = 40
seconds, q = 1.6 x 10-19 C (for each
electron)
• I = q/t
• = [(1.6 x 10-19 C)(5.0 x 1019)]/40 s =
8.0C/40s
• = 0.20 A
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.1
8 | 10
Electric Force – Law of Charges
• An electric force exists between any
two charged particles – either attractive
or repulsive
• Law of Charges – like charges repel,
and unlike charges attract (this gives
the direction of electric force)
– Two positives repel each other
– Two negatives repel each other
– Positive and negative attract each other
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.1
8 | 11
Coulomb’s Law
• Force of attraction/repulsion between two
charged bodies is directly proportional to the
product of the two charges and inversely
proportional to the square of the distance
between them
• F = (kq1q2) / r2
–
–
–
–
–
F = force of attraction or repulsion
q1 = magnitude of the first charge
q2 = magnitude of the second charge
r = distance between charges
k = constant = 9.0 x 109 N-m2/C2
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.1
8 | 12
Comparison of Coulomb’s Law &
Newton’s Law of Universal Gravitation
• Equations look similar
– F = (kq1q2) / r2 & F = Gm1m2/ r2
• Both depend on r2
• Coulomb’s law can describe either an
attractive or repulsive force – gravity is
always positive
• Electrical charges are much stronger
than gravitational forces
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.1
8 | 13
Negative/Positive
• An object with an excess of electrons is
said to be negatively charged
• An object with a deficiently of electrons
is said to be positively charged
• Photocopier (xerography) – practical
and widespread use of electrostatics
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.1
8 | 14
Repulsive and Attractive Electrical Forces
Two negative charges
repel
Two positive
charges repel
Repulse
Repulse
Copyright © Houghton Mifflin Company. All rights reserved.
One negative and one
positive attract
Attract
Section 8.1
8 | 15
Electric Potential Energy
• When work is done to separate positive
and negative charges, we have electric
potential energy
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.2
8 | 16
Voltage
• Instead of measuring electric potential
energy, we measure the potential difference,
or voltage
• Voltage – the amount of work it would take to
move a charge between two points, divided
by the value of the charge
• Voltage = work / charge = V = W/q
• Measured in volts (V) = 1 joule/Coulomb
• When we have electric potential energy, this
may be used to set up an electrical current
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.2
8 | 17
Ohm’s Law
• Whenever there is an electrical current, there
is resistance (R) within the conducting
material
– R is due to atomic/subatomic collisions
• Georg Ohm (1787-1854) – formulated a
simple relationship between voltage, current,
and resistance
• Ohm’s Law  V = IR
– V = voltage in volts, I = current in amperes, and
R = resistance in ohms
• 1 ohm = 1 volt/1 ampere (R=V/I)
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.2
8 | 18
Simple Electrical Circuit
• Electrons flow from negative terminal to
positive terminal (provided by the
chemical energy of the battery) -negative to positive
• Open switch – not a complete circuit
and no flow of current (electrons)
• Closed switch – a complete circuit and
flow of current (electrons) exists
• Closed Circuit Required – to have a
sustained electrical current
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.2
8 | 19
Simple Electrical Circuit
The light bulb offers resistance. The kinetic energy of the
electric energy is converted to heat and radiant energy.
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.2
8 | 20
Electrical Circuit & Waterwheel Analogy
Insert figure 8.5, p.189, 11/e
Section 8.2
Electrical Power
•
•
•
•
•
•
•
•
•
Recall: P = W/t (Power = work/time) – Ch. 4
V = W/q (Voltage = work/charge) – Ch. 8
Rearrange V=W/q  W = qV
Substitute W = qV into P = W/t equation
Result  P = qV/t
Recall that I = q/t (Current = charge/time)
\ P = IV (Electric power = current x voltage)
also P = I2R (substitute in V = IR)
P in watts, I in amperes, R in ohms, V in volts
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.2
8 | 22
Finding Current in Resistance -Example
• Find the current and resistance of a 60-W, 120V light bulb in operation.
