VII. Give the account of the discovery from the point of view of Michael Bowers (about 12 sentences).

1. Text VIII

2. Electric properties of matter. Piezoelectricity [pietsəuelik’tricəti]

1. Some solids, notably certain crystals, have permanent electric polarization. Other crystals become electrically polarized when subjected to stress. In electric polarization, the centre of positive charge within an atom, molecule, or crystal lattice element is separated slightly from the centre of negative charge. Piezoelectricity (literally “pressure electricity”) is observed if a stress is applied to a solid, for example, by bending, twisting, or squeezing it. If a thin slice of quartz is compressed between two electrodes, a potential difference occurs; conversely, if the quartz crystal is inserted into an electric field, the resulting stress changes its dimensions. Piezoelectricity is responsible for the great precision of clocks and watches equipped with quartz oscillators. It also is used in electric guitars and various other musical instruments to transform mechanical vibrations into corresponding electric signals, which are then amplified and converted to sound by acoustical speakers.

2. A crystal under stress exhibits the direct piezoelectric effect; a polarization P, proportional to the stress, is produced. In the converse effect, an applied electric field produces a distortion of the crystal, represented by a strain proportional to the applied field. The basic equations of piezoelectricity are P = d × stress and E = strain/d. The piezoelectric coefficient d (in metres per volt) is approximately 3 × 10⁻¹² for quartz, 5 × −10⁻¹¹ for ammonium dihydrogen phosphate, and 3 × 10⁻¹⁰ for lead zirconate titanate.

3. For an elastic body, the stress is proportional to the strain—i.e., stress = Ye × strain. The proportionality constant is the coefficient of elasticity Ye, also called Young's modulus for the English physicist Thomas Young. Using that relation, the induced polarization can be written as P = dYe × strain, while the stress required to keep the strain constant when the crystal is in an electric field is stress = −dYeE. The strain in a deformed elastic body is the fractional change in the dimensions of the body in various directions; the stress is the internal pressure along the various directions. Both are second-rank tensors, and, since electric field and polarization are vectors, the detailed treatment of piezoelectricity is complex. The equations above are oversimplified but can be used for crystals in certain orientations.

4. The polarization effects responsible for piezoelectricity arise from small displacements of ions in the crystal lattice. Such an effect is not found in crystals with a centre of symmetry. The direct effect can be quite strong; a potential V = Y_edδ/ε₀K is generated in a crystal compressed by an amount δ, where K is the dielectric constant. If lead zirconate titanate is placed between two electrodes and a pressure causing a reduction of only 1/20th of one millimetre is applied, a 100,000-volt potential is produced. The direct effect is used, for example, to generate an electric spark with which to ignite natural gas in a heating unit or an outdoor cooking grill.

5. In practice, the converse piezoelectric effect, which occurs when an external electric field changes the dimensions of a crystal, is small because the electric fields that can be generated in a laboratory are minuscule compared to those existing naturally in matter. A static electric field of 10⁶ volts per metre produces a change of only about 0.001 millimetre in the length of a one-centimetre quartz crystal. The effect can be enhanced by the application of an alternating electric field of the same frequency as the natural mechanical vibration frequency of the crystal. Many of the crystals have a quality factor Q of several hundred, and, in the case of quartz, the value can be 10⁶. The result is a piezoelectric coefficient a factor Q higher than for a static electric field. The very large Q of quartz is exploited in electronic oscillator circuits to make remarkably accurate timepieces. The mechanical vibrations that can be induced in a crystal by the converse piezoelectric effect are also used to generate ultrasound. The reflected sound is detectable by the direct effect. Such effects form the basis of ultrasound systems used to fathom the depths of lakes and waterways and to locate fish. Ultrasound has found application in medical imaging (e.g., fetal monitoring and the detection of tumours). The use of ultrasound makes it possible to produce detailed pictures of organs and other internal structures because of the variation in the reflection of sound from various body tissues. Thin films of polymeric plastic with a piezoelectric coefficient of about 10⁻¹¹ metres per volt are being developed and have numerous potential applications as pressure transducers.

I. In paragraphs 1 – 5 find the terms that correspond to these definitions:

Paragraph 1

16. Existing or intended to exist for an indefinite period - ________.

17. The condition of having or giving polarity - _________.

18. The difference in electric potential between two points in an electric field - ___________.

19. The product or the quotient of the fundamental physical quantities (such as mass, length, or time) - ________.

Paragraph 2

20. To manifest; display; show - ________.

21. A mathematical statement that two expressions are equal - ________.

Paragraph 3

22. Capable of returning to its original shape after compression, expansion, stretching, or other deformation - ________.

23. Secondary - ____________.

24. A set of components, functions of the coordinates of any point in space, that transform linearly between coordinate systems - __________.

Paragraph 4

• An array of objects or points in a periodic pattern in two or three dimensions - _______.

• To catch fire or set fire to; burn or cause to burn - ________.

• An appliance used for heating rooms - _________.

Paragraph 5

• Very small - _________.

• Any device, such as a microphone or electric motor, that converts one form of energy into another - ____________.

II. Decide whether the statements are true or false.

1. Piezoelectricity occurs due to the permanent polarization of some solids.

2. The words “stress” and “strain” can be considered as synonyms.

3. Displacement in the crystal lattice can produce piezoelectricity.

4. The electric fields generated in a laboratory cannot be compared to those existing in matter.

5. The converse piezoelectric effect can produce mechanical vibrations.

III. Speaking

1. What other electric properties of matter are familiar to you? Use any extra sources you can.

2. What other applications of piezoelectricity can you mention? Use any extra sources you can.

3. Give a presentation of piezoelectricity for a popular scientific radio programme.

4. Write a small abstract for the article.

IV. Role-Play

-You’re a tutor giving a seminar on the electric properties of matter. Make a list of questions you would like to ask your students. Role-play the seminar. Don’t forget about the introductory speech and the encouraging comments you need to make to involve your students into a conversation.