Question 1

List the name of 6 methods of generating an emf and the level of voltage you would expect from each. Is each voltage AC, DC or both.

NOTE: These are six methods of generating an EMF, there may be others.

Method Mechanism Range of EMF
piezoelectric Applied physical force causing physical deformation of a crystal with piezoelectric properties. The physical deformation generates a voltage. Examples include quartz (silicon dioxide) and PZT (lead zirconate titanate).
 There is no lower limit on EMF (there are piezoelectric microphones), but the EMF generated depends on the force and speed that the crystal is moved. A barbecue igniter has a typical EMF of a few thousand volts.
photoelectric
 Electromagnetic radiation (e.g. light) strikes a material that uses the energy in light to generate an EMF.
 Usually a few volts.
friction Friction between two substances can cause a separation of charge. This is called the triboelectric effect. In metals, the charge difference rapidly dissipates. In insulators, the charge gets "trapped", and results in an EMF buildup.
 Any voltage, becomes noticeable from a few thousand volts upwards.
thermocouple When two dissimilar metals are connected together at one end (the measurement junction), and exposed to a temperature gradient, an EMF results. The EMF corresponds depends on the metals used and the temperature difference between the measurement junction and the reference junction.
 About 10-100 mV. It depends on the metals used and the temperature difference between the measurement junction and the reference junction.
chemical A chemical reaction is used to generate electricity. Such a device is called a 'cell'. One or more cells with terminals for connection to external loads is referred to as a 'battery'.
 The EMF depends on the components of the cell. A single cell usually generates less than 5 V. Cells may be connected in series for higher voltages.
electromagnetic induction
 Subjecting a conductor to a changing magnetic field produces an electromotive force. The changing magnetic field can come from a coil subject to a magnetic field varying in strength, or a coil that moves through a magnetic field, even if the magnetic field is constant strength.
 There is no lower limit on EMF. In the electrical grid, generators operate at about 11 kV.


Question 2

Give an example of an application each method could be used for.

Method Example
piezoelectric Barbecue igniter.
photoelectric
 Photovoltaic panel (solar panel).
friction The triboelectric effect is more often obtained by deliberately applying charges to insulators via high voltages.

Static electricity is used to apply a charge to paint droplets. This causes the droplets to adhere more uniformly to an oppositely charged object.

Static electricity is also used in laser printers. A static charge in the same pattern is used to adhere toner powder to the paper before the toner is fixed to the paper by melting by a high temperature roller.
thermocouple Thermocouples are widely used for temperature measurement.

Large arrays of thermocouples can be used to generate power. The heat source is a radioactive element. Such "radioisotope thermal generators" are used to power items like space probes. The power lasts as long as there is sufficient heat from the radioactive decay, and the thermocouples are in good condition.
chemical Batteries.
electromagnetic induction
 Induction generators. All AC generators that use magnetism use electromagnetic induction.


Question 3

Which system of emf production produces the most energy in NZ?

Electromagnetic induction. This is the operating mechanism of all the largest power plants in New Zealand.


Question 4

Which method uses gold in the production device?

NOTE: This question is quite subjective. The use of gold is dependent on the design of the device itself, rather than its mechanism.

Notwithstanding the note above, the photocell as described in the notes uses gold.


Question 5

What is the energy origin of the electric flow in a photocell?

The energy origin is electromagnetic radiation as sunlight.


Question 6

Why do you think static is an issue when servicing electronic equipment?


Static electricity can involve very high voltages, which can easily damage some types of semiconductor devices.


Question 7

Which method of emf production produces the smallest emf?

NOTE: This question is quite subjective. For example, there is no lower limit on the EMF generated using electromagnetic induction. The scope of this question will be limited to DC sources.

Of the DC sources, the thermocouple has the smallest EMF.


Question 8

What does \( E = B l v \) mean?


This equation describes the voltage generated by electromagnetic induction, such a rotary alternator.

\( E \) is the EMF generated. Its unit is volt (V).

\( B \) is the magnetic field strength. Its unit is tesla (T).

\( l \) is the length of conductor at right angles to the magnetic field. Its unit is metres (m).

\( v \) is the speed of the of conductor at right angles to the magnetic field. Its unit is metres per second (m.s-1).


Question 9

Overleaf construct a sketch of each method labelling the main features and a brief description of how it works.


The intention of this question is to reproduce the sketches in the book.


Last modified: Thursday, 28 May 2020, 12:12 PM