Electrical Production of Sound1 Electric Circuits 2 Electric Current3 Resistance4 Voltage5 Ohm’s Law6 Series and Parallel Circuits7 Electric Energy and Power8 Alternating currents and Household Current9 AC and DC10 Faraday’s law11 Guitar12 Tape Deck13 Microphone14 Loudspeaker15 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.

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.

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Time

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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 orElectromotive 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 (1745-1827) who invented the electric battery. 1 volt = 1 J/C.

Ohm’s LawGeorg 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 = I R

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,

.time

EnergyP

Electric Power:.

time

EnergyP

Since the electrical energy is charge times voltage (QV), the above equation becomes,

.t

QVP

Since the current is charge flow per unit time (Q/t), the above equation becomes,

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Q

t

QVP

Since V = IR, the above equation can also be written as,

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2

R

VRIIVP

Killowatt-hour (kWh)

The SI unit of power is watt, after James Watt (1736-1819), who developed steam engines.

Utility companies use the unit kilowatt-hour to measure the electrical energy used by customers. One kilowatt-hour, kWh is the energy consumed for one hour at a power rate of 1 kW.

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jouleWwatt

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,

.; ABt

N

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: