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Electronics Paper 2, Nov/Dec. 2007  
Questions: 1 2 3 4 5 6 7 Main
General Comments
Weakness/Remedies
Strength



















































Question 2

In Fig. 6~ if R2 is connected in parallel with one of the series resistors; the current
flowing through the circuit increases to 62.5 mA .. Calculate the            .

(a)        current flowing through R2;
(b)        value ofR2.

V1 . -=-2'il


_____________________________________________________________________________________________________
OBSERVATION

According to the statements' of the question, pnor"to the inclusion ofR2in
,
(           the circui~ the current through the two resistances then in the circuit and
connected in series was 50~ and maintained by an e.m.fof 12V. Therefore~ it follows from Ohm's low that

J .

 

 

 

 

 

 

 

. ,

 

 

 

 

 

 

 

 

50

 

10-3

 

 

..L

, .

 

x

=

 

 

 

 

 

 

 

 

2Rl

:n

 

RI

=

.w:n

=

200

 

 

 

 

50

 

 

'j'. '

,Rz is connected in parallel as shown, the current in the, cirqJit increases to
62.SinA, which means that the total resistance in the circuit iS,now by Ohm's law
                           RT=        '2              = 320
                                        62.S x 10.3'
It f'OlIows therefore, the parallel combmation ofR} and R2 will have a re~istance
   of(32 - 20).0 or 12n. Therefore,                                                                ,


,R2 = 240 0 = 300 8
, The, current through R2 is readily computed from the current divider rule as

 

IR2

=

2Q..

x

62.SA

=

25mA

; ;(

 

 

50

 

 

 

 

(

 

 

 

 

 

 

 

 

IR2 =

25mA

 

 

 

 

(b)', As thoroughly calculated in part (a), R2 = 200
This question was basic electrical theory associated with resistors in circuits when connected in series/parallel. The question seemed quite unpopular with the candidates, with the perfonnance very poor.
, The poor performance was not unconnected with inadequate preparation instead of full coverage of the syllabus.

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