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Question
State the principle of the step-down transformer and its working.
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Solution
Principle: A transformer is based on the principle of electromagnetic mutual induction, i.e., whenever the amount of magnetic flux linked with a coil changes, an e.m.f. is induced in the neighboring coil.
Working
Alternating emf is supplied to the primary coil PP’. The resulting current induces a current in the secondary coil.
Magnetic flux linked with primary is also linked with the secondary. The induced emf in each turn of the secondary is equal to that induced in each turn of the primary.
Let
EP − Alternating emf applied to primary
nP − Number of turns in the primary
`(dphi)/dt-"Rate of change of flux through each turn of primary coil"`
`:.E_p=-n_p (dphi)/dt" ......(1)"`
Es− Alternating emf of secondary
ns − Number of turns in secondary
`:.E_s=-n_s (dphi)/dt" ....(2)"`
Dividing equation (2) by (1),
`E_s/E_p=n_s/n_p=k`
For step-up transformer, K > 1
∴ Es > Ep
For step-down transformer, K < 1
∴ Es < Ep
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RELATED QUESTIONS
The primary coil of an ideal step-up transformer has 100 turns and the transformation ratio is also 100. The input voltage and power are 220 V and 1100 W, respectively. Calculate the
(a) number of turns in secondary
(b) current in the primary
(c) a voltage across secondary
(d) current in secondary
(e) power in secondary
The teachers of Geeta’s school took the students on a study trip to a power generating station, located nearly 200 km away from the city. The teacher explained that electrical energy is transmitted over such a long distance to their city, in the form of alternating current (ac) raised to a high voltage. At the receiving end in the city, the voltage is reduced to operate the devices. As a result, the power loss is reduced. Geeta listened to the teacher and asked questions about how the ac is converted to a higher or lower voltage.
1) Name the device used to change the alternating voltage to a higher or lower value. State one cause for power dissipation in this device.
2) Explain with an example, how power loss is reduced if the energy is transmitted over long distances as an alternating current rather than a direct current.
3) Write two values each shown by the teachers and Geeta.
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A transformer lowers e.m.f. 220 V to 12 volts. If the number of turns in primary are 8800, how many turns are in secondary coil?
A 100% efficient transformer has 100 turns in the primary and 50 turns in its secondary coil. If the current in the secondary coil is 6 A, then the current in the primary coil is ______.
Name one electrical device which works on Faraday's law of electromagnetic induction principle.
Two coils P and Q are kept near each other. When no current flows through coil P and current increase in coil Q at the rate 10A/s, the emf in coil P is 15mV. When coil Q carries no current and current of 1. 8A flows through coil P, the magnetic flux linked with the coil Q is ______.
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