BOSTES HSC Physics 2015 Question 35

Question: 
Students are expected to assess the impact on society in the use of transducers. The answer should include one input transducer and one output transducer (excluding thermistors).

Marking Guidelines:

Criteria	Marks
Provide applications of transducers (excluding thermistors). Assess the impact on society in the use of these transducers. The answer includes the application of an input transducer and an output transducer.	6

(Source: https://www.boardofstudies.nsw.edu.au/hsc_exams/2015/guides/2015-hsc-mg-physics.pdf)

Comments:

According to Usher (1985), the term ‘sensor’ is widely used in the United States, whereas ‘transducer’ is more commonly used in Europe. Furthermore, sensor is derived from the Greek word sentire which means “to perceive” and transducer is originated from the Greek word trans-ducere which means “to lead across.” Currently, a transducer is sometimes defined as a device that transforms electrical energy into non-electrical energy, or vice versa. Alternatively, the American National Standards Institute (ANSI) standard MC6.1 defines a transducer as “a device which provides a usable output in response to a specific measurand (Instrument Society of America, 1975).” In this definition, an output is an electrical quantity, and a measurand is a physical quantity which is measured. However, ANSI’s definition of transducer is not widely adopted. Essentially, the transducer may be considered to be a sensor that converts energy from one form to another.

In this question, students are expected to discuss the impacts of an input transducer and an output transducer on society. However, the types of transducers can be classified as active and passive. Furthermore, we can distinguish the types of transducers according to the quantity that is measured: temperature transducers (e.g. a thermocouple), pressure transducers (e.g. a diaphragm), displacement transducers (e.g. linear variable differential transformer), and flow transducers. More importantly, with the use of a control system, the input transducer can convert a measurable quantity (temperature, pressure, displacement, flow rate) into an electrical quantity (voltage, current, resistance, capacitance) that can be processed by an electronic instrument.

In the marking guidelines, possible answers include solar cells (input transducer) and current meters (output transducer). For solar cells, one may state that there is a conversion of light energy to electrical energy. The impacts of solar cells to society include the reduction of global warming and the improvement in the quality of life by gaining access to communications technologies. For current meters, we can use them to detect electrical energy in a circuit and it can be viewed by connecting to a control system. The impacts of current meters to society include an increase in safety and efficient control of systems. However, students should understand and explain the principles of operations in the use of these transducers.

Feynman’s insights or goofs?:

Feynman has a good explanation on the operation of photoconductive cells (input transducer). He explains that “photons of light (or x-rays) can be absorbed and create a pair if the photon energy is above the energy of the gap. The rate at which pairs are produced is proportional to the light intensity. If two electrodes are plated on a wafer of the crystal and a ‘bias’ voltage is applied, the electrons and holes will be drawn to the electrodes. The circuit current will be proportional to the intensity of the light. This mechanism is responsible for the phenomenon of photoconductivity and the operation of photoconductive cells (Feynman et al., 1966, section 14–1 Electrons and holes in semiconductors).” In short, the operation of solar cells is based on the photoelectric effect. Nevertheless, photoelectric devices can be classified as photo-emissive cells, photovoltaic cells, and photoconductive cells, which may be confusing to introductory students.

Interestingly, one may quibble whether the operation of solar cells is related to photoelectric effect or photovoltaic effect. Currently, the term photoelectric effect is commonly used when the electron is ejected out of the metal into a vacuum, whereas photovoltaic effect is sometimes used when the electron is still contained within the photoelectric or photovoltaic devices. Specifically, one may define the photoelectric effect as the emission of electrons from a metal surface when light shines upon it. However, Feynman explains that photons of light can be absorbed and generate electron-hole pairs if the photon energy is greater than the “energy gap.” Furthermore, he adds that the electron-hole pairs will be drawn to the electrodes if there is a “bias” voltage. More importantly, it is possible that the operating conditions of photoelectric devices can be zero-bias, reverse bias, or high reverse bias.

Feynman also has an insightful explanation on the operation of current meters (output transducer). In his own words, “[t]he same idea can be used for making a sensitive instrument for electrical measurements. Thus the moment the force law was discovered the precision of electrical measurements was greatly increased. First, the torque of such a motor can be made much greater for a given current by making the current go around many turns instead of just one. Then the coil can be mounted so that it turns with very little torque—either by supporting its shaft on very delicate jewel bearings or by hanging the coil on a very fine wire or a quartz fiber. Then an exceedingly small current will make the coil turn, and for small angles the amount of rotation will be proportional to the current. The rotation can be measured by gluing a pointer to the coil or, for the most delicate instruments, by attaching a small mirror to the coil and looking at the shift of the image of a scale. Such instruments are called galvanometers. Voltmeters and ammeters work on the same principle. (Feynman et al., 1964, section 16–1 Motors and generators).” In this case, an important principle of operation is related to the magnetic force on current carrying wire.

To a certain extent, Feynman has sufficiently elaborated the principle of operation pertaining to current meters such as ammeters and voltmeters. Unfortunately, it could be unclear to students when he simply mentions that the discovery of the force law increases the precision of electrical measurements greatly. One may guess whether the force law refers to Lorentz’s force law (F = qE + Bqv), Coulomb’s law of magnetic force (F = kM₁M₂/r²), or Ampère’s force law (F = μ₀I₁I₂/2πr). As usual, Feynman would not care to name the force law (F = BIL or F = Bqv) involved in the magnetic force on current carrying wire as Ampère’s law or Laplace force. Similarly, he prefers to say “the law of inertia” instead of Newton’s first law of motion or Newton’s first law of dynamics. In essence, it is good to focus on the principle of operation; however, it can be confusing to students when the force law is not specified as F = BIL or F = Bqv, though it seems futile to debate whether it should be labeled as Ampère’s force law or Laplace force.

References: 

1. Feynman, R. P., Leighton, R. B., & Sands, M. (1964). The Feynman Lectures on Physics, Vol II: Mainly electromagnetism and matter. Reading, MA: Addison-Wesley.

2. Feynman, R. P., Leighton, R. B., & Sands, M. (1966). The Feynman lectures on physics Vol III: Quantum Mechanics. Reading, MA: Addison-Wesley.

3. Instrument Society of America (1975). Electrical Transducer Nomenclature and Terminology. ANSI Standard MC6.1. Research Triangle Park, North Carolina: Instrument Society of America.

4. Usher, M. J. (1985). Sensors and transducers. Macmillan: London.