There are two types of guitars, acoustic guitars and electric guitars. In acoustic guitars resonance occurs between vibrating strings and other parts of the guitar.

(i) Define resonance.  

Resonance is the transfer of energy between two bodies of the same natural frequency of vibration.

(ii) Describe a laboratory experiment to demonstrate resonance.

Method:

• Take two tuning forks of the same natural frequency. Hold one against a hollow wooden box, as shown.
• Strike the other so that it vibrates. Hold it against the box so that you can hear the sound. Then remove this fork.
• The other tuning fork will continue to audibly vibrate, demonstrating resonance.

A guitar string has length 2 m and mass 0.88 g.  It is stretched across two fixed points which are 65.1 cm apart on a guitar. It is then plucked and it vibrates at a fundamental frequency of 330 Hz. 

(iii) Draw a labelled diagram to show a guitar string vibrating at its fundamental frequency.

Calculate 

(iv) the tension in the string, 

Mass per unit length:

(v) the speed of sound in the string. 

Wavelength:

Speed:

In an electric guitar a magnetic pickup detects the vibration in the string. The pickup consists of a stationary magnet and a coil around the magnet. When the string vibrates an emf is induced.

(vi) Draw the magnetic field around a bar magnet.  

(vii) Explain how an emf is induced in the coil.  

Faraday’s Law states that when there is a change in the magnetic flux linking a coil, an emf is induced in that coil. The strength of the emf is proportional to the rate of change of the flux linking the coil.

In an electric guitar, the string is magnetised by the presence of a magnet just beneath it, usually built into the structure of the guitar. When the string vibrates, it essentially becomes a vibrating magnet, inducing an emf and a current in the coil beneath.

(viii) Sketch a graph to show how the output current varies with time.