Very Short Question Answers

Q1. A transformer converts 200v ac into 2000ac.Calculate the number of turns in the secondary if the   primary has 10 turns.

Ans.  $N_{s}  =    \frac{V_{s}}{V_{p}   ×N_{p}}   = 2000/200  ×10 = 100   $     
       $ ∴Ns  = 100 turns$

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Q2. What type of transformer is used in a 6V bed lamp?

Ans. Step – down transformer

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Q3. What is the phenomenon involved in the working of a transformer?

Ans. Mutual induction

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Q4. What is the transformerratio?

Ans. The ratio between the number of turns in secondary coil and the number of turns in primary coil   is called transformer ratio. 

 Transformer ratio (k) $=\frac{N_{s}}{N_{p}} =    \frac{V_{s}}{V_{p}}$

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Q5. Write the expression for the reactance of i ) an inductor and ii) a capacitor ?

Ans. $1)\:\;  X_{L}  =  \:ω L      \:\:\:2)  X_{C}   =   \:1/ωC$

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Q6. What is the phase difference between ac emf and current in the following: pure resistor, pure inductor and pure capacitor.

Ans. Pure resistor, phase difference is zero  
Pure inductor, phase difference is  $π/2$
 Pure capacitor, phase difference is  $π/2$

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Q7. Define power factor. On which factors does the power factor depend?

Ans. Theaverage  power over a complete cycle is $ P = VI\: cosϕ$    ( or )$ P= I ^{2} Z\, cosϕ  $ in this,  The term $cosϕ$ is called power factor.  It depends on phase difference $\phi$ between current and voltage.

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Q8. What is meant by wattless component of current?

Ans. When  $cosϕ=0$ and no power so dissipated even though a current is flowing in the circuit. This is called wattles component of current.

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Q9. When does a LCR series circuit has minimum impedance?

Ans.  When $X_{L}=  X_{C }\:\,  ,   \:\,  ω L  =   1/ωC$

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Q10. What is the phase difference between voltage and current when the power factor in LCR series circuit is unity?

Ans. If the power factor $cosϕ=1$.Then phase difference between voltage and current $ϕ=0$

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Q11. What is step-up transformer How it differences from step-down transformer.

Ans. If the secondary coil has a more number of turns than the primary is called step-up transformer.    $ N_{S}>N_{P}$.
If the primary coil has more turns than the secondary is called step – down transformer.  $N_{S}<N_{P}$

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