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Electrical Production of Sound
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Electric Circuits
Electric Current
Resistance
Voltage
Ohm’s Law
Series and Parallel Circuits
Electric Energy and Power
Alternating currents and Household Current
AC and DC
Faraday’s law
Guitar
Tape Deck
Microphone
Loudspeaker
AM-FM Tuners
Electric Circuits
How to Get the Bulb to Light?
How to Get the Bulb to Light?
Electric Current
The electric current, I is the amount of charge per unit time
that passes through a surface that is perpendicular to the
motion of the charges.
Ch arg e
I
.
Time
The SI unit of electric current is the ampere (A), after the French
mathematician André Ampére (1775-1836). 1 A = 1 C/s. Ampere is a
large unit for current. In practice milliampere (mA) and microampere
(μA) are used.
Direction of Current Flow
Electric current is a flow of electrons. In a circuit, electrons
(negatively charged) actually flow through the metal wires.
Conventional electric current is defined using the flow of
positive charges.
It is customary to use a conventional current I in the
opposite direction to the electron flow.
Direction of Current Flow
What Limits the Flow of Current?
What Limits the Flow of Current?
A: Resistance
Electric Current Is Analogous to Water
Flow
Voltage or
Electromotive Force (emf)
The energy needed to run electrical devices comes from
batteries.
Within a battery, a chemical reaction occurs that transfers
electrons from one terminal (leaving it positively charged) to
another terminal (leaving it negatively charged).
Because of the positive and negative charges on the battery
terminals, an electric potential difference exists between them.
The maximum potential difference is called the electromotive
force* (emf) of the battery.
The electric potential difference is also known as the voltage, V.
The SI unit for voltage is the volt, after Alessandro Volta (17451827) who invented the electric battery. 1 volt = 1 J/C.
Ohm’s Law
Georg Simon Ohm (1787-1854), a German physicist, discovered
Ohm’s law in 1826.
This is an experimental law, valid for both alternating current (ac)
and direct current (dc) circuits.
When you pass an electric current (I) through a resistance (R)
there will be a potential difference or voltage (V) created
across the resistance.
Ohm’s law gives a relationship between the voltage (V), current (I),
and resistance (R) as follows:
Voltage = Current X Resistance
V=IR
What Is the Current?
Circuits
Series Circuit
Parallel Circuit
Electrical Energy
Electrical Energy and Power
Our daily life depends on electrical energy. We use many
electrical devices that transform electrical energy into other
forms of energy. For example, a light bulb transforms electrical
energy into light and heat. Electrical devices have various power
requirements. Electrical power, P is defined as the electrical
energy transfer per unit time,
Energy
P
.
time
Electric Power:
Energy
P
.
time
Since the electrical energy is charge times voltage (QV), the
above equation becomes,
QV
P
.
t
Since the current is charge flow per unit time (Q/t), the
above equation becomes,
QV Q
P
 V  I V .
t
t
Since V = IR, the above equation can also be written as,
V2
P  IV  I R 
.
R
2
Killowatt-hour (kWh)
The SI unit of power is watt, after James Watt (17361819), who developed steam engines.
joule
J
watt  W 
 .
sec ond s
Utility companies use the unit kilowatt-hour to measure
the electrical energy used by customers. One kilowatthour, kWh is the energy consumed for one hour at a
power rate of 1 kW.
Household Circuits
AC adapter
INPUT: AC 120 V, 60 Hz, 15 W
OUTPUT: DC 9V, 1A
Direct and Alternating Current
Current from a battery
flows steadily in one
direction (direct current,
DC). Current from a
power plant varies
sinusoidally (alternating
current, AC).
Alternating Current
Alternating Voltage
Effective voltage = 115 V
Electromagnetic Induction
Induced Current
(a) When there is no relative motion between the coil of wire and
the bar magnet, there is no current in the coil. (b) A current is
created in the coil when the magnet moves toward the coil. (c) A
current also exists when the magnet moves away from the coil,
but the direction of the current is opposite to that in ( b).
Faraday's Law of Electromagnetic
Induction
Michael Faraday found experimentally that the magnitude
of the induced emf is proportional to the rate at which the
magnetic flux changed. Faraday’s law can be written as,

  N
;   B A.
t
where N is the number of turns in the loops, A is the area
of one loop, ξ is the induced emf, Φ is the magnetic flux,
and B┴ is the perpendicular component of the magnetic
field.
Guitar
The Magnetic Playback Head of a
Tape Deck
A Moving Coil Microphone
Loudspeaker
Loudspeakers use the
principle that a magnet
exerts a force on a
current-carrying wire to
convert electrical
signals into mechanical
vibrations, producing
sound.
Radio and Television; Wireless
Communication
This figure illustrates the process by which a
radio station transmits information. The audio
signal is combined with a carrier wave:
Radio and Television; Amplitude
Modulation
The mixing of signal and carrier can be done
two ways. First, by using the signal to modify
the amplitude of the carrier (AM):
Radio and Television; Frequency
Modulation (FM)
Second, by using the signal to modify the
frequency of the carrier (FM):
Radio and Television; Receiver
At the receiving end, the wave is received,
demodulated, amplified, and sent to a
loudspeaker:
Radio and Television; Tuner
The receiving antenna is bathed in waves of
many frequencies; a tuner is used to select the
desired one: