1970-05-21 — Page 27

育教偶服,其三第張七第 日七十月四年戌庚夏 WAH KIU YAT PO

(搜第七張第二六)

報日橋

四期星

日一廿月五年〇七九一曆公年九十五國民華中

170英文中學會考試題預習專欄

堅道英文書院王編

物理科

( 二十九 )

(2-2)2. (500) (3600)

4.2

(2.2)(2,2)(12)(10*)

4.2

#152000.eal..

ELECTRICAL MEASUREMENTS

Measuring Instruments are:

Aureter

Voi tule t

amme ter..

voltmeter

ga ivanometer

It will be noted that the current 4 is divineq at A into two branches I, and f. Similarly at D, the branch currents I and I unite to give i. More- over, if the point C is at the same potential as point B, there will be no flow registered by the galvanometer G.

whereupon

and

9 x 152000

and also.

1188000 cal.

wheatstone Bridge Potentiometer.

has a low resistance;

should be placed in series with the resistance. in which the current is to be measured.

-uus a high resistance

placed in parallel with the resistance. between whose terminals the ».ù, is measured,

ovanerese the range of all ammeter,

Fistance is connected in parallel with the 'an 196

netei

To increase tie range of a voltueter, a suitable. resistance is connected in series with the volt-

meter.

alvanometer -- to measure await current

Valvanometer as an ammeter

If a galvanometer is placed in a circuit ita de- flection is determined by the current passing through its coil. The size of current passing determines the angle of deflection. if the currant

19 large which the galvanometer cannot register. it any be danged, fo fulfil this purpose, the instrument should be shunted by a resistor whi a known proportion of current passes.

*xample 29-5

andame ter has a resistance of 2.4 olius and gives full-acale deflection when the instrument current 18 0.02 amp. What shunt resistance will cause the instrument to deflect full scale when there are 100 amperes in the line?

whence Zero

Metre Bridge

an

if the galvanometer registers

R

Since only the ratio HR rather than the actual

are involved,

the resistance and R, can be replaced in the circuit by a single wire and the circuit becomes a meter-bridge circutti

The pointer Pis slide along the wire to locate the point C at which the galvanometer ú reads zero whence

Thus the resistance of an "unknown" resistor determined in terms of two meaaurable lengths and, and a laboratory standard resistor

Potentiometer

It is a device for comparing two potential differ ences. It consists of a uniform straight vire AB rests on a metre rule.. A working battery drives a steady current through AB,

The cells whose emf are to be compared are con nected in turn to the circuit. The polarities of these cells are in opposite to the working cells..

point where the sensitive galvanometer reads,

zero

ications.

Electric Bell He lay

Special attention should bef

given to the direction of windings and current flow

Field strength can be in- creased by:

(1) increasing current (2)

increasing the turni, of coil

(3) soft magnetic ma ter

Magnetre separator Telephone earniece

Motor Principle

Lines of force are concentric circles around a current-carrying conductor, a free magnetic poles tends to move around a current condeator. Convers ly, a movable current-carrying conductor directed; perpendicularly to a magnetic field is moved side-

Such a wire is Wise across the field as follows.

D) Motion

field

The direction of thrust can be forced downward. determined by the Left-lland Rule, which states that if the forefinger of the left ahad points in the i direction of the field, and the middle finger pointa at right angle to it and in the direction of the current (conventional current), then the thumb points in the direction of the motion when these fingers are mutually perpendicular.

ELECTROMAGNETIC INDUCTION

·PIECEromagnetic inquction

An ent is induced in a conductor whenever there is

chance in the magnetic flux linking the conductor,

Lanz's Law

The induced current Lows 10 such a direction as to oppose, by its electromagnetic action. the motion or effect by which it is produced.

Amount of Ind

The magnitude

portional to

the induced eat in A COIL 18 Dro

the time rate of change of flux. the number of turns in the coil and the nature of the circuit involved

The Trangformer

The transformer utilizes the principle of induces emf to step up or step down vary-ing voltages, "by winding two coils with different numbers of turns on the same core: the primary and the secondary coila or windings.

99.980

shunt

when the meter 18 shunted, it is simply a circuit with two resistors in parallel. 100 kaptres is to be split into two branches that are inversely

roportional to their resistances.

Besistance of the skuné. esistance of the.neter

02

100 0.02

0.000430 ima

99.98

The potential difference between A.

AC

k(length AC)

where k is a constant or the wire

The potential difference between AC' is.

k(length AC1Ÿ

and thus the ratio of the two p.d.s

AC..

AC!

Lengtır AC

length AC

ㄍㄨㄥ

A source of varying electromotive force impressed “ across the primary coil produces, in the primary circuit, a varying current which in turn produces a varying field that is containing largely in theo common core. The turns of the secondary coil loop this varying field and experience an induced emf proportional to its number of turns.

Valvanometer as a voltmeter

A galvanometer, as well as an azimeter, has low resistance, It cannot tolerate high voltage at the two terminals unless a large resistor is in series with it

Example 29-4

A voltmeter has a resistance of 4000, olims and reads 1 volt per scale division. How can the

range of the voltmeter be increased to read 10 volts per s

scale division?

Solution

R

The range of a voltmeter is increased by insert- ing a resistance it in series with the voltmeter. Since the range is increased from 1 to 10, 1/10 of the total "petiential drop must. be in the volts aster, and 9/10 in the series resistance R.

Since current in voltmeter is the same as the current in the series resistance B, the potential drop in each varies as the resistance of each,

P.d. in R

pid. in voltneter

o.y of the total. p.d.

D. of the Enfoil p,d.

4000

4000-

36,000 ohm

30 kilolms

Wheatstone" Bridge

It provides a method of measuring resistance, uti- lizing the concepts of potential drop and divided. currents, if a circuit is arranged as

MAGNETIC EFFECT OF CURRENT

currents, that 19 moving charges, in a wire produce a magnetic field, which is determined by the cur- rent strength (in field strength) and by the dir- ection of current flow (in sense).

Direction of the magnetic field is determined by the Xight-hand Rule which states

grasp the conductor with your right hand and stretch your thumb in the direction of the current, the other fingers noint in the direct- ion of the angnetic field.

pingnetic field produced

by a straight wire wi th current goiing into the Daer

represents cur

Electromagnet

Field produced by solenoid

represents current an

current out

Therefore

iņus voltages can be stepped up or down în direct proportion to the number of turns in the secondary as compared to the number of turns in the primary. Transformer cannot be used with steady D.C.. current unless it is supplemented by a mechanical breakers system.

Perfect or Indeal Transformer

By the Principle of the Conservation of Energy.

Power Input Power Outout

PP

If voltage is stepped up, the current 19 stepped. down.

Fractical Trangı ormer

In practice, Power Input is greater than Uutput

Loss of anergy is due to

heating effect in the windings

heating in the core

hysteresis loss of the 'core flux leakage

(5) eddy current loss

for power transmission, alt, voltages are stepped up and the current is limited to a small amount, The line-loss (I2R) is thus minimized to a much less account.