Lesson 10.
Topic “Electromotive force and resistance”.
Grammar material: Articles.
Text : “Electromotive force and resistance”.
As was previously stated, there is always a disorderly movement of free electrons
within all substances, especially metals.
Let us assume that there is a movement of electrons through the wire, say from point A
to point B. What does it mean? It means that there is an excess of electrons at point A.
Unless there were a flow of electric current between A and B in any direction, it would
mean that both the former and the latter were at the same potential. Of course the greater
the potential difference, the greater is the electron flow.
The electromotive force (e.m.f.) is the very force that moves the electrons from one
point in an electric circuit towards another. In case this e.m.f. is direct, the current is direct.
On the other hand, were the electromotive force alternating, the current would be
alternating, too. The e.m.f. is measurable and it is the volt that is the unit used for
measuring it.
One need not explain to the reader that a current is unable to flow in a circuit
consisting of metallic wires alone. A source of an e.m.f. should be provided as well. The
source under consideration may be a cell or a battery a generator, a thermocouple or a
photocell, etc.
In addition to the electromotive force and the potential difference reference should be
made here to another important factor that greatly influences electrical flow, namely,
resistance. So, to resistance shall we turn our attention now. The student probably
remembers that all substances offer a certain amount of opposition, that is to say
resistance, to the passage of current. This resistance may be high or low depending on the
type of circuit and the materials employed. Take glass and rubber as an example. They
offer a very high resistance and, hence, they are considered as good insulators.
Nevertheless, one must not forget that all substances do allow the passage of some current
provided the potential difference is high enough.
Imagine two oppositely charged baits suspended far apart in the air. In spite of our
having a difference of potential, no current flows. How can we explain this strange
behaviour? The simple reason is that the air between the balls offers too great a resistance
to current flow However, the electrons could certainly flow from the negatively charged
ball towards the positively charged one provided we connected them by a metal wire. As a
matter of fact, it is not necessary at all to connect both balls in the manner described in
order to obtain a similar result. All that we have to do is to increase the charges. If the
potential difference becomes great enough, the electrons will jump through the air forming
an electric spark.
One should mention in this connection that certain factors can greatly influence the
resistance of an electric circuit. Among them we find the size of the wire, its length, and
type. In short, the thinner or longer the wire, the greater is the resistance offered. Besides,
could we use a silver wire, it would offer less resistance than an iron one,
TEST. ELECTROMOTIVE FORCE AND RESISTANCE
I. Connect the word combinations:
Potential
factor
Electromotive
resistance
Direct
behavior
Important
difference
High
charged
Good
force
Strange
insulator
Positively
difference
II. Open the brackets , use the correct form of the verbs in conditional
sentences:
1 If I live in the South, I (to bathe) every day.
2.If she finds my book, she (to give ) it to me.
3.If the weather is fine, we ( to play) outside.
4.If you rang me up, I (to tell) you something.
5.If I had seen her, I (to be) glad.
6. If you had been Free last weekend, I ( to come) to see you.
III. Find the end of the sentences:
1.The greater the potential difference…
1. the very force that moves the
2.The e.m.f. is…
electrons from one point in an electric
3.One must not forget that all substances…
circuit towards another.
4. It was in 1800 that…
2. .the greater is the electron flow.
5.If coal were not used as fuel …
3.we should get more valuable products.
6.It was Lomonosov…
4.who was the inventor of the radio.
7.It was Popov…
5.do allow the passage of some current
provided the potential difference is
high enough.
6. Volta first produced a continuous
current.
7.who stated that heat phenomena were
due to molecular motion.
IV. Translate the following phrases:
1. On the contrary_
2. To meet requirements_
3. Electrical engineering_
4. As a result_
5. Under such conditions_
V. Fill in the blanks with the words and phrases:
______________________________________________________________________
Excess, in case, influents, suspended, offer some resistance, unless, provided, potential
difference.
______________________________________________________________________
1. The current will not flow … there is a potential difference in the circuit.
2. How can we use the … of generated electrical energy?
3. All substances … to the passing current.
4. The current flows ... the circuit is closed.
5. The electrical current flows if there is a … .
6. Insulator strings are … from the towers of transmission lines.
7. The temperature … the conductance of metals.
8. The resistance in electrolytes decreases … their temperature increases.
Grammar.
I.Decide whether to use the definte article >the< or not. If you do not need the article
>the<, use x.
1) My grandmother likes … flowers very much.
2) I love … flowers in your garden.
3) See you on … Wednesday.
4) I always listen to … radio in the morning.
5) Alex goes to work by … bus.
6) Don't be late for … school.
7) Listen! Dennis is playing … trumpet.
8) We often see our cousins over … Easter.
9) She has never been to …Alps before.
10) What about going to Australia in … February?
II. Decide whether to use the definte article >the< or not. If you do not need the
article >the<, use x.
1. Last year we visited … St. Paul's Cathedral and … Tower.
2) … Mount Everest is … highest mountain on earth.
3) … Loch Ness is … most famous lake in Scotland.
4) … most children like … sweets.
5) … summer of 1996 was hot and dry.
6) … Plaza Hotel is on the corner of … 59th Street and … 5th Avenue.
7) My sister often stays at …Uncle Tim's in Detroit.
8) Our friends … Millers moved to Florida last August.
9) … smog is a problem in … big cities.
10) Our children go to … school by … bus.
III. Use >a< or >an<. Write the correct forms of the indefinte articles into the gaps.
Example: We read __ book.
Answer: We read a book.
1) Lucy has … dog.
2) Let's sing … song.
3) Emily needs … new desk in her room.
4) I need … blue pen.
5) Ben has … old bike.
6) Peter has … aunt in Berlin.
7) We listen to … English CD.
8) She has … exercise book in her school bag.
9) The speed of this car was 160 miles … hour.
10) They finished … unit.