Interference & Stationary Waves (Edexcel A Level Physics)

The photograph shows a guitar.

When a guitar string is plucked, a standing wave is created.

Explain how a standing wave is created on the string.

The diagram shows a standing wave on a guitar string.

The oscillating length of the guitar string is 66 cm.

(1)

Wavelength = ....................................................................

tension in guitar string = 88.6 N

mass per unit length of guitar string = 4.47 × 10⁻³ kg m⁻¹

(3)

Frequency = ................................................

One end of the guitar string is wrapped around a cylindrical tuning peg. Turning the peg changes the total length of the string and hence changes the tension in the string. This changes the frequency of vibration of the string.

Calculate the extension required to produce a tension of 93.4 N in the string.

Young modulus of string material = 1.8 × 10⁹ N m⁻²

cross-sectional area of string = 6.6 × 10⁻⁷ m²

(4)

Extension = ...........................................

Explain the effect that tightening the string has on the frequency of the sound produced.

(2)

A simple loudspeaker consists of a cone, a coil of wire and a magnet. The cone and coil are attached to each other and are free to move. An alternating current in the coil causes the cone to oscillate. The loudspeaker is mounted in a wooden box. A cross-section through the loudspeaker is shown.

A student made the following observations:

•   when an alternating potential difference (p.d.) is applied to the coil, the cone oscillates

•   the frequency of oscillation is the same as the frequency of the p.d.

•   at particular frequencies, the box vibrates with a large amplitude.

Explain these observations.

The student connected a signal generator to the loudspeaker, and placed the loudspeaker near to one end of a long tube containing sand. The student adjusted the signal generator until the sand collected in small heaps as shown.

(4)

Describe the procedure she should follow to determine an accurate value for d.

(3)

signal generator frequency = 3.25 kHz.

d = 5.1 cm

(3)