• Given: P = 60W, V = 120 V
• Find: I (current in amperes), R (resistance in
ohms)
• Since P = IV  I = P/V = 60 W/120 V = 0.50 A
• Since V = IR  R = V/I = 120 V/0.50A = 240 W
• Since P = I2R  R = P/I2 = 60 W/(0.50 A)2 =
240 W
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.2
8 | 23
Equation Review
• Current: I = q/t
– I = current (amperes),
– q = electric charge (coulombs),
– t = time (seconds)
•
•
•
•
Coulomb’s Law: F = kq1q2/r2
Voltage: V = W/q [Work is in joules (J)]
Ohm’s Law: V= IR
Electrical Power: P = IV = I2R
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.2
8 | 24
Forms of Electric Current
• Direct Current (dc) – the electron flow is
always in one direction, from (-) to (+)
– Used in batteries and automobiles
• Alternating Current (ac) – constantly
changing the voltage from positive to
negative and back
– Used in homes.
– 60 Hz (cycles/sec) and Voltage of 110-120 V
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.3
8 | 25
Simple Electric Circuits
in Series
• Plugging appliances into a wall outlet
places them in a circuit – either in series
or in parallel
• In a series circuit, the same current (I)
passes through all the resistances.
– The total resistance is simply the sum of
the individual resistances.
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.3
8 | 26
Series Circuit
• Rs = R1+R2+R3+…
• If one bulb were to burn out the circuit would be
broken, and all the lights would go out
Same current
all the way
Section 8.3
Simple Electric Circuits in Parallel
• In a parallel circuit, the voltage (V)
across each resistance is the same, but
the current (I) through each may vary.
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.3
8 | 28
Parallel Circuit
I = I1+I2+I3+… for R in parallel: 1/Rp = 1/R1+1/R2+…
for 2 R in parallel: Rp = (R1R2)/(R1+R2)
Current Divides, Voltage is the same
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.3
8 | 29
Resistances in Parallel - Example
• Three resistors have values of R1=6.0W,
R2=6.0W, and R3=3.0W. What is their total
resistance when connected in parallel, and
how much current will be drawn from a 12-V
battery if it is connected to the circuit?
• 1/Rp = 1/R1+1/R2+1/R3 = 1/6 + 1/6 + 1/3
• 1/Rp = 1/6 + 1/6 + 2/6 = 4/6 W
• Rp = (6 W)/4 = 1.5 W = Total Resistance
• I = V/Rp = (12 V)/(1.5 W) = 8.0 A = current
drawn
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.3
8 | 30
Household Circuits110-120 V
• Wired in parallel independent branches any particular
circuit element can operate when others in the same
circuit do not.
Section 8.3
Electrical Safety
• Wires can become hot as more and more
current is used on numerous appliances.
• Fuses are placed in the circuit to prevent
wires from becoming too hot and catching
fire.
• The fuse filament is designed to melt (and
thereby break the electrical circuit) when the
current gets too high.
• Two types of fuses: Edison and S-type
• Circuit Breakers are now used.
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.3
8 | 32
Fuses
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.3
8 | 33
Electrical Safety with Dedicated Grounding
• A dangerous shock
can occur if an
internal ‘hot’ wire
comes in contact
with the metal
casing of a tool.
• This danger can be
minimized by
grounding the case
with a dedicated
wire through the
third wire on the
plug.
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.3
8 | 34
Magnetism
• Closely associated with electricity is
magnetism.
• In fact electromagnetic waves consists
of both vibrating electric and magnetic
fields. These phenomena are basically
inseparable.
• A bar magnet has two regions of
magnetic strength, called the poles.
• One pole is designated “north,” one
“south.”
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.4
8 | 35
Magnetic Poles
• The N pole of a magnet is “north-seeking”
– it points north.
• The S pole of a magnet is “south-seeking”
– it points south.
• Magnets also have repulsive forces,
specific to their poles, called …
• Law of Poles – Like poles repel and
unlike poles attract
– N-S attract
– S-S & N-N repel
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.4
8 | 36
Law of Poles
All magnets have
two poles – they
are dipoles
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.4
8 | 37
Magnetic Field
• Magnetic field - a set of imaginary lines
that indicates the direction in which a
small compass needle would point if it
were placed near a magnet
• These lines are indications of the
magnetic force field.
• Magnetic fields are vector quantities.
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.4
8 | 38
Magnetic Field
• The arrows indicate the direction in which the north
pole of a compass would point.
