A 240v mains transformer has 800 turns in the primary and N turns in the secondary. It is used to supply energy to a 12v/21w lamp

(a) A 240v mains transformer  has 800 turns in the primary and N turns in the secondary. It is used to supply energy to a 12v/21w lamp. ​

(i) Calculate the number of turns in the secondary (2mks)

(ii) What is the efficiency of the transformer if the current drawn from the 240V mains is 100mA (2mks)

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The circuit above shows a step-down transformer used to light a lamp of resistance.

​Calculate.

(i) the reading of voltmeter, V, with, S, open (2mks)

(ii) the reading of the ammeter A with S closed (neglect the effective resistance of the secondary winding) (2mks)

(iii) the power dissipated in the lamp (2mks)

(iv) the primary current if the transformer is 90% efficient (3mks)

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