Unit 7. Induction. Types of Magnets. Active vocabulary

1. (To) align [ə'laɪn] вирівнювати

2. Alignment [ə'laɪnmənt] вирівнювання, регулювання

3. Curie temperature ['kjuərɪ] температура (крапка) Кюри

4. Diamagnetic [ˌdaɪəmæg'netɪk] діамагнітний, діамагнетик

5. Diamagnetism [ˌdaɪə'mægnɪˌtɪzm] діамагнетизм

6. Domain [dəu'meɪn] домен

7. Electromagnetic induction електромагнітна індукція

[ɪˌlektrəmæg'netɪk ɪn'dʌkʃən]

8. Ferromagnetic [ˌferəumæg'netɪk] феромагнітний, феромагнетик

9. Ferromagnetism [ˌferəu'mægnɪˌtɪzm] феромагнетизм

10. Induce [ɪn'djuːs] індукувати

11. Magnetic moment [mæg'netɪk'məumənt] магнітний момент

12. Meissner effect ['maɪsnə ɪ'fekt] ефект Мейснера

13. Paramagnetic [ˌpærəmæg'netɪk] парамагнітний, парамагнетик

14. Paramagnetism [ˌpærə'mægnɪˌtɪzm] парамагнетизм

15. Retentivity [ˌriːtenˈtɪvɪtɪ] залишкове намагнічування,

залишкова магнітна індукція

16. Self-induction [ˌselfɪn'dʌkʃ(ə)n] самоіндукція

17. Susceptibility [səˌseptə'bɪlɪtɪ] сприйнятливість, чутливість

Pre-reading task

Exercise 89. Mankind uses the earth magnetic fields and forces to create tools and instruments that are useful in many different ways. Thus magnets have many uses. Work in pairs and make a list of items where magnets are used in everyday life. Compare your list with other groups’ lists.

Exercise 90. Read and translate the text and find the answers for the questions below the text.

Electromagnetic induction

Electromagnetic induction is the production of voltage across a conductor moving through a magnetic field. It underlies the operation of generators, all electric motors, transformers, induction motors, synchronous motors, solenoids, and most other electrical machines.

W

Michael Faraday

British Physicist and Chemist (1791–1867)

Faraday is often regarded as the greatest experimental scientist of the 1800s. His many contributions to the study of electricity include the invention of the electric motor, electric generator, and transformer, as well as the discovery of electromagnetic induction and the laws of electrolysis. Greatly influenced by religion, he refused to work on the development of poison gas for the British military.

hile Oersted's surprising discovery of electromagnetism paved the way for more practical applications of electricity, it was Michael Faraday who gave us the key to the practical generation of electricity: electromagnetic induction. Faraday discovered that a voltage would be generated across a length of wire if that wire was exposed to a perpendicular magnetic field flux of changing intensity.

An easy way to create a magnetic field of changing intensity is to move a permanent magnet next to a wire or coil of wire. The magnetic field must increase or decrease in intensity perpendicular to the wire, or else no voltage will be induced.

Michael Faraday stated that electromotive force (EMF) produced around a closed path is proportional to the rate of change of the magnetic flux through any surface bounded by that path. In practice, this means that an electric current will be induced in any closed circuit when the magnetic flux through a surface bounded by the conductor changes. This applies whether the field itself changes in strength or the conductor is moved through it.

In mathematical form, Faraday's law states that . For the special case of a coil of wire, composed of N loops with the same area, the equation becomes , where ɛ is the electromotive force, is the magnetic flux, "d" representing rate of change of flux over time, N stands for the number of turns, or wraps, in the wire coil (assuming that the wire is formed in the shape of a coil for maximum electromagnetic efficiency).

A sequence of Faraday's Law, together with Ampère's law and Ohm's law is Lenz's law. So, there are three laws of electromagnetic induction:

Faraday’s first law states that the voltage appears between the ends of a conductor when there is a change in the magnetic field around it. (In other words, any magnetic field that changes over time will produce an electric field in the space around it). It explains what electromagnetic induction is.

