Page 530 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
518
incident Z1 wave
angle of incidence
reflected wave
In a piezo-electric transducer, an alternating voltage is applied across the crystal, which then acts as the vibrating source of ultrasound waves. A brief pulse of ultrasound waves is sent into the patient’s body; the transducer then receives an extended pulse of reflected ultrasound waves.
Detecting ultrasound
The transducer also acts as the detector of reflected ultrasound waves. It can do this because the piezo-electric effect works in reverse: a varying stress applied to the crystal produces a varying e.m.f. across the crystal – see Figure 32.22b. To maximise the effect, the frequency of the waves must match the resonant frequency of the crystal.
The optimum size of the crystal is half the wavelength λ2 of the ultrasound waves.
Figure 32.23 shows the construction of a piezo-electric ultrasound transducer. Note the following features:
■■ The crystal is now usually made of polyvinylidene difluoride. Previously, quartz and lead zirconate titanate were used.
■■ The outer case supports and protects the crystal.
■■ At the base is the acoustic window, made from a material
that is a good transmitter of ultrasound.
QUESTIONS
11 Quartz is an example of a piezo-electric material. The speed of sound in quartz is 5700 m s−1.
a Calculate the wavelength of ultrasound waves of frequency 2.1 MHz in a quartz crystal.
b If the crystal is to be used in an ultrasound transducer, its thickness must be half a wavelength. Calculate the thickness of the transducer.
12 Piezo-electric crystals have many applications other than in ultrasound scanning. For example, they are used:
a in gas lighters (to produce a spark)
b in inkjet printers (to break up the stream of ink
into droplets)
c in guitar pickups (to connect the guitar to an amplifier)
d in the auto-focus mechanism of some cameras (to move the lens back and forth).
For each of these examples, state whether the piezo-electric effect is being used to convert energy in the vibrations of the crystal to electrical energy or the other way round.
■■
Behind the crystal is a large block of damping material (usually epoxy resin). This helps to stop the crystal vibrating when a pulse of ultrasound has been generated. This is necessary so that the crystal is not vibrating when the incoming, reflected ultrasound waves reach the transducer.
connector outer case
damping material
crystal
acoustic window
Figure 32.23 A section through an ultrasound transducer. Echo sounding
The principle of an ultrasound scan is to direct ultrasound waves into the body. These pass through various tissues and are partially reflected at each boundary where the wave speed changes. The reflected waves are then detected and used to construct an internal image of the body.
Figure 32.24 shows what happens when a beam of ultrasound reaches a boundary between two different media. The beam is partially refracted (that is, the transmitted beam has changed direction) and partially reflected. This diagram should remind you of the way
in which a ray of light is refracted and reflected when it strikes the boundary between two media. It is the change in speed which causes the refraction of a wave.
Z2
Figure 32.24 An ultrasound wave is both refracted and reflected when it strikes the boundary between two different materials.
angle of refraction
refracted wave (transmitted)