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.4
8 | 39
Source of Magnetism
• The source of magnetism is moving and
spinning electrons.
• Hans Oersted, a Danish physicist, first
discovered that a compass needle was
deflected by a current-carrying wire.
– Current open  deflection of compass needle
– Current closed  no deflection of compass needle
• A current-carrying wire produces a magnetic
field: stronger current  stronger field
• Electromagnet – can be switched on & off
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.4
8 | 40
Magnetic Materials
• Most materials have many electrons going in
many directions, therefore their magnetic
effect cancels each other out  nonmagnetic
• A few materials are ferromagnetic (iron,
nickel, cobalt) – in which many atoms
combine to create magnetic domains (local
regions of magnetic alignment within a single
piece of iron)
• A piece of iron with randomly oriented
magnetic domains is not magnetic.
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.4
8 | 41
Magnetization
• The magnetic
domains are
generally
random, but
when the iron is
placed in a
magnetic field
the domains line
up (usually
temporarily).
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.4
8 | 42
Electromagnets
• A simple electromagnet
consists of an insulated
coil of wire wrapped
around a piece of iron.
• Stronger current 
stronger magnet
• Electromagnets are made
of a type of iron that is
quickly magnetized and
unmagnetized – termed
“soft.”
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.4
8 | 43
Curie Temperature & Permanent Magnets
• Materials cease to be ferromagnetic at very
high temperatures – the “Curie temperature”
• Permanent magnets are made by
permanently aligning the many magnetic
domains within a piece of material.
• One way to create a permanent magnet is to
heat the ferromagnetic material above the
Curie temperature and then cool the material
under the influence of a strong magnetic field.
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.4
8 | 44
Earth’s Magnetic Field
• This planet’s magnetic field exists within the
earth and extends many hundreds of miles
into space.
• The aurora borealis (northern lights) and
aurora australis (southern lights) are
associated with the earth’s magnetic field.
• Although this field is weak compared to
magnets used in the laboratory, it is thought
that certain animals use it for navigation.
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.4
8 | 45
Earth’s Magnetic
Field
• Similar to the
pattern from a
giant bar
magnet being
present within
the earth (but
one is not
present!)
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.4
8 | 46
Earth’s Magnetic Field
• The origin of the earth’s magnetic field is not
completely understood.
– Probably related to internal currents of electrically
charge particles in the liquid outer core of the
earth, in association with the earth’s rotation
• The temperatures within the earth are much
hotter than the Curie temperature, so
materials cannot be ferromagnetic.
• The positions of the magnetic poles are
constantly changing, suggesting “currents.”
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.4
8 | 47
Magnetic Declination
• Magnetic declination – the angle between
geographic (true) north and magnetic north
• The magnetic declination varies depending
upon one’s location on Earth.
– In the northern hemisphere the magnetic North
Pole is about 1500 km from the geographic North
Pole.
• Therefore, a compass does not point to true
north, but rather magnetic north.
– An adjustment (magnetic declination) must be
made to determine true north from a compass.
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.4
8 | 48
Magnetic Declination
Needs to be known for proper navigation
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.4
8 | 49
Useful Equations
• Resistance in Series: Rs =
R1+R2+R3+…
• Resistance in Parallel: 1/Rp =
1/R1+1/R2+…
• For 2 R in parallel: Rp =
(R1R2)/(R1+R2)
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.4
8 | 50
Electromagnetism
•
•
Electromagnetism – the interaction of
electrical and magnetic effects
Two basic principles:
1) Moving electric charges (current) give
rise to magnetic fields (basis for an
electromagnet).
2) A magnetic field will deflect a moving
electric charge (basis for electric motors
and generators).
