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A transformer is a device which increase...

A transformer is a device which increases or decreases alternating voltage through mutual induction. It consists of two coils wound on the same code. One of the coil is connected to input alternating supply. This is called primary coil. The other coil gives the output. i.e. it is connected to external load. This coil is called secondary coil. The alternating emf applied across the primary current causes an alternating current to flow through it. This alternating current creates a continuously changing magnetic flux through the core. The changing magnetic flux is linked with secondary coil. If there is no leakage of flux, then the flux linked per unit turn with primary coil is equal to the flux linked per unit turn with secondary coil. Let `phi_(S)` is flux linked in secondary coil and `phi_(P)` is flux linked in primary coil then `(phi_(S))/(N_(S))=(phi_(P))/(N_(P))` (where `N_(S)` and `N_(P)` are respective number of turns in secondary and primary of transformer)
`:. (1)/(N_(s))(d phi_(S))/(dt)=(1)/(N_(P))xx(d phi_(P))/(dt)`
or `(1)/(N_(S)) E_(S)=(1)/(N_(P))E_(P)` (`:' E = - (d phi)/(dt) = ` emf in a coil )
If the voltage in secondary is more than that in primary, the transformer is step-up. If voltage in secondary is less than that in primary, the transformer is called step down. Select the correct alternative

A

For step-up transformer `N_(S) gt N_(P)`

B

For a step-up transformer `N_(S) lt N_(P)`

C

For a step-down transformer `N_(S) gt N_(P)`

D

For a step-down transformer `N_(S)=N_(P)`

Text Solution

Verified by Experts

The correct Answer is:
A
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A transformer is a device which increases or decreases alternating voltage through mutual induction. It consists of two coils wound on the same code. One of the coil is connected to input alternating supply. This is called primary coil. The other coil gives the output. i.e. it is connected to external load. This coil is called secondary coil. The alternating emf applied across the primary current causes an alternating current to flow through it. This alternating current creates a continuously changing magnetic flux through the core. The changing magnetic flux is linked with secondary coil. If there is no leakage of flux, then the flux linked per unit turn with primary coil is equal to the flux linked per unit turn with secondary coil. Let phi_(S) is flux linked in secondary coil and phi_(P) is flux linked in primary coil then (phi_(S))/(N_(S))=(phi_(P))/(N_(P)) (where N_(S) and N_(P) are respective number of turns in secondary and primary of transformer) :. (1)/(N_(s))(d phi_(S))/(dt)=(1)/(N_(P))xx(d phi_(P))/(dt) or (1)/(N_(S)) E_(S)=(1)/(N_(P))E_(P) ( :' E = - (d phi)/(dt) = emf in a coil ) If we assume that there is no loss of power in a transformer, and the currents in the primary and secondary coils are respectively i_(P) and i_(S) , then

A transformer is a device which increases or decreases alternating voltage through mutual induction. It consists of two coils wound on the same code. One of the coil is connected to input alternating supply. This is called primary coil. The other coil gives the output. i.e. it is connected to external load. This coil is called secondary coil. The alternating emf applied across the primary current causes an alternating current to flow through it. This alternating current creates a continuously changing magnetic flux through the core. The changing magnetic flux is linked with secondary coil. If there is no leakage of flux, then the flux linked per unit turn with primary coil is equal to the flux linked per unit turn with secondary coil. Let phi_(S) is flux linked in secondary coil and phi_(P) is flux linked in primary coil then (phi_(S))/(N_(S))=(phi_(P))/(N_(P)) (where N_(S) and N_(P) are respective number of turns in secondary and primary of transformer) :. (1)/(N_(s))(d phi_(S))/(dt)=(1)/(N_(P))xx(d phi_(P))/(dt) or (1)/(N_(S)) E_(S)=(1)/(N_(P))E_(P) ( :' E = - (d phi)/(dt) = emf in a coil ) A transform is used to light 140 W, 24 V lamp from 240 V ac mains. The current in the mains cable is 0.7 A. The efficiency of the transformer is

Knowledge Check

  • Mutual inductance of two coils can be increased by

    A
    decreasing the number of turns in the coils
    B
    increasing the number of turns in the coils
    C
    winding the coils on wooden cores
    D
    none of these

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