English for students of Physics_Unit 14

144
Unit Fourteen

ELECTRIC CHARGE
READING PASSAGE
Electric charge and a measure for the quantity of charge
Any household electrical appliance – be it a light bulb, a motor, or a television set – has
two contacts which have to be plugged in to get the device to operate.
This common characteristic, that an electrical apparatus must have two wires connecting
it to a source of electricity, gave the rise in the eighteenth century to the idea that when an
electrical device is working, something is moving through it. That something is called electric
charge. When you pull out a plug, turn off a switch, or disconnect a battery, the flow of
electric charge stops and with it the operation of the apparatus.
The idea of a flowing electric charge is quite attractive, because it permits us to draw in
our minds a mental picture which may eventually lead to a useful model. To develop the
intuitive idea of a flowing charge into a model, we must find a way to measure the quantity of
electric charge that flows through a bulb, a motor, or any other device. We have to look for an
effect produced by moving charge which can be measured quantitatively. You have used such
indirect methods many times before, probably without noticing it. For example, you can not
see temperature directly. To measure temperature, we use the fact that substances expand
when heated, and we can construct various kinds of thermometers using thermal expansion.
We shall use a similar method to build a charge meter.
In the experiment on the decomposition of water, the longer the time the electrodes are
connected to the battery, the greater the volume of both gases produced. This suggests that
more charge must have flowed through the apparatus when it was connected for a longer time.
Thus it seems reasonable to use the quantity of either gas produced in the reaction as a
measure of the quantity of electric charge that passes through the water. We shall choose the
quantity of hydrogen, since we get twice as much of this gas as we do of oxygen, and this
makes it easier to detect small quantities of charge. This apparatus, which we shall use as a
charge meter, we shall refer to as a ‘hydrogen cell’. Notice that since we shall not be

measuring the amount of oxygen, we have made no provision to collect it.
A source of electricity, such as a battery or a wall outlet, and one or more electrical
devices connected to the source make up what is called an electric circuit. If we want to know
how much charge flows through a given part of an electric circuit, we break the circuit at that
place and insert the hydrogen cell. The amount of hydrogen collected tells us how much
charge passed through the cell.

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You will recall that volume is not a reliable measure of the quantity of matter,
particularly in the case of a gas, since a gas expands and contracts appreciably as the pressure
and the temperature change. Thus, to be accurate, we should measure the quantity of electric
charge in terms of the mass, rather than the volume, of hydrogen collected in the test tube. But
we can be quite sure that the temperature and pressure of the hydrogen are nearly the same all
over the classroom for a short time. Therefore, as long as we are interested only in comparing
quantities of charge measured almost at the same time, we can be satisfied with simply
comparing the volumes of hydrogen collected in the test tube of different hydrogen cells. We
can choose any convenient volume of hydrogen in a test tube as our unit of electric charge.
We shall use our unit the charge needed to produce 1.0 cm
3
of hydrogen.
(
From
Uri Haber-Schaim. et al;
Introductory Physical Science
;

Prentice Hall, Inc;

Englewood Cliffs, New Jersey 07632; 1987).
READING COMPREHENSION

Exercise 1:
Answer the following questions by referring to the reading passag
e
1.
How do the electrical appliances operate?
.........................................................................................................................................
.........................................................................................................................
2.
How was the idea of electric charge provoked?
.........................................................................................................................................
.........................................................................................................................
3.
What do you think is the function of a switch?
.........................................................................................................................................
......................................................................................................................
4.
Is electric charge measured directly or indirectly? Explain.
.........................................................................................................................................
.........................................................................................................................
5.
What name is given for the device to measure charge?
.........................................................................................................................................
.........................................................................................................................
6.
What is the relationship between the volume of the gases produced and the time taken
for the production?
.........................................................................................................................................
.........................................................................................................................
7.
Why should we take use of the quantity of hydrogen to measure charge?

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.........................................................................................................................................
.........................................................................................................................
8.
What is an electric circuit?
.........................................................................................................................................
.........................................................................................................................
Exercise 2:
Find the words/phrases in the text which have similar meaning or implication to
the followings

1.
domestic .................................
2.
popular .................................
3.
a piece of equipment .................................
4.
give birth to .................................
5.
cell .................................
6.
performance .................................
7.
sensible .................................
8.
proper/appropriate .................................
9.
truthful .................................

Exercise 3:
Finish each of the following sentences by circling the best choice or by phrases
from the reading passage.

1.

To find a method of measurement of charge,
a.

We should look for an effect produced by moving charge which can be measured
quantitatively.
b.

We should use an indirect method.
c.

Either way of the above expressions is correct
2.

The direct proportion between the time the electrodes are connected to an electric
source and the volume of oxygen and hydrogen produced in the experiment of
decomposition of water results in
....................................………….....................................................................................
.........................................................................................................................................
.................................................................................................................
3.

An electric circuit consists of
.................................................................................................................................
....................................................................................…………………………....

4.

The quantity of charge flowing through a given part of an electric circuit is measured
with .......................................................................………………………………….....

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.................................................................................................................................
5.