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 51
Telephone – Electromagnet Application
• Telephone transmission involves the
conversion of sound waves into varying
electric current
• Sound waves at the transmitting end vibrate a
diaphragm that puts pressure on carbon
grains
• The varying pressure on the carbon, results
in varying amounts of resistance in the circuit
• By Ohm’s law (V=IR), varying resistance
results in varying current
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 52
Telephone – Electromagnet Application
• The varying current gives an
electromagnet at the receiving end
varying magnetic strengths
• The electromagnet is drawn
toward/away a permanent magnet as a
function of the electric current in the line
• The electromagnet is attached to a
diaphragm that vibrates, creating sound
waves that closely resemble the original
sound waves
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 53
Simple (one-way) Telephone
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 54
Magnetic Force on Moving Electric Charge
• When a moving charge (a current)
enters a magnetic field, the moving
charge will be deflected by the magnetic
field
• The magnetic force (Fmag) is
perpendicular to the plane formed by
the velocity vector (v) of the moving
charge and the magnetic field (B)
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 55
Magnetic
Deflection
• Electrons
entering a
magnetic field
experience a
force Fmag that
deflects them
“out of the page”
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 56
Magnetic Force on a Moving Charge
• A beam of electrons is
deflected downward
due to the introduction
of the magnetic field of
a bar magnet
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 57
Magnetic Field and Force on a CurrentCarrying Wire
• Since a magnetic field deflects a moving
charge, a current-carrying wire loop will
experience a force
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 58
A stationary wire with no current will not
experience a force in a magnetic field
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 59
A current-carrying wire in a magnetic field
experiences a force, out of the page as
shown here
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 60
Motors
• Electric motor – a device that converts
electrical energy into mechanical work
• When a loop of coil is carrying a current
within a magnetic field, the coil
experiences a torque and rotates
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 61
A dc Motor
• The split-ring commutator
reverses the loop current
every half-cycle, enabling
the loop to rotate
continuously
• The inertia of the spinning loop carries it through the
positions where unstable conditions exist
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 62
Motion and Induced Current
The Basic Principle of the Electric Generator
• When a wire is
moved perpendicular
to a magnetic field,
the magnetic force
causes the electrons
in the wire the move
– therefore a current
is set up in the wire
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 63
Generators
• Generator – a device that converts
mechanical energy into electrical energy
• Generators operate on the principle of
electromagnetic induction
• Electromagnetic induction was discovered by
the English scientist, Michael Faraday in
1831
– When a magnet is moved toward a loop or coil of
wire, a current is induced in the wire
– The same effect is obtained if the magnetic field is
stationary and the loop is rotated within it
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 64
Electromagnetic Induction
• A current is induced in the circuit as the magnet is
moved toward and into the coil
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 65
An ac Generator
• When the loop is
rotated within the
magnetic field a
current is induced in
the loop
• The current varies in
direction every halfcycle and is termed
alternating current
(ac)
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 66
Electrical Transformers
• Transformers serve to step-up or step-down
the voltage of electric transmissions
• A transformers consists of two insulated coils
of wire wrapped around an iron core
– The iron core serves to concentrate the magnetic
field when there is a current in the input coil
• The magnetic field resulting from the primary
coil goes through the secondary coil and
induces a voltage and current
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 67
A Step-up Transformer
• Since the
secondary coil has
more windings, the
induced ac voltage
is greater than the
input voltage.
• The factor of
voltage step-up
depends on the
ratio of the
windings on the
two coils.
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 68
Why Step-up the Voltage?
• The main reason is to reduce the amount of
current going through the lines
– As the current is reduced, the amount of
resistance is also reduced
– If the amount of resistance is reduced, the amount
of heat (called ‘joule heat’ or ‘I2R’ losses)
• For example, if the voltage is stepped-up by a
factor of 2, the resulting I2R losses would be
reduced four-fold
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 69
Voltage Change for a Transformer
Equation
• The voltage change for a transformer is
given by the following:
• V2 = (N2/N1)V1
• V1 = primary voltage
• V2 = secondary voltage
• N1 = Number of windings in primary coil
• N2 = Number of windings in secondary
coil
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 70
Finding Voltage Output for a Transformer
An Example
• A transformer has 500 windings in its primary
coil and 25 in its secondary coil. If the
primary voltage is 4400 V, find the secondary
voltage.
• Use equation V2 = (N2/N1)V1
• Given: N1 = 500, N2 = 25, V1 = 4400 V
• V2 = (25/500)(4400 V) = 220 V
• This is typical of a step-down transformer on
a utility pole near homes
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 71
Electrical Transmission System
Voltage is dramatically stepped-up at the generating plant to minimize joule
heat loss during long-distance transmission. The voltage must then be
stepped-down for household use.
Copyright © Houghton Mifflin Company. All rights reserved.
Section 8.5
8 | 72