Transcript Chap 1.3 notes

Chapter 1 – Section 3
Voltage in Electrical Systems
Objectives
• Explain the similarities and differences between
Newton’s law of universal gravitation and
Coulomb’s law.
• Describe the force between like and unlike
electric charges.
• Describe how to create an electric field.
• Define electric potential difference (voltage).
• Differentiate between AC and DC current.
• Identify common sources of DC current.
• Describe how to connect DC voltage sources so
that voltages will add.
Gravitational Force
• Newton’s Law of Universal Gravitation
states that every object in the universe is
attracted to every other object
• The force felt is proportional to the product
of the mass of each object.
• It is inversely proportional to the square of
the distance between them.
Gravitational Force Formula
FG = G m1m2 / r2
Where
FG = force of gravity
G = universal gravitational constant =
6.67x10-11 N m2/ kg2
m1 and m2 = the masses of the two objects
in kilograms
r = the center to center distance between
them in meters
Force of gravity is always attractive
Example – Force of the Earth on
the Moon
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•
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•
Earth’s mass ME = 6 x 1024 kg
Moon’s mass Mm = 7.4 x 1022 kg
Average separation distance = 3.9 x 108m
Calculate the force of the Earth on the
Moon.
Electric Charge
• The origin of electric charge is found in the
atom.
• The nucleus of the atom contains
positively charged protons (p+) and neutral
neutrons (n0).
• The nucleus is surrounded by negatively
charged electrons (e-).
Electric Force
• Electric charge can be positive or
negative.
• Unlike charges attract, like charges repel.
• 18th century French scientist Charles
Coulomb did experiments with charged
spheres (Saturday nights were pretty dull
in the 18th century).
• He discovered the relationship between
charge, force and distance.
Coulomb’s Law
• The force felt is proportional to the product
of the charge on each body.
• It is inversely proportional to the square of
the distance between them.
Coulomb’s Law Formula
Fe = K q1q2 / r2
Where
Fe = electrostatic force
K = electrostatic constant = 9 x 109 Nm2/ c2
q1 and q2 = the charges of the two objects,
measured in coulombs (c)
r = the center to center distance between
them in meters
Electric force may be attractive or
repulsive
Gravitational & Electrical Fields
• The gravitational field is an expression of
the force felt per unit of mass in the vicinity
of another mass.
• G = F / m, where G = gravitational field
strength, F = force & m = a small test
mass.
• The units for G are N / kg which also
equals m / s2.
• On Earth, G = 9.8 N/ kg = g = 9.8 m / s2 .
Electrical Fields
• The electric field is an expression of the
force felt per unit of charge in the vicinity
of another charge.
• E = F / q , where E = electric field strength,
F = force and q = a small test charge.
• The units for E are N / c.
Fields – cont.
• Both G & E are vectors. Neither depends
upon the size of the test mass or test
charge.
• G always points in the direction of the
mass causing the field.
• E points in the direction that a positive test
charge would move.
Gravitational Field
Electric Field
Electric Potential Difference
• Imagine two flat metal plates, one charged
positively, the other negatively.
• Between the plates is a uniform electric field.
• In this field, we place a positively charged
particle. The particle will cling to the negatively
charged plate.
• Now, we move the particle from point A to point
B, some distance, d, towards the positively
charged plate.
Electric potential difference
Electric Potential Difference - cont
• If released, the particle will accelerate
back towards the negative plate, i.e point
A.
• There exists, therefore, an electric
potential difference between points A & B.
• The magnitude of this potential difference,
or voltage, depends upon the magnitude
of the electric field and the distance
between the two points.
Potential difference formula
• DVAB = Ed where,
• DVAB = the potential difference between A
& B, measured in volts.
• E = the electric field, measured in
Newtons/ coulomb (N/c) .
• d = distance, measured in meters (m).
Electric Current
• If the two charged plates are connected by
a wire, the charge will flow between them
until there is no potential difference.
• The flow of electric charge is called
current.
• The current can be maintained by a
potential difference source, or voltage
source, such as a battery.
Prime movers
• In a fluid system, pressure difference is
the prime mover. It is what causes fluids
to flow.
• In an electrical system, potential
difference, or voltage, is the prime mover.
It is what causes charges to flow.
Components of Electrical Systems
• Electrical systems usually contain four
major components:
• At least one voltage source such as a
battery or generator;
• Conductors, such as wires or connections
on printed circuit boards;
• At least one load, and
• One or more control elements, such as a
switch.
Electric components - cont
• The load is usually an appliance or
machine – motors, lights, heaters, TV sets,
computers, air conditioners, etc.
• Conductors are materials through which
charge can easily flow, usually metal.
• Control elements may be simple switches,
variable controls, diodes, transistors, etc.
AC/DC
• An Australian rock band formed in 1973 by brothers
Malcolm and Angus Young, who are considered pioneers
of heavy metal. They developed the idea for the band's
name after their older sister, Margaret, saw the initials
"AC/DC" on a sewing machine.
• "AC/DC" is an abbreviation for "alternating current/direct
current" electricity. They felt the name symbolized the
band's raw energy and power-driven performances. The
band is colloquially known as "Acca Dacca" in Australia.
• DC – direct current; the charges flow in only one
direction. Batteries are a common source of DC.
• AC – alternating current; the charges reverse direction
on a regular basis. In North America, common
household AC current is 60 cycles per second, or Hertz
(Hz). This means the current reverses 60 times each
second. European current is usually 50 Hz. AC is
produced by an alternator or generator.
Batteries
• Typical sources of DC voltage
• Cell – a single unit that holds chemicals that
react to separate electrons and positive ions
causing a potential difference.
• Battery – a collection of cells.
• Anode – the positive electrode of a battery
• Cathode – the negative electrode.
• Batteries can be connected, positive to negative,
in order to add their voltages together.
Circuit diagrams
• Circuit diagrams are used to design, build,
diagnose and repair circuits. They are like
a road map to show the path of the
current.
• Circuit diagrams use symbols to show how
a circuit is constructed.
• Some common symbols used in circuit
diagrams are shown below.
A simple DC circuit
Summary
• Newton’s law of gravitation & Coulomb’s law are
both inverse square laws, the magnitude of the
forces decreasing with the square of the
distance.
• Atoms are composed of p+, n0, & e- .
• Electron flow is called current
• Like charges repel, unlike attract.
• Electric fields exist around charges. The
magnitude depends upon the size of the charge
and the distance from it.
More summary
• Potential difference, voltage, between two points
in a uniform electric field is the product of the
field strength and the distance.
• Voltage is the prime mover in electrical systems.
• A battery is a DC voltage source. It can maintain
current in a circuit.
• Batteries or cells can be connected in series to
increase voltage.