Electronics 2 May/June 2010

Electronics Paper 2, May/June 2010

Questions:		1	2	3	4	5, 6 &7	8	9	10	Main

General Comments
Weakness/Remedies
Strength

Question 8

(a) Explain briefly the difference in the principle of operation of moving-coil and moving-iron instruments.
(b) A galvanometer with full-scale deflection of SOIlA internal resistance 10n is required to measure 10V.
(i) Draw the circuit arrangement.
(ii) Calculate the value of the resistor required to extend the meter's range.

_____________________________________________________________________________________________________

OBSERVATION

The expected answers were:
(a) In a moving-coil meter, the coil through which current flows moves in a magnetic field created by permanent magnets, whiles in a moving-iron meter, two pieces of iron attract or repel each other when magnetism is induced in them due to current flow in a stationary coil.

ii)

	Rm	=100
	I.d	= SOmA
	Vmeter	= 10 x 50 mA
		= sOOmV= 0.5mV
	Ish	=9.95mA
	Rsh	O __ S~nJ"
		---
		S".S'&n-tA

= 0.050
Vmu1tl = 10 V - O.smV= 9.999sV
98995
Value of R =-=--o
;,. mA
=199.990
The question was on Measuring Instruments. It was not popular.
None of the candidates gave a satisfactory answer to the question concerning the principles of operation for the moving-coil and moving-iron measuring instruments. This was reported as a recurring incidence where candidate could not distinguish between 'multiplier' and 'shunt.'