A transformer consisting of two coils wrapped around an iron core is connected t

Question

A transformer consisting of two coils wrapped around an iron core is connected to a generator and a resistor, as shown in the drawing. There are 19 turns in the primary coil and 20 turns in the secondary coil. The peak voltage across the resistor is 71.5 V. What is the peak emf of the generator? In a television set the power needed to operate the picture tube comes from the secondary of a transformer. The primary of the transformer is connected to a 120-V receptacle on a wall. The picture tube of the television set uses 86 W, and there is 5.5 mA of current in the secondary coil of the transformer to which the tube is connected. Find the turns ratio N_s/N_o of the transformer.

Explanation / Answer

Let the emf in the primary coil be E1 which is the emf of the generator

Let the emf in the secondary coil be E2, which is the voltage across resistance R.

The turns in primary coil are NP = 19

The turns in secondary coil are NS = 20

Then we have

E2 = - (NS/NP)E1

or 71.5V = -(20/19)E1

E1 = -71.5V(19/20) = -67.925V

So the peak emf of the generator is -67.925V,

where the negative sign shows that the voltage E2 across the resistor or the emf induced in the secondary coil is 1800 out of phase with E1.

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Here we have E1 = 120V

using the relation Power = VoltageXCurrent or P = VI which gives V = P/I, we have

E2 = (86W)/(5.5mA) = (86W)/(5.5X10-3A)

or E2 = 15.63X103V

Using the formula;

E2 = - (NS/NP)E1

we have;

15.63X103V = - (NS/NP)(120V)

or (NS/NP) = 0.13X103 = 130, we ignored the negative sign as turns ratio have to be positive.

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This concludes the answers. Check the answer and let me know if it's correct. If you need anymore clarification or correction I will be happy to oblige....

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