An electric guitar is similar to an acoustic guitar in that both create sound by the vibration of strings through strumming. The structure of both electric guitars and acoustic guitars can vary dramatically based on style, purpose, and personal preference, but the distinguishing feature of a electric guitar is its pickup. Unlike its acoustic brother, the electric guitar doesn't require a hollow body to amplify sound waves; rather it amplifies sound by feeding electric signals to an amplifier. Utilizing the principle of electromagnetic induction, an electric signal is generated from the vibration of the guitar's strings, thus providing the ability to amplify and modify the sound produced. The electric guitar is one of the most influential inventions of the music history. It plays a crucial role in the development of rock and roll and many other genres of music.  

Parts of the electric guitar

Legend and brief descriptions on selected parts:
1. Headstock:
     1.1 machine heads

          Machine heads are used for tuning.
     1.2 truss rod cover
     1.3 string guide
     1.4 nut

          A nut is used to support the strings.
2. Neck:
     2.1 fretboard (also called fingerboard)
     2.2 inlay fret markers
     2.3 frets
     2.4 neck joint
3. Body
     3.1 "neck" pickup
     3.2 "bridge" pickup

           Pickups are magnets that send electric signals to the amps.
     3.3 saddles
     3.4 bridge

          A bridge is a device for supporting the strings.
     3.5 fine tuners
     3.6 tremolo arm

          A tremolo arm is used to add vibrato to the sound.
     3.7 pickup selector switch
     3.8 volume and tone control knobs
     3.9 output connector
     3.10 strap buttons
4. Strings:
     4.1 bass strings
     4.2 treble strings

          There are six strings on an electric guitar. They initiate the process of sound production.

Electromagnetic Induction

Figure 1

Faraday’s law of electromagnetic induction states that “an electric current is induced in a conductor whenever the magnetic field in the region of the conductor changes with time”. This law can be demonstrated by plunging a bar magnet into a solenoid connected to an ammeter, as shown in theFigure 1. As the north pole enters the solenoid, the magnetic field in the region of the conductor changes and a current is induced.

Electromotive force

In physics, electromotive force (EMF,  ) is any source that produces a potential difference that can be used in a circuit to drive current. For example, batteries and mitochondrion generate a electromotive force that is utilized as a power resource. Similarly, electromotive force is produced in electromagnetic induction as well. When a conductor moves across a magnetic field, an electric current is produced, and that electric field pushes that charges through the wire. This is the induced electromotive force. We are able to determine the electromotive force if we know the strengths of electric and magnetic fields by using a formula that contains differential equations.

Pickups 

In class we learned that passing a magnet through an electromagnet can produce current and vice versa. This is very similar to how an electric guitar's pickup works. Electric pickups work on the principle of electromagnetic induction. The law of electromagnetic induction states that "an electric current is induced in a conductor whenever the magnetic field in the region of the conductor changes with time". 

Sound is produced by vibrations in the guitar strings over electromagnetic pick ups. These vibrations are then sent electronically to an amp and then to a speaker. The pickup consists of a bar magnet wrapped around thousands of turns of copper wire and is located underneath the guitar strings. The strings themselves are usually made of ferromagnetic materials, combinations such as nickel and steel.This means that the strings are attracted to the bar magnet in the pickup. As the string vibrates (when its strummed) it causes disturbances in the magnetic field. These disturbances create a changing magnetic field which in turn produces a current. This is based off the law of electromagnetic induction where a fluctuating magnetic field creates an electric field. This in turn creates current (electrons being pushed by the electric field) in a conductor. In the case of an electric guitar the fluctuating magnetic field is caused by the vibrating strings and the conductor is the coils of copper wire. The signal or current created is uniquely defined by the frequency of the string that was struck, and can be interpreted to produce a specific sound. Different frequencies, created by strumming different strings (among other things such as finger placement), result in different signals being sent, and thus different sounds being produced.The current or electric signal is then sent to an amp and ultimately converted into audible sound via the speaker.

Variations in pickups

Not all pickups are made the same. For example some guitars have pickups with separate magnets (fig 3) where the height of each magnet can be changed using a screw piece while others feature a single bar magnet (Fig 2). The closer the magnet is to the string the stronger the signal and vice versa. Additionally the type of magnet used can affect the pickup structure and ultimately the sound. Some commons types used are aluminum nickel cobalt (cheap), samarium cobalt (strong & expensive) or neodymium boron iron (strongest & most expensive). Pickups with stronger magnets can be smaller and need less coils. However this does not mean that a pickup with a strong magnet is a good idea as the strings themselves are ferromagnetic. If the pickup is too strong it may affect the vibrations in the strings. Generally speaking a pickup with a weaker magnet is more lenient to the vibrating strings but there is no "correct" pickup; rather it depends on the user's tastes.

 Figure 2                                                                                                                                                        Figure 3

Amplifiers 

Unlike its acoustic cousin, the electric guitar alone is incapable of producing any adequately audible sounds due to the fact that it does not have a hollow body where the vibrations from the strings would be amplified. Instead, the signal from the electric guitar is sent to the amplifier where the signal must be boosted before it can visit the speaker and produce audible sound. Due to the dependency of the guitar on the amplifier, amps are often considered a part of the electric guitar. An amplifier is composed of two parts, a pre-amp, and a main amp. The pre-amp boosts (increases the amplitude of) the electric signal from the guitar, as often the amount of current induced by strumming is too small to drive the main amplifier.  The main amplifier is then responsible to further increase the amplitude of the signal while maintaining its frequency, thus outputting a louder version sound of the input sound. In modern times, guitarists are inclined towards modifying the input sound with effects such as distortion, reverb, etc. Applying these effects after the signal has been boosted from the pre-amp may result in uncontrolled distortion - a result of the current in the circuitry being too large for the circuitry. Therefore, these effects are done by exterior effects boxes (sometimes built into the main amplifier) before the pre-amp boost. In special cases, the final sound produced can be altered by the amplifier to be of the right frequency and of high enough amplitude that it causes the original string on the guitar itself to vibrate. This creates what is known as a feedback loop; the string vibration causes sound output and the sound output causes more string vibrations. These special effects, among others, are what the electric guitar has become known for and are part of the reason why electric guitars are so popular around the globe. 

Assembly

Check out the following videos that show how electric guitars are made in an assembly line:

Sources

http://entertainment.howstuffworks.com/electric-guitar.htm

http://en.wikipedia.org/wiki/Electric_guitar 

http://www.aqdi.com/pickups.htm

http://en.wikipedia.org/wiki/Guitar_amplifier

http://www.ehow.com/how-does_4596787_guitar-amps-work.html

http://entertainment.howstuffworks.com/electric-guitar2.htm

http://entertainment.howstuffworks.com/electric-guitar1.htm

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