Plus Two Physics Chapter Wise Previous Questions Chapter 7 Alternating Current is part of Kerala Plus Two Physics Chapter Wise Previous Questions and Answers Kerala. Here we have given Plus Two Physics Chapter Wise Questions and Answers Chapter 7 Alternating Current.
Kerala Plus Two Physics Chapter Wise Previous Questions Chapter 7 Alternating Current
Question 1.
Which one of the following can produce the maxi¬mum induced e.m.f? (March – 2009)
a) 50A, DC
b) 50A, 50HzAC
c) 50A, 500HzAC
d) 100A, DC
Answer:
c) 50A, 500 Hz AC
Question 2.
A voltmeter reads 80V when connected across the terminals of AC voltage source with frequency f = 100Hz (March – 2009)
a) What is the RMS value of the voltage in the above case?
b) Write the equation of the instantaneous voltage provided by the source.
c) If the above voltage is connected across a 40Ω resistor, find the current through the resistor. How will the current change if the same voltage is applied at 100kHz (1 kHz = 1000 Hz)?
d) If the resistor is replaced by a 2mH inductor, comment on the current at 100 Hz and 100kHz.
Answer:
a) Vms = 80V
c) The current through resistor I = V/R = 80/40 = 2A l0 = 2A. For pure resistance, V = IR & there is no frequency dependence. Hence current is same in both cases.
d) The inductive reactance (XL = Lω) depends on frequency. When frequency increases 1000 times, reactance also increases 1000 times and current reduces by the same factor.
Question 3.
Seema constructed a series LCR circuit in the laboratory as shown in the diagram. She found that the voltages across the inductor and capacitor are equal when the circuit is connected to an ac source. (March – 2010)
a) State the condition at which the voltage across L and C become equal.
b) Obtain an expression for the frequency at which this situation occurs in a series LCR circuit.
c) Find the voltmeter and ammeter readings in the circuit.
Answer:
a) Resonance
c) Ammeter reading
Voltmeter reading
Question 4.
A student connects an ac source of emf V = V0 sinet to a circuit having a capacitor of capacitance ‘C’ as shown below. (Say – 2010)
a) If the instantaneous current from ac source is 10 sin 314 tA, what will be the effective current in the circuit?
b) If he replaces the ac source by a dc source and connects a bulb in the above circuit, whether the bulb glows or not. Justify your answer.
c) In the circuit shown above, find the expression for instantaneous current through the capacitor.
d) Find the average power consumed in the above circuit.
Answer:
b) Bulb will not glow. The capacitative reactance of capacitor for dc flow is infinity. i.e. Xc = ∞
Consider a circuit containing a capacitor of capacitance ‘C’ connected to alternating voltage. Let the applied voltage be
d) The instantaneous value of voltage and current of a capacitor is y = V0sinct, i l0cos ot Hence the instantaneous power is
Question 5.
Aseries LCR AC circuit has great practical importance. It is used for tuning radio, TV, wireless sets etc. (March – 2011)
a) Obtain an expression for current in a series LCR ac circuit using phasor diagram.
b) Linder what condition, this circuit is used for tuning?
Answer:
a) Phasor Diagram
The instantaneous current lags the voltage by an angleϕ.
If V = V0 sinωt is the applied voltage, the current at any instant can be written as
I = I0 sin (ωt – ϕ) ……………….. (3)
Where l0 is the peak value of current it’s value can be written as
Question 6.
……………….. signals are continuous variations of voltage or current. (Say – 2011)
Answer:
Analog signals.
Question 1.
a) Fill in the blanks: (March – 2012)
If ‘ω’ is the angular frequency of a.c., then the reactance offered by inductance ‘L’ and capacitance ‘C’ are respectively, XL = ……………… and XC = …………….
b) An electric bulb ‘B’ and a parallel plate capacitor ‘C’ are connected in series as shown in figure. The bulb glows with some brightness. How will the glow of the bulb affected on introducing a dielectric slab between the plates of the capacitor? Give reasons in support of your answer.
c) Given belowaretwoelectnccircuitsAandB. What is the ratio of power factor of the circuit 5to that of A?
Answer:
b) Glow increases.
When we introduce a dielectric slab in between the plates, the capacitance of the capacitor increases. When capacitance increases capacitative reactance (Xc) decreases. Hence current through capacitor increases.
Question 7.
The schematic diagram of the generator is show below. (Say – 2012)
a) State the law which governs the working of a generator.
b) An ac voltage is applied to an LCR circuit. Draw the phasordiagram showing the voltages across the components.
c) The variation of maxjmum current iandfreqiemcu ‘co’ in an LCR circuit fortwo different values of the resistances?
d) The quality factor Q determines the starpness of resonance of an ac circuit. Obtain a relation that shows the dependence of Q on Resistance (R).
Answer:
a) Faraday’s law of induction The magnitude of the induced emf in a circuit is equal to the time rate of change of magnetic flux through the circuit.
c) The value of resistance R1 is less than R2. We know that the current in the LCR circuit
When bandwidth increases, the sharpness of resonance decreases. ¡e. the tuning of the circuit will not be good.
Question 8.
a) An alternating voltage is applied across on LCR circuit as shown below. Draw the phasor diagram for the circuit. (March – 2013)
b) Prove that an inductor offers easy path to d.c. and a resistive path to a.c.
c) In the above circuit L = 100 mH, C= 100 μ F, R = 120 Ω and E = 30 sin (100 t) find the
i) impedance
ii) reactance
iii) peak current and
iv) resonant frequency of the circuit.
Answer:
Refer SAY 2012, Question 1b,
b) Inductive reactance XL = Lω; XL = L2πf Case – 1
For dc current, frequency f = 0
Question 9.
