Chapter 08

```James T. Shipman
Jerry D. Wilson
Charles A. Higgins, Jr.
Chapter 8
Electricity and Magnetism
Sec 8.1Electric Charge, Electric Force,
and Electric Field
• 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
Intro
Sec 8.1Electric Charge, Electric Force,
and Electric Field
• 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)
• The unit of charge is the Coulomb (C)
– The charge on the electron is 1.6x10-19C
Section 8.1
Sec 8.1Electric Charge, Electric Force,
and Electric Field
Two negative charges
repel
Repulse
Two positive
charges repel
One negative and one
positive attract
Repulse
Attract
Section 8.1
Sec 8.1Electric Charge, Electric Force,
and Electric Field
Section 8.1
Sec 8.1Electric Charge, Electric Force,
and Electric Field
• An electric force exists between any two
charged particles – either attractive or repulsive
– Like charges repel (positive to positive, or negative to
negative)
– Unlike charges attract (positive to negative)
• Coulomb’s Law relates the magnitude of the
force between two electric charges and:
– Their separation
– Their charge
=

122

, k is coulomb’s constant,  =

9.0109 2

Section 8.1
2
Sec 8.1Electric Charge, Electric Force,
and Electric Field
Coulomb’s apparatus
Sec 8.1Electric Charge, Electric Force,
and Electric Field
Comparison between Coulomb’s Law and
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
Section 8.1
Sec 8.1Electric Charge, Electric Force,
and Electric Field
The Electric Field
Action-at-a-distance
concept replaced
by the Electric
Field which
surrounds the
charge and
represents the
physical effect in
nearby space.
Section 8.1
Sec 8.2 Current, Voltage, and Electrical
Power
• I = the rate of flow of electric charge =
charge/time = 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
Section 8.2
Sec 8.2 Current, Voltage, and Electrical
Power
• 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)
Section 8.2
Sec 8.2 Current, Voltage, and Electrical
Power
• When work is done to separate positive and
negative charges, we have electric potential
energy
Section 8.2
Sec 8.2 Current, Voltage, and Electrical
Power
• 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
Section 8.2
Sec 8.2 Current, Voltage, and Electrical
Power
• 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)
Section 8.2
Sec 8.2 Current, Voltage, and Electrical
Power
In a simple electric 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
Section 8.2
Sec 8.2 Current, Voltage, and Electrical
Power
The light bulb offers resistance. The kinetic energy of the
electric energy is converted to heat and radiant energy.
Section 8.2
Sec 8.2 Current, Voltage, and Electrical
Power
The water wheel analogy
Section 8.2
Sec 8.3 Simple Electric Circuits and
Electrical Safety
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
Section 8.2
Sec 8.3 Simple Electric Circuits and
Electrical Safety
A series circuit
• 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
Sec 8.3 Simple Electric Circuits and
Electrical Safety
A parallel circuit: Current Divides, Voltage is the
same
Section 8.3
Sec 8.3 Simple Electric Circuits and
Electrical Safety
• Household circuits are wired in parallel
independent branches any particular circuit
element can operate when others in the same
circuit do not.
Section 8.3
Sec 8.3 Simple Electric Circuits and
Electrical Safety
• 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.
Section 8.3
Sec 8.4 Magnetism
• Closely associated with electricity is magnetism.
• Electric fields come from stationary electric
charges
• Magnetic fields come from moving electric
charges
• A bar magnet has two regions of magnetic
strength, called the poles.
– One pole is designated “north,” one “south.”
Section 8.4
Sec 8.4 Magnetism
• 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.
Section 8.4
Sec 8.4 Magnetism
• The N pole of a magnet is where magnetic fields
emanate (or come out of…).
• The S pole of a magnet is where magnetic fields
terminate (or go into…).
• 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
Section 8.4
Sec 8.4 Magnetism
All magnets have
two poles – they are
dipoles
Section 8.4
Sec 8.4 Magnetism
• The arrows indicate the direction in which the
north pole of a compass would point.
Section 8.4
Sec 8.4 Magnetism
• The source of magnetism is moving and
spinning electrons.
• First discovered in rocks in the Greek province
of Magnesia
– Magnetic rock: magnetite
Section 8.4
Sec 8.4 Magnetism
• 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
Sec 8.4 Magnetism
• 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.”
Section 8.4
Sec 8.4 Magnetism
• Similar to the
pattern from a
giant bar
magnet being
present within
the earth (but
one is not
present!)
Section 8.4
Sec 8.4 Magnetism
Section 8.4
Sec 8.5 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).
Section 8.5
Sec 8.5 Electromagnetism
• 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)
Section 8.5
Sec 8.5 Electromagnetism
• Electrons entering
a magnetic field
experience a force
Fmag that deflects
them “out of the
page”
Section 8.5
Sec 8.5 Electromagnetism
• 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
Section 8.5
Sec 8.5 Electromagnetism
• 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
Section 8.5
Sec 8.5 Electromagnetism
• 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
Section 8.5
Sec 8.5 Electromagnetism
• 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)
Section 8.5
Sec 8.5 Electromagnetism
• 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.
Section 8.5
Sec 8.5 Electromagnetism
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.
Section 8.5
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