Download Chapter 5 - Bakersfield College

Outline Chapter 5 Electricity and Magnetism
5-1. Positive and Negative Charge
A. Electric charge is a fundamental property of certain of the elementary
particles of which all matter is composed.
B. There are two types of electric charge: negative charge and positive
charge.
C. Like charges repel one another; unlike charges attract one another.
D. An object having equal amounts of positive and negative charges is said
to be electrically neutral.
5-2. What Is Charge?
A. Every substance is composed of tiny bits of matter called atoms, which
are composed of three kinds of elementary particles:
1. The positively charged proton
2. The negatively charged electron
3. The electrically neutral neutron
B. The structure of an atom consists of a central nucleus of protons and
neutrons with electrons moving about the nucleus.
C. An atom is electrically neutral since its number of protons (positive
charges) is equal to its number of electrons (negative charges).
D. The coulomb (C) is the unit of electric charge.
E. The basic quantity of electric charge (e) is 1.6 x 10-19 C.
5-3. Coulomb's Law
A. Coulomb's law states that the force between two electrical charges is
inversely proportional to the square of the distance between the charges and
directly proportional to the product of the magnitude of the
charges:
where F = force, K = a constant (9 x 109 N  m2/C2), Q = magnitude of
charge, and R = distance between charges.
B. The coulomb is a very large unit. Even the most highly charged objects
contain a small fraction of a coulomb of charge.
5-4. Force on an Uncharged Object
A. An electrically charged body attracts small uncharged objects such as
dust particles and bits of paper.
B. The attractive force results from a separation of charge (positive and
negative charges) induced in the uncharged object(s) by the charged body.
5-5. Matter in Bulk
A. Coulomb's law resembles the law of gravity; however, gravitational forces
are always attractive, whereas electric forces may be attractive or repulsive.
B. Gravitational forces dominate on a cosmic scale; electric forces dominate
on an atomic scale.
5-6. Conductors and Insulators
A. A conductor is a substance through which electric charge flows readily.
B. An insulator is a substance that strongly resists the flow of electric
charge.
C. Semiconductors are substances whose electrical conductivity is
between that of conductors and insulators.
D. Transistors are semiconductor devices whose conductivity can be
changed at will.
E. The conduction of electricity through gases and solids involves the
movement of charged atoms or molecules called ions.
1. A positive ion results when an atom or molecule loses electrons.
2. A negative ion results when an atom or molecule gains electrons.
3. The process of forming ions is called ionization.
5-7. Superconductivity
A. Superconductivity refers to the loss of all electrical resistance by certain
materials at very low temperatures.
B. Superconductors allow the transmission of electric current without energy
loss.
C. The use of superconductors was limited because they had to be cooled to
extremely low temperatures; however, superconductors have been
discovered that do not need the extreme cold of earlier ones.
D. The goal is to discover superconductors that operate at room
temperatures.
5-8. The Ampere
A. Electric current is the rate of flow of charge from one place to another.
Currents are by conventiion assumed to flow from the + terminal of a battery
or other source to its - terminal.
B. A closed path that conducts an electric current is called a circuit.
C. The ampere (A) is the SI unit of electric charge and is equal to a flow of 1
coulomb per second:
1 A = 1 C/s
5-9. Potential Difference
A. Potential difference, or voltage, is the electrical potential energy per
coulomb of charge.
1. Potential difference is thought of as the work a unit charge can
perform as the charge travels from one point to another.
2. The volt (V) is the unit of potential difference and is equal to one
joule per coulomb:
1 V = 1 J/C
B. The voltages of two or more batteries are added when the method of
connection between the batteries is — terminal to + terminal.
C. Storage batteries can be recharged and used again.
5-10. Ohm's Law
A. Ohm's law, discovered by Georg Ohm (1787-1854), states that current in
an electrical circuit is equal to voltage across the circuit divided by the
resistance of the circuit:
I = V/R
where I = current, V = voltage, and R = resistance.
B. Resistance is a measure of opposition to the flow of charge.
1. The ohm () is the unit of resistance and is equal to 1 volt per
ampere.
1  = 1 V/A
2. The resistance of a metal wire depends on its:
a. Composition: Some materials offer more resistance than
others.
b. Length: The longer the wire, the greater the resistance.
c. Cross-sectional area: The greater the wire's cross-sectional
area, the less the resistance.
d. Temperature: The higher the temperature, the more the
resistance.
