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.  $N_s=\frac{V_s}{V_p\times N_p}=2000/200\times 10=100$     
       $\therefore Ns=100turns$

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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. $1)X_L=\omega L2)X_C=1/\omega C$

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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  $\pi /2$
 Pure capacitor, phase difference is  $\pi /2$

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

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

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Q8. What is meant by wattless component of current?

Ans. When  $cos\varphi =0$ 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 $X_L=X_C,\omega L=1/\omega C$

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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 $cos\varphi =1$.Then phase difference between voltage and current $\varphi =0$

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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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