Basic Electricity Paper 2 , May/June 2014

Questions:		1	2	3	4	5	6	7		Main

General Comments
Weakness/Remedies
Strength

Question 4
Explain damping as applied to electrical measuring instruments. (b) State two advantages of a digital meter over an analogue meter. (c) A moving-coil instrument has a resistance of 10 Ω with a full-scale deflection (f.s.d.) of 8 mA. With the aid of a sketch, calculate the value of a multiplier that will enable it read up to 100 V
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Observation
The expected answers were: (a) In an electrical measuring instrument, the combination of the inertia of the moving system and the controlling torque of the spiral springs or of gravity gives the moving system a natural frequency of oscillation called damping. (b) (i) Digital meter has high impedance hence less error (ii)Digital meter can be read by an inexperienced person from a far (c)V = Va + Vm = Ira + IRm 100 = 0.008 x 10 + 0.008Rm 100 – 0.08 = 0.008Rm Rm= = 12490 = 12.49 kΩ The question was on Measuring Instruments. It was very popular with the candidates. Those who attempted this question did better in part (c) than they did in parts (a) and (b).