In an A.C. circuit the flow of current is opposed by inductors and capacitors. This is called reactance. (Say – 2013)
a) Fill in the blanks:
If W’ ¡s the angular frequency of A.C, then the reactance offered by L and C are respectively XL = ………….. and Xc = …………….
b) An elednc bulb ‘B’ and a parallel plate capacitor ‘C’ are connected ¡n series to an a.c. mains as shown in the given figure. The bulb glows with some brightness.
How will the glow of the bulb be affected on introducing a dielectric slab between the plates of the capacitor? Give reasons in support of your answer.
c) Given below are two electric circuits A and B.
Calculate the ratio of power factor of the circuit B to that of A.
Answer:
b) The bulb glows more. Because on introducing a dielectric slab the capacitance increases and hence the capacitive reactance decreases. So the current experiences less opposition which makes the bulb glow more.
Question 10.
A fascinating behaviour of the series RLC circuit is the phenomenon of resonance. (March – 2014)
a) Explain Resonance in an LCR circuit.
b) Draw a graphical representation of variation of current amplitude m with frequency o.
c) What do you mean by sharpness of resonance? Explain it.
Answer:
a) At resonance, inductive reactance will be equal to capacitive reactance.
Hence impedence will be minimum (Z = R) and current will be maximum.
The frequency of applied signal at which impedence of LCR circuit is minimum and current becomes maximum is called resonance frequency.
C) Sharpness of resonance,
When band width increases, sharpness of resonance decreases. ie, tuning of circuit will not be good.
Question 11.
An AC. generator is used to convert mechanical energy into electrical energy. (Say – 2014)
a) But what does a transformer do?
b) What are the different types of transformers.
Answer:
a) To change the voltage level
b) Step up transformer, stepdown transformer
Question 12.
We usually ‘tune the radio to hear a programme clearly (Say – 2014)
a) By ‘tuning’ what we are doing actually?
b) What are the essential components in a tuning circuit?
c) What phenomefl’on can be observed in a tuned circuit?
d) Howdoesit happen?
Answer:
a) When we tune a radio circuit, we adjust the value of L or C till we get
b) Resistor Capacitor and Inductor
c) We get maximum current for resonance frequency. (Resonance takes place)
d) When 
the impedance of the LCR circuit becomes minimum. Hence current becomes maximum. This phenomenon is called resonance. The frequency of the applied signal at which the impedance of LCR circuit is minimum and current becomes maximum is called resonance frequency.
Question 13.
Transformers either increase or decrease AC voltage. (March – 2015)
a) State the principle of a transformer.
b) Explain with the help of a labelled diagram the working of a transformer.
c) Explain briefly any three energy losses in a transformer.
Answer:
a) Mutai induction
Whenever a flux linked with first coil changes, an emf is induced in the second coil. The phenomenon production of emf in the second coil due to change in flux in first coil is called mutual induction.
A transformer consists of two insulated coils wound over a core. The coil to which energy is given is called primary and that from which energy is taken is called secondary.
Working and mathematical expression
Let V1, N1 be the voltage and number of turns in the primary. Similarly let V2, N2 be voltage and number of turns in the secondary.
When AC is passed, a change ¡n magnetic flux is produced in the primary. This magnetic flux passes through secondary coil.
If ϕ1 and ϕ2 are the magnetic flux of piimary and secondary, we can write ϕ1α N1 and ϕ1α N2 Dividing ϕ1 and ϕ2
c) 1) JoulelossorCoierloss When current passes through a coil heat ¡s produced. This energy loss is called Joule loss. It can be minimized by using thick wires,
2) Eddycurrent loss Can be minimized by using laminated cores. Laminated core increases the resistance of the coil. Thus eddy current decreases.
3) Hysteresis loss When the iron core undergoes cycles of magnetization, energy is lost. This loss is called hysteresis loss. This is minimized by using soft iron core.
Question 14.
a) The core of a transformer has the following properties: (March – 2016)
i) core is laminated.
ii) hysteresis loop is narrow.
Explain the significance of each property,
b) What is meant by resonance in an LCR circuit?
Answer:
a)
- To reduce eddy current loss
- To reduce hystersis loss
b) When 
the impedance of the LCR circuit becomes minimum. Hence current becomes maximum. This phenomena is called resonance. The frequency of the applied signal at which the impedance of LCR circuit is minimum and current becomes maximum is called resonance frequency.
Question 15.
a) The electrical analog of mass is (March – 2016)
i) diode
ii) capacitance
iii) inductance
iv) resistance
b) A 2 m long solenoid having diameter 6 cm and 2000 turns has a secondary of 500 turns wound closely near it mid-point. Calculate the mutual inductance between the two coils.
Answer:
a) Inductance, (iii)
Question 16.
a) The S.l. unit of inductive reactance is (Say – 2016)
(i) Henry
(ii) Ohms
(iii) Volt
(iv) No unit
b) Figure given below shows a series LCR circuit connected to a variable frequency source.
Determine the source frequency at resonance.
Answer:
a) Ohms
Question 17.
a) At resonance, in an LCR circuit, the emf and current are (March – 2017)
i) in phase
ii) out of phase
iii) having a phase difference of π/2
iv) having a phase difference of π/6
b) In the following circuit, find the value of V.
OR
a) In a circuit carrying an ideal coil with negligible resistance, the power dissipated is …….
b) In the following circuit, find the impedance.
Answer:
Question 18.
A series LCR circuit is connected to a DC source. The magnitude of inductive reactance is …………… (Say – 2017)
Answer:
Diamagnetic Material – Meissner Effect
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