Unit 10 self-induction

An emf can be induced by varying the number of magnetic lines threading through a circuit, the induced current always opposing the change that is occurring, no matter what causes the change of magnetic flux. It may be due to the motion of a magnet or to the change of current in a nearby electrical circuit as in the transformer. The change of magnetic flux may also be due to a change of current in the coil itself, this effect being known as self-induction.

Suppose several hundred feet of wire, in a single loop, to be connected in series with an incandescent lamp, a 115-volt direct-current source, and a switch. The switch being closed, the current in the circuit will increase, in a few millionths of a second, to a steady value determined by Ohm's law. Now let this wire be wound onto an iron rod to form a coil. When the switch is again closed, the current will increase to the same final value as before, but the time required will be several hundredths of a second. In the coil there are hundreds of turns of wire, side by side. The current in each turn causes magnetic lines that thread through the other turns. An increase of current in any loop varies the flux through all the others, the change of flux of magnetic lines generating an emf. This induced emf opposes the change of current.

Self-induction is known to oppose not only the increase of current in a coil but the decrease also. The circuit being opened, the current will not stop instantly. The forward induced emf will cause a spark to appear at the switch.

In order to demonstrate self-induction, connect a large electromagnet and an incandescent lamp, in parallel with each other, through a resistor to a direct-current source. When you close the switch, at first the increasing current through the coils of the electromagnet increases the flux, thus generating an opposing emf. Self-induction impedes the current through the coil. Most of the current flows through the lamp, which glows brightly. The current having become constant, most of the current flows through the coils, and the lamp becomes dim. The switch being opened, the flux through the coils will decrease rapidly. The induced emf will make the lamp glow brightly for an instant.

EXERCISES I. Find in the text Infinitives, Gerunds and Participles, state their forms and functions and analyse them.

2. Give synonyms for the following words:

motion, to oppose, to occur, to be due to, to call, several, to connect, steady, to determine, to require, value, to vary, to decrease, to stop, constant, rapidly.

2. Find in the text antonyms for:

never, increase, to open, to start, different, backward, to separate, slowly, bright, before, unparallel

2. State the kind of the subordinate clauses and translate them properly:

1. The relative motion of the coil and magnet is what produces the current. 2. What Ohm discovered was that the ratio of the potential difference between the ends of a metallic conductor and the current flowing through the metallic conductor is a constant. 3. When the tem­perature is reduced, scattering by the impurities is predominant. 4. As the temperature of the diode is increased, the lattice atoms are in more rapid motion. 5. Once a molecule has formed, it will move about and behave as a unit particle under various physical conditions. 6. To avoid any trouble the operator should always check up whether the devices are in order. 7. That the coefficients are positive follows from the problem statement. 8. It is very important that the programmer should understand some of the control logic on the interface card. 9. The phenomenon we are going to observe is of great practical importance. 10. The operator could not state the exact moment that phenomenon occurred.

2. Answer the following questions:

1. How can an e. m. f. be induced? 2. What does the induced current always oppose? 3. What is the change of magnetic flux due to? 4. What do we call self-induction? 6. When will the current in the circuite increase to a steady value determined by Ohm's law? 6. How many turns are thrtr in a coil? 7. What does the current in each turn cause?8. What generates an emf? 9. What does self-induction oppose? 10. How is the opposing emf generated? 11. When does the lamp glow brightly? 12. When does the lamp become dim? 13. What happens when the switch is open? 14. What will make the lamp glow brightly for an instant?

2. Be ready to speak about self-induction making use of your answers to the questions.

VILTranslate into English the summary of the text:

Е. р. с. може бути наведена шляхом зміни числа магнітних силових ліній, що проходять через ланцюг. Наведений струм завжди протидіє змінам, що відбуваються. Показано, як наводиться е.р. с. і як вона протидіє зміні струму, як його збільшенню, так і зменшенню. Пояснюється, як відбувається (виникає) самоіндукція.

4. Supplementary reading.