Though volume is unreliable measure of the quantity of matter, hydrogen volume is
still used to measure quantities of electric charge measured almost at the same time
because
............................................................………………………………………….....
...........................................................................................................................................
.........................................................................................................................
GRAMMAR IN USE
A review of prepositions
As a review on prepositions, the following just gives a summary on what types of
prepositions there are, basing on the function of each.
First we should go through briefly about prepositions in general
1. A preposition usually comes before a noun phrase, sometimes an adverb
Example:

in
our minds
into
a model
through
a bulb

at
once
up to
now
through
there
2. Prepositions and their object to form a prepositional phrase functioning as an
adverbial
Example:
1.
We all live on terra firma, the 29 percent of our planet’s solid crust that lies
above
sea
level
.
2.
Rubber bands, books, and the clothes you wear- these flexible materials maintain
their shape
to
some degree
.
3.
We’ve seen that at the atomic level, the atoms or molecules bonded together in a solid
stay in place
with respect to

their neigh
bors.
3. Some prepositions can also be adverbs, many forming phrasal verbs
Example:

1.
Whether a solid is crystalline or amorphous
depends
on
how it is formed.
2.
Especially, slow cooling can sometimes
results
in
very large crystals.
4. Some prepositions of time can also be conjunctions
Example:
1. After
preparing carefully, he successfully detected the questionable element in the
compound.

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2.
He had made lots of observations
before
he made such a report.
5. Types of prepositions
5.1.

Prepositions of place:
In/inside on/ on top of under(neath) above/over
under/below up/down through off out of
at next to/by/beside close to/near in front of
behind between opposite among round
beyond against

Example:
To understand interference, we must go
beyond
the restrictions of geometrical optics and
employ the full power of wave optics.

5.2.

Prepositions of direction/movement:

onto into to away from from along past towards
across around through off
Example:
Toss your keys along the floor, they’ll skitter
along
for a bit as friction does negative
work on them, reducing their kinetic.

5.3.

Prepositions of time:
at on in during over since/for
till/until before/ after by from between
Example:
But we can be quite sure that the temperature and pressure of the hydrogen are nearly the
same all over the classroom
for
a short time.
5.4.

Prepositions with other meanings
about according to against as/as for
for by instead of because of with respect to
on on behalf of up to with of
Example:
We can choose any convenient volume of hydrogen in a test tube
as
our unit of electric
charge.
5.5.

Idiomatic phrases with prepositions

149
at top speed from……..point of view by mistake
in advance out of order at risk on average
Example:
The equipment is
out of order
now, you should call for an engineer.
PRACTICE
Exercise 1:
Fill in the blank with suitable prepositions
1.
You will be familiar (1).............the idea that, when you use a power supply or other
source (2)............e.m.f., you can not assume that it is providing you (3)............ the
exact voltage that its controls suggest. You need to measure the voltage to be sure
(4).............its value. There are two reasons (5)............this. First, the supply may not
be made (6)............a high degree of precision, batteries become flat, and so on.
However, there is a second, more important, reason for measuring the voltage

(7)................the supply to be sure of its value. Experiments show that the supply
voltage depends (8)............the circuit of which it is part. (9)..............particular, the
voltage of a supply decreases if it is required to supply more current.
2.
Matter is made up (1).............three types of particles: electrons (which have negative
charge), protons (positive) and neutrons (neutral). An uncharged object has equal
numbers (2).............protons and electrons, whose charges therefore cancel out.
When one material is rubbed (3)..............another, there is a force of friction
(4).............them, and electrons may be rubbed off one material (5).............the other.
The material that has gained electrons is now negatively charged, and the other
material is positively charged.
If a positively charged object is brought close (6)..............an uncharged one, the electrons
(7)..............the second object may be attracted; we observed this (8)………..a force of
attraction between two objects. (This is electrostatic induction)
Note that it is usually electrons that are involved (9).............. moving within a
material, or (10)............one material to another. This is because electrons, which are
(11)............the outside of atoms, are less strongly held within a material than protons;
they may be free to move about within a material (like the conduction electrons in a
metal), or they may be relatively weakly bound within atoms.
Exercise 2:
Complete each of the following statements with suitable phrases from the list
given

A. between two parallel metal plates I. on each other
B. between them
C. by showing lines of forces J. spreads outwards in all directions

150
D. due to an excess of electrons. K. with the addition of a small amount
of energy

E. in moving from one plate to the other. M. to that point
F. In order to observe the field N. being pulled upwards
G. once summoned by friction O. to some degree
H. to leak away into the air P. in the process of doing work
1.
........................, we need to put something in it that will respond to the field
.
2.
If you rub a strip of plastic so that it becomes charged and then hold it close to your
hair, you feel your hair ………….............
3.
We can draw electric fields in much the same way that we can draw gravitational and
magnetic fields................................
4.
A radial field....................., for example from a point charge or from a charged sphere.
5.
You can set up a uniform field ........................... by connecting them to the terminals
of high- voltage power supply.
6.
Any two charges particles exert a force ........................that is proportional to each of
their charges and inversely proportional to the square of the distance between them.
7.
It is quite tricky to investigate the force between charged particles, because charge
tends ........................during the course of any experiment.
8.
........................., energy is transferred from you to the charge that you are pushing.
9.
The potential difference (voltage) between the plates tells us the energy charge per
coulomb.........................
10.

The electric potential at a point is equal to the work done per unit charge in moving a
positive charge from infinity..........................
11.
Long before the days of plastic pens and rubber balloons, investigators found that
charges, …........................., could be transferred from one material to another.
12.
To day, we know that any material can become charged ............................by bringing
it into contact with a different material.
13.
The negative charge on a piece of rubbed amber or rubber is...............................
14.
A semi-conductor is an electrical insulator that, ....................................., becomes a
conductor.
15.
A proton repels another proton and attracts any electron, though nothing tangible
bridges the distance....................................