class 12 physics chapter 7 ALTERNATING CURRENT important questions
Dear students class 12 physics chapter 7 ALTERNATING CURRENT important short questions with answers pdf have been collected at one place to get you all prepared for your 12th Bihar board exam, as well as for UP board exams ,Jharkhand board exam ,MP board exams ,Rajsthan board exams ,cbse board exams and other board exams. Visit my nawendu classes youtube channel for more help.
Q.(1) What is difference between alternating current and direct current ?
Answer
Alternating current
(i)The current which flows in a direction for a definite time interval and in opposite direction for the same time interval of time, is called alternating current.
(ii)AC is changed into DC by rectifier.
(iii)AC does not show chemical effect of current.
(iv)AC cannot be used to make permanent magnets.
Direct current
(i)The current which always flows in particular direction, is called direct current.
(ii)DC is changed into AC by inverter.
(iii)DC shows chemical effect effect of current.
(iv)DC is used to make permanent magnets.
Q.(2) Define mean value and root mean square value of alternating current.
Bihar Board  2013
Answer Read in nawendu classes notes
Q.(3) Establish the relation between mean value and peak value of AC.
Bihar Board  2019
Answer Read in nawendu classes notes
Q.(4) What do you mean by inductive reactance?
Bihar Board  2020
OR
What is inductive reactance?
Bihar Board  2014
Answer Effective resistance of L AC (inductor AC) circuit is called inductive reactance.
OR
Opposition offered by an inductor to the flow alternating current is called inductive reactance.
* It is denoted by ${{X}_{L}}$.
${{X}_{L}}=\omega L$=$2\pi fL$
* SI unit of inductive reactance is ohm.
Q.(5) What are reactance and impedance in alternating current circuit ?
Bihar Board  2019
Answer Reactance Effective resistance of L AC circuit or CAC circuit is called reactance.
OR
Opposition presented to alternating current by an inductor or a capacitor is called reactance.
* SI unit of reactance is ohm.
*Reactance is of two types
(i) Inductive reactance It is denoted by ${{X}_{L}}$.
${{X}_{L}}=\omega L$=$2\pi fL$
(ii) Capacitive reactance It is denoted by ${{X}_{C}}$.
${{X}_{c}}=\frac{1}{\omega C}$=$\frac{1}{2\pi fC}$
Impedance Effective resistance of LCR AC circuit is called impedance.
OR
Opposition to alternating current presented by the combined effect of resistance and reactance (by an inductor or by a capacitor or by both) is called impedance.
* SI unit of impedance is ohm.
* Impedance is denoted by Z.
$Z=\sqrt{{{R}^{2}}+{{\left( {{X}_{L}}{{X}_{C}} \right)}^{2}}}$=$\sqrt{{{R}^{2}}+{{\left( \omega L\frac{1}{\omega C} \right)}^{2}}}$
Q.(6) What is meant by resonant circuit and resonant frequency ?
Answer Resonant circuit  The circuit, in which inductive reactance and capacitive reactance are same (equal) is called resonant circuit.
* For a resonant circuit
${{X}_{L}}={{X}_{C}}$
* In resonant circuit impedance is minimum which is equal to resistance.
* In resonant circuit current in the circuit is maximum.
* In resonant circuit ,the applied voltage is equal to the voltage across resistance.
Resonant frequency The frequency of alternating current, for which inductive reactance and capacitive reactance are equal (same), is called resonant frequency.
For resonant frequency
${{X}_{L}}={{X}_{C}}$
$\omega L=\frac{1}{\omega C}$
$\omega L=\frac{1}{\omega C}$
${{\omega
}^{2}}=\frac{1}{LC}$
$\omega
=\frac{1}{\sqrt{LC}}$
$2\pi
{{f}_{r}}=\frac{1}{\sqrt{LC}}$
${{f}_{r}}=\frac{1}{2\pi \sqrt{LC}}$
${{f}_{r}}=\frac{1}{2\pi \sqrt{LC}}$
This is expression for resonant frequency.
Q.(7) Explain electrical resonance.
