Transcript Chapter 2: Voltage and Current

Chapter 2
Voltage and Current
Atomic Theory
• An atom consists of a nucleus of
protons and neutrons surrounded by a
group of orbiting electrons.
• Electrons are negative, protons are
positive.
• In its normal state, each atom has an
equal number of electrons and protons.
Atomic Theory
• Electrons orbit the nucleus in discrete
orbits called shells.
• These shells are designated by letters
K, L, M, N, etc.
• Only certain numbers of electrons can
exist within any given shell.
Atomic Theory
• The outermost shell of an atom is called
the valence shell.
• The electrons in this shell are called
valence electrons.
• No element can have more than eight
valence electrons.
• The number of valence electrons affects
its electrical properties.
Conductors
• Materials that have large numbers of
free electrons are called conductors.
• Metals are generally good conductors
because they have few loosely bound
valence electrons.
• Silver, gold, copper, and aluminum are
excellent conductors.
Insulators
• Materials that do not conduct because their
valence shells are full or almost full are called
insulators.
• Glass, porcelain, plastic, and rubber are good
insulators.
• If high enough voltage is applied, an insulator
will break down and conduct.
Semiconductors
• Semiconductors have half-filled valence
shells and are neither good conductors nor
good insulators.
• Silicon and germanium are good
semiconductors.
• They are used to make transistors, diodes,
and integrated circuits.
Electrical Charge
• Objects become charged when they
have an excess or deficiency of
electrons.
• An example is static electricity.
• The unit of charge is the coulomb.
• 1 coulomb = 6.24 × 1024 electrons.
Voltage
• When two objects have a difference in
charges, we say they have a potential
difference or voltage between them.
• The unit of voltage is the volt.
• Thunderclouds have hundreds of
millions of volts between them.
Voltage
• A difference in potential energy is defined as
voltage.
• The voltage between two points is one volt if
it requires one joule of energy to move one
coulomb of charge from one point to another.
• V = Work/Charge
• Voltage is defined between points.
Current
• The movement of charge is called
electric current.
• The more electrons per second that
pass through a circuit, the greater the
current.
• Current is the rate of flow of charge.
Current
• The unit of current is the ampere (A).
• One ampere is the current in a circuit
when one coulomb of charge passes a
given point in one second.
• Current = Charge/time
• I = Q/t
Current
• If we assume current flows from the positive
terminal of a battery, we say it has
conventional current flow.
• In metals, current actually flows in the
negative direction.
• Conventional current flow is used in this
course.
• Alternating current changes direction
cyclically.
Voltage Sources
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Electronic Power Supplies
Batteries: AA, AAA, C, D
Solar Cells
DC Generators
How to Measure Voltage
• Measure voltage by placing voltmeter
leads across the component.
• The red lead is the positive lead; the
black lead is the negative lead.
• If leads are reversed, you will read the
opposite polarity.
How to Measure Current
• The current you wish to measure must
pass through the meter.
• You must open the circuit and insert the
meter.
• Connect with correct polarity.
Switches
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Single-pole, single-throw
Single-pole, double-throw
Double-pole, single-throw
Push-button - Normally open or
Normally closed.