Faraday’s second law states that the induced electromotive force in a wire loop is proportional to the rate of change of magnetic flux through the loop.

Lenz’s law states that the direction of induced current is always such as to oppose the cause which produces it.

Thus, Faraday’s second law gives the size of the effect and Lenz’s law gives its direction. This phenomenon is put into obvious practical use in the construction of electrical generators, which use mechanical power to move a magnetic field past coils of wire to generate voltage.

If we recall that the magnetic field produced by a current-carrying wire was always perpendicular to that wire, and that the flux intensity of that magnetic field varied with the amount of current through it, we can see that a wire is capable of inducing a voltage along its own length simply due to a change in current through it. This effect is called self-induction: a changing magnetic field produced by changes in current through a wire inducing voltage along the length of that same wire. If the magnetic field flux is enhanced by bending the wire into the shape of a coil, and/or wrapping that coil around a material of high permeability, this effect of self-induced voltage will be more intense. A device constructed to take advantage of this effect is called an inductor (www.allaboutcircuits.com).

1. What is electromagnetic induction?

2. Who discovered electromagnetic induction?

3. What did he discover about the voltage?

4. What is an easy way to create a magnetic field of changing intensity?

5. What do Faraday’s laws state?

6. What does Lenz’s law state?

7. What is self-induction?

Exercise 91. Give the corresponding terms for the following definitions.

1. _______________________ – a magnet that retains its magnetic properties in the absence of an inducing field or current.

2. _______________________ – a component in an electric or electronic circuit that possesses inductance.

3. ________________________ – a difference in potential that tends to give rise to an electric current.

4. _______________________ – a cylindrical coil of wire acting as a magnet when carrying electric current.

5._________________________– the production of an electromotive force either by motion of a conductor through a magnetic field so as to cut across the magnetic flux or by a change in the magnetic flux that threads a conductor.

6. ________________________ – a device that converts mechanical energy to electrical energy, generally using electromagnetic induction.

7. ______________________ is a type of alternating current motor where power is supplied to the rotor by means of electromagnetic induction (also called asynchronous motor).

Exercise 92. Circle the correct answer.

1. According to Lenz's law, the resulting force produced by an induced current __________.

a) opposes the current c) contributes to the motion of the wire

b) reduces the resistance d) opposes the motion of the wire 2. __________ is the generation of current due to relative motion between a wire and a magnetic field.

a) Electromagnetic induction c) Electrolysis

b) Magnetic flux d) Electrospectrometry

3. A wire is moving toward you through a magnetic field directed to your right. In what direction is the force?

a) downward c) toward you

b) to the right d) upward

4. A transformer is a device to increase or decrease AC _________.

a) power c) heat

b) resistance d) voltage

5. What is the unit for EMF?

a) N b) C/J c) V d) N • m

Reading and Writing

Exercise 93. Read, translate the text and find the terms which correspond with these descriptions.

1. ________________ - the property that certain substances have of being weakly repelled by both poles of a magnet.

2. ________________ - the ratio of electric or magnetic polarization in a material to the strength of the field producing that polarization.

3. ________________ - a region in a ferromagnetic material within which the atoms are magnetically aligned: alignment of these regions results in the material being magnetized.

4. ________________ - the temperature at which the magnetic properties of a substance change from ferromagnetic to paramagnetic, usually lower than the substance's melting point.

5. ________________ - the property of a magnet that interacts with an applied field to give a mechanical moment.

6. ________________ - material, as aluminum or platinum, having a magnetic permeability slightly greater than unity and varying to only a small extent with the magnetizing force.

7. ________________ - material, as iron, nickel, or cobalt, having a high magnetic permeability which varies with the magnetizing force.

8. ________________ - the principle that the magnetic susceptibility of a paramagnetic substance is inversely proportional to its thermodynamic temperature.

9. ________________ - the expulsion of magnetic flux when a material becomes superconducting in a magnetic field.

10. ________________ - any material that exhibits superconductivity.