Bihar Board  2021
Answer The phenomena, in which inductive reactance and capacitive reactance of (AC) electrical circuit are (same or) equal to each other, is called electrical resonance.
* For electrical resonance
${{X}_{L}}={{X}_{C}}$
* While resonance impedance is minimum which is equal to resistance.
* While resonance current in the circuit is maximum.
* While resonance the applied voltage is equal to the voltage across resistance.
Q.(8) What do you meant by Qfactor ?
Qfactor= potential difference across L (or C) / potential difference across R
Hence,
* Qfactor is unitless and dimensionless.
In LCR circuit
$Z=\sqrt{{{R}^{2}}+{{\left(
{{X}_{L}}{{X}_{C}} \right)}^{2}}}$
Z = R
Now , applied voltage= I X Z=IR
potential difference across ends of the resistance= I X R= applied voltage
potential difference across ends of the inductance=$I{{\omega }_{r}}L$
potential difference across ends of the capacitance=$I\times
\frac{1}{{{\omega }_{r}}C}$
Q=$\frac{I{{\omega
}_{r}}L}{IR}$= $\frac{{{\omega
}_{r}}L}{R}$
OR
Q=$\frac{I\times
\frac{1}{{{\omega }_{r}}C}}{IR}$=$\frac{1}{RC{{\omega
}_{r}}}$
But ${{\omega
}_{r}}=\frac{1}{\sqrt{LC}}$
So,
$Q=\frac{1}{R}\sqrt{\frac{L}{C}}$
Q.(9) What is choke coil ?
Answer Choke coil is a device which is used to control current in ac circuit.
Choke coil is made, to wrap copper wire on laminated soft iron core. Choke coil works on the principle wattless current. Because current lags by 90 degree behind emf.
So it controls alternating current without any loss of energy.
Q.(10) Explain the use of motor starter or choke coil.
Bihar Board  2016
Answer motor starter is used to provide initial high current (or high voltage) to start the working of tube light.
while on the other hand choke coil is used to control current (to maintain working voltage) while working of the tube light.
Q.(11) Hot wire instrument is used to measure AC, why ?
Answer In A.C. circuit , current flows in a direction for half cycle and in the opposite direction for rest half cycle. This is why alternating current is measured by hot wire instrument based on heating effect of current , which does not depend on the direction of current.
Q.(12) What is transformer ? What is transformation ratio?
Bihar Board  2014
Answer Transformer Transformer is a device which is used to transform low AC voltage into high AC voltage and high AC voltage into low AC voltage.
Transformation ratio The ratio of output voltage (or turn of secondary coil) to input voltage (or turn of primary coil) of a transformer is called ratio of transformation.
It is denoted by k or r.
\[r=\frac{{{V}_{s}}}{{{V}_{p}}}=\frac{{{n}_{s}}}{{{n}_{p}}}\]
*for stepup transformer $r\rangle 1$
*for stepdown transformer $r\langle 1$
Q.(13) What is transformer ? What do you meant its efficiency ?
Bihar Board  2013
Answer Transformer Transformer is a device which is used to transform low AC voltage into high AC voltage and high AC voltage into low AC voltage.
Efficiency of a transformer Ratio of output power to input power of a transformer is called efficiency of a transformer.
* It is denoted by $\eta $.
efficiency of a transformer= output power/input power=$\frac{{{P}_{o}}}{{{P}_{i}}}$
*It is also expressed in percentage.
efficiency of a transformer=$\frac{{{P}_{o}}}{{{P}_{i}}}$x100%
* Efficiency of a transformer is generally from 95% to 99%.
Q.(14) What is the use of stepup transformer?
Bihar Board  2020
Answer Stepup transformer is used to transform low AC voltage into high AC voltage.
Q.(15) Name the energy losses in a transformer.
Bihar Board  2015
Answer There are following four losses of energy in a transformer
(i)Copper loss copper coils of a transformer have some resistance due to which energy losses in the form of heat energy. This loss of energy is called copper loss.