C. Ohm's law holds only for solid metallic conductors.
5-11. Electric Power
A. The power of an electric current is the rate at which it does work and is
equal to the product of the current and the voltage of a circuit:
P = IV
where P = power, I = current, and V = voltage.
B. The unit of electric power is the watt.
C. The commercial unit of electric energy is the kilowatthour (kWh).
5-12. Magnets
A. Every magnet has a north pole and a south pole.
B. Like magnetic poles repel one another; unlike poles attract.
C. Magnetic poles always come in pairs. There is no such thing as a single
north pole or south pole.
D. All substances have magnetic properties, though generally to a very slight
extent.
E. In a magnet, all of the atoms are aligned with their N poles in the same
direction.
5-13. Magnetic Field
A. A force field is a region of altered space around a mass, an electric
charge, or a magnet that exerts a force on appropriate objects in that region.
B. The force field around a magnet is called a magnetic field.
C. Field lines are imaginary lines in a field of force; they help us visualize a
magnetic field.
5-14. Oersted's Experiment
A. Hans Christian Oersted discovered in 1820 that an electric current near a
compass causes the compass needle to be deflected.
B. Oersted's experiment showed that every electric current has a magnetic
field surrounding it.
C. According to the right-hand rule, the electron current in a wire and the
magnetic field it generates are perpendicular to each other.
D. All magnetic fields originate from moving electric charges.
1. A magnetic field appears only when relative motion is present
between an electric charge and an observer.
2. Electric and magnetic fields are different aspects of a single
electromagnetic field.
5-15. Electromagnets
A. The magnetic field strength of a wire coil carrying an electric current
increases in direct proportion to the number of turns of the coil.
B. An electromagnet consists of an iron core placed inside a wire coil.
5-16. Magnetic Force on a Current
A. A magnetic field exerts a sideways push on an electric current with the
maximum push occurring when the current is perpendicular to the magnetic
field.
B. Currents exert magnetic forces on each other. The forces are attractive
when parallel currents are in the same direction and are repulsive when the
parallel currents are in opposite directions.
5-17. Electric Motors
A. An electric motor uses the sideways push of a magnetic field to turn a
current-carrying wire loop.
B. To prevent the loop from coming to rest when it is perpendicular to the
magnetic field, direct current electric motors use a commutator to change
the direction of the current in the loop.
C. Alternating current electric motors do not use commutators since the
alternating current automatically changes the direction of the current in the
wire loop.
5-18. Electromagnetic Induction
A. Michael Faraday discovered that an induced current is produced in a
wire moved across a magnetic field.
1. The direction of the induced current can be reversed by reversing
the motion of the wire or reversing the field direction.
2. The strength of the current depends on the strength of the
magnetic field and the speed of the wire's motion.
B. The effect of producing an induced current is known as electromagnetic
induction.
1. Alternating current (ac) is current that flows in a back-and-forth
manner; household current changes direction 120 times each second
(60 Hz).
2. Direct current (dc) flows in one direction.
C. The ac generator (or alternator) produces an ac current and can be
modified to produce dc current by
1. Use of a commutator.
2. Use of a rectifier which permits current to pass through it in only
one direction.
5-19. Transformers
A. A transformer is a device composed of two unconnected coils, usually
wrapped around a soft iron core, that can increase or decrease the voltage
of ac current.
B. The induced voltage in the secondary coil can be higher, lower, or the
same as that of the primary coil depending on the ratio of turns between the
primary and secondary coils.
1. If the number of turns of both coils is equal, the induced voltage is
also equal.
2. If the secondary coil has more turns, the induced voltage is higher.
3. If the secondary coil has fewer turns, the induced voltage is lower.
C. Electric power (P= IV) is always the same in both coils; thus the
relationship between the number of turns, voltage, and current of a
transformer is represented by the formula:
5-20. Tape Recorders
A. Sound waves are converted into alternating current by a microphone.
B. The electrical signals are amplified and converted by the recording head
of the tape recorder into a magnetic pattern on the recording tape.
C. The magnetic fields on the tape induce an alternating current in the
playing head. The current is amplified and fed into a loudspeaker.
D. The loudspeaker converts the electrical signals into sound waves.