1973-04-17 — Page 21

育教僑華 頁一第張六第日五十月三年丑癸醫夏 WAH – KIU YAT: PO

उस

報日僑華

voltage. Thus winding consisting of a source, i.e. an a.c. input is called the primary coil, the

winding from which the output is

drawn is the secondary coil.

二期星

on the same sott iron core which is laminated to reduce the eddy current.

Es

secondary

soft iron

日七十月四年三七九一届公年二十六國民華中

the charge and fall to the funne

where they discharge. The charge

then flows along the wire to the gold leaf which then diverges.

charged ebonite plate

1973英文中學會考試題預習高橋

物理科(廿五)

Physics (25)

Electromagnetic induction

Faraday observed that when-

ever there was a relative motion between a magnetic field and a conductor connected to a galvano- meter, there was always a deflect- ion in the galvanometer. If the two were relatively at rest, the galvanometer did not deflect. Also the deflection was proport- ional to the relative speed be tween the magnetic field and conductor and to the direction of motion. Reversed the direction of motion would reverse the de- flection. Faraday explained his observations in terms of the magnetic flux lines linking with the conductor. The deflection of the galvanometer was attributed to an induced current which arises because of an emf induced in the

The deflection increased circuit. with speed because the rate of change of flux linking of the conductor changes with speed. The greater the speed, the greater is the rate of change of flux. linkage and therefore the greater is the induced emf. Faraday - summarized his explanations in a law known as the Faraday's law of electromagnetic induction. The law states that the induced emf is proportional to the rate of change of flux linkage with the coil. At about the same time as Faraday proposed his law on electromagnetic induction, a Russian scientist Lena proposed a law determining the direction of the induced emf. The law is known as the Lenz's law which says that the induced emf is always in such a direction as to oppose the change giving rise to it. Thus the magnitude and direct -ion of the induced emf 19 comp letely determined by the above 2 laws.

Fleming's right hand rule.

Consider the diagram shown. fig. 1.

ACEP

I soft non care

The coils are generally wound on soft iron cores. The basic trans- former equation is

where E is primary voltage and E

p is the secondary voltage. N. is the number of tums in the primary and N the number of tums in the

8 secondary. For an ideal transfor -er, the power input is equal to the power output. If I is the

Ip P primary and 1 is the secondary current then

12

If N

= E I

is stepped down. But Ip

S

the voltage

Igs 1.9 Conver

the

But

the current is stepped up. -sely, if N N

P voltage is stepped up. the current is stepped down.

Losses in a transformer.

(1) Copper loss: the resistance in the copper wire used in the wind- ings causes joule's heating effect. Hence some input electrical energy is dissipated.

(2) flux leakage: the imperfect coupling between the secondary and primary causes some of the magnet- 10 flux leaking from the coil. Hence not all of the magnetic energy in the primary is converted into electrical energy in the secondary.

(3) Core loss: the changing magnet. -ic field in the primary produces an induced emf in the iron core and an induced current is set up in the core. This current is called the e eddy current which has the effect of producing heat. i again it produces the joule's

To remedy this,

heating effect.sformer is

the core of a

laminated.

Ip

primary

The number of secondary windings is much greater than that of the primary winding.

When the primary circuit is closed, the flow of I

f I produces a magnetic field in the soft iron core, i.e. the latter becomes an electromagnet. The magnet attract -s the soft iron contact towards it, hence breaks the contact. The rise in the magnetic field when the primary is closed induces an emf in the secondary. A similar emf but in the opposite sense is induced in the secondary when the primary is opened, by the collapse of the magnetic field. The second

ary emf is in general of the order of thousands of volts.

When the contact is broke, a spark is often observed between the contact

This spark will assist the conduction of the primary current the collapse of the magnetic field will be slow and hence the magnit- ude of induced emf is small. To remedy this a condenser C is con- nected across the contact so that as a spark is formed, the charges produced are stored in the condens- er and do not exist between the contacts.

-S.

N

notion direction

coil stationary

Of

The field line pattem is as shown.

3. The needle is induced negative- ly. The sharp point of the needle has a very high negative charge density which will repel electrons from the surrounding molecules. The electrons are repelled to the The cap of the electroscope and hence neutralize the positive charge

The divergence there-. fore decreases.

-Ye changes

repelled towards

cáp

resence.

Solution to ex. 3

1. The circuit is as shown. fig 12.

300V.

JokЛ

www

30kn

The voltmeter is in parallel with the 60 K. resistance and the two are in series with the battery. The equivalent resistance is

S

or r

thin layers of soft vron

Velocity

direction

R 10K +

30 KIL

60K + 30 K

G

300

Current in the circ

30.K

10 mA.

If the rod AB is slided along the rails towards the inside of a horseshoe magnet, the deflection in the galvanometer shows that this would result in a current Flowing in the direction shown. The current is the induced current If the direction of motion is reversed, the deflection is rever sed and the induced current is also reversed. The direction of -induced current, the magnetic

field, and the direction of motion are related by the Fleming's right hand rule, which as given in fig 2 states that put out the first three fingers of the right hand so that they are at right angles to each other; the direction of thumb is the direction of motion, the direction of fore finger is the direction of magnetic field and the direction of the second. finger gives the direction of induced current.

magnetic field

induced current

3. Transformer

↑

motion

transformer is a device for stepping up or down an alternating

lamination

1.e. made up of a large number of mutually insulated soft iron plat- es. This will reduce the eddy current to a minimum..

Action of a transformer.

The input current is variable the magnetic field produced by this input is also variable. There is a certain rate of change of flux linkage with the secondary. The result is to set up an induced

The enf in the secondary coil. magnitude of the induced emf depends on the number of tums in the secondary and the magnitude of the input. The use of the soft iron core is to concentrate the field lines, so that a maximum flux linkage with the secondary can be achieved.

4. Induction coil.

The induction coil is a de- vice for getting a high voltage from a low one. The primary and the secondary windings are wound

motion

Exercise

(fig 7)

M

1. Give the direction of induced emf in the given figures. (6.7.8)

2. The efficiency of a transformer is defined as the ratio

E=

output power input power

* 100%

A transformer whose primary wind- ing is 100 and secondary winding 200 is connected to an a.c. input. of 200 V peak value. What is the output voltage? If the efficiency of the transformer is 80%, what is the secondary current when the. primary current is 1AÄ?

Solution to ex.

1. The Jet is induced with a posiv -ive charge. The water droplets emerge from the jet carry away,

The internal drop is 10 mA x 10KЛ

The voltmeter reading

100 V.

300 100 200 v.

The circuit for (a) is as shown. in fig 13.

loov

N

The effective resistance

The current in the circuit is

100

3K

37.5 mA.

The current thro' the voltmeter is

1K 37.5 mA x = 12.5 mA

The voltmeter reading is

12.5 mA x 2 Kл 25

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