(ii)Iron loss When direction and magnitude of the current flowing through the coils of a transformer change then flux linked to soft iron core also changes due to which eddy currents induced in the soft iron cores. Due to large amount of eddy currents soft iron core becomes hot. So energy losses in the form of heat energy. This loss of energy is called iron loss.
(iii)Flux loss Total number of magnetic field lines passing through secondary coil is a bit less than that passing through primary coil. Because some of the magnetic field lines passes through air medium instead of soft iron core. So flux linked to secondary coil is not exactly equal to that linked to primary coil.
Loss of energy due to loss of flux in secondary coil is called flux loss.
(iv)Hysteresis loss Due to change in direction of alternating current flowing through the coils of a transformer the soft iron core undergoes a number of cycle of magnetization due to which the soft iron core becomes hot .i.e. energy losses in the form of heat energy. This loss of energy is called hysteresis loss.
Q.(16) Explain copper loss in a transformer.
Bihar Board  2021
Answer Copper loss copper coils of a transformer have some resistance due to which energy losses in the form of heat energy. This loss of energy is called copper loss.
Q.(17) Why is core of transformer laminated ?
Bihar Board  2012
Answer We know that, If flux linked
with a conducting plate changes, eddy currents are produced. Eddy currents loss
energy in the form of heat energy.
So to
minimize eddy currents and energy loss due to eddy currents, cores of
transformers are made laminated.
Q.(18) Why does the core of a transformer becomes hot, if current flows through its coils ?
Answer When current flows throng the coils of a transformer, its core frequently gets magnetized and demagnetized. In this process energy losses in the form of heat energy. Due to which, core of a transformer becomes hot.
Q.(19) What is transformer steel ?
Answer If 4% silicon is mixed in soft iron then it is called transformer steel. Transformer steel is more suitable for cores of a transformer because it minimizes hysteresis loss. Q.(20) What is use of transformer in long distance electricity transmission ?
Ans.
Let power of produced electricity = 20000w
Let power of produced electricity = 20000w
Potential difference = 200v
P = V I
20000 = 200 × I
.......(I)
It 200v of a.c. is converted into 1000v of a.c. by a transformer
P = V I
20000 = 1000 × I
I = 100A

It 200v of a.c. is converted into 1000v of a.c. by a transformer
P = V I
20000 = 1000 × I
I = 20 A

..........(II)
From eqᶯ  (I) & eqᶯ – (II) ,
It is clear
when low AC voltage is converted into high AC voltage by a transformer,
value of current decreases in the circuit. Due to low current loss of energy (into heat energy) also becomes low ( according to formula H= I²Rt).Thin wire are required to conduct low current, which minimizes the cost of wire purchasing.
Hence, transformer is very useful in long distance electricity transmission with low purchasing cost and low electricity loss .
Q.(21) Explain capacitive reactance?
Bihar Board  2022
Answer Effective resistance of C AC (capacitor AC) circuit is called capacitive reactance.
OR
Opposition offered by an capacitor to the flow alternating current is called capacitive reactance.
* It is denoted by ${{X}_{C}}$.
${{X}_{C}}=\frac{1}{\omega
C}=\frac{1}{2\pi fC}$
chapter 1 ELECTRIC CHARGES AND FIELDS short QA
chapter 2 ELECTROSTATIC POTENTIAL AND CAPACITANCE short QA
chapter 3 CURRENT ELECTRICITY short QA
chapter 4 MOVING CHARGES AND MAGNETISM short QA
chapter 5 MAGNETISM AND MATTER short QA
chapter 6 ELECTROMAGNETIC INDUCTION short QA
chapter 8 ELECTROMAGNETIC WAVES short QA
chapter 2 ELECTROSTATIC POTENTIAL AND CAPACITANCE short QA
chapter 3 CURRENT ELECTRICITY short QA
chapter 4 MOVING CHARGES AND MAGNETISM short QA
chapter 5 MAGNETISM AND MATTER short QA
chapter 6 ELECTROMAGNETIC INDUCTION short QA
chapter 8 ELECTROMAGNETIC WAVES short QA
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