# 12th Class Physics Chapter 5 Alternating Current Short Questions Answer

1.A sinusoidal current has rms value of 10 A.What is the maximum or peak value? 2.Name the device that will (a) permit flow of direct current but oppose the flow of alternating current.(b) permit flow of alternating current but not the direct current?
(a) An inductor is a device which will permit flow of direct current but oppose the flow of alternating current. (b) A capacitor is a device which will permit the flow of alternating current but oppose the flow of direct current.
3.How many time per second will an inductance lamp reach maximum brilliance when connected to a 50Hz source?
The brilliance of lamp will become maximum twice in one A.C. cycle because the current also become maximum two time in a cycle. As the frequency f of A.C cycle is 50Hz.So Maximum brilliance will be maximum 100 times in one second.
4.A circuit an iron – cored inductor a switch and a D.C source arranged in series.The switch is closed and after an interval reopened.Explain why a spark jumps across the switch contacts?
Consider a series circuit containing an iron – core inductor a switch and D.C source.When the switch is closed then the current increases from zero to maximum value.This changing current produces change of magnetic flux and an induced emf is produced. When the switch is reopened after an interval then again current changes from its maximum value to zero.Once again the emf is produced across the inductor.It is in the form of back emf.As a result it is flux change which causes an emf induced leading to a spark across the switch contacts.
5.How does doubling the frequency affect the reactance of (a) an inductance (b) a capacitor? 6.In a R- L circuit will the current lag or lead the voltage?IIIustrate your answer by a vector diagram?
In R – L series circuit the current will lag behind the voltage by π/2 in its phase as shown in vector diagram.In this case phase VL leads current by π/2.
7.Explain the conditions under which electromagnetic waves are produced from source?
When alternating voltage (V = Vo sin2 πft) is applied across the ends of a metallic antenna an oscillating electric field comes into existence which accelerates the electric again and again as the polarities of the antenna change after half a cycle.If the electric charge on the antenna is +q at one moment which it becomes – q at the other moment.The accelerated electrons radiate energy carried by electric field.The changing electric field produces changing magnetic field.Thus each field produces the other turn by turn and so this process continues all along the direction of propagation.The changing electric and magnetic fields both confined to a plane perpendicular to the direction of propagation produe electromagnetic waves. 9.What is meant by A.M and F. M?
i) Amplitude Modulation (A.M):- A type of modulation in which amplitude of the carrier wave is increased or decreased as the amplitude of the superposing modulating signal increases or decreases. ii) Frequency Modulation (F.M):- A type of modulation in which frequency of the carrier wave is increases or decreased as the modulating signal amplitude increases or decreases but the carrier wave amplitude remains constant.
10.Name a device that will (a) permit the flow of direct current but not the alternating current (b) permit alternatig current but not the direct current?
a) Inductor:- Inductor is a device which permits the flow of direct current but opposes the flow of alternating current passing through it. b) Capacitor:- Capacitor is a device which permits the flow of alternating current but not the direct current.
11.An inductor draws 30 ampers when connected across the terminals of generator A and 6 ampers when connected to B.Assuming the same impressed voltage can you tell which of the two generators A or B is an A.C machine?Give reasons for your answer?
We can find the difference between an A.C and D.C generator by an inductor. We know that an inductor is a device which oppose the flow of A.C but not that of D.C. therefore the inductor should draw less current when connected across the terminals of an A.C.generator as compared to the current when connected across a D.C.generator.In the present case the inductor draws 30 amperes from the generator and 6 amperes from the generator B.Hence generator B is an A.C machine.
12.Can electrolysis take place with the help of an Source?
No the ions inside the electrolyte will continue vibrating about its mean position due to continuous reversal of polarity of A.C.
13.Differentiate between Peak value and Peak to Peak value?
Peak Value:- The highest or maximum value reached by the voltage or current during one cycle is called as peak value.It is denoted by Vo for voltage and Io for current respectively. Peak to Peak value:- The sun of positive and negative peak values of voltage is called as peak to peak value.It is denoted by a symbol p-p.The p-p value of voltage is 2V.
14.What is reactance? 15.Describe the condition which will make the reactance small? 16.What do you understand by inductive reactance? 17.What is a choke?
Choke is like a inductor coil of thick copper wire wound closely in a large number of turns over a soft iron laminated core.Such a inductor coil i.e. chock does not consume energy when the current flows through A.C.circuit.
18.What do you understand about three phases A.C supply?
The source of electrical power consists with a pair of slip rings.But in a three phase A.C. generator instead of one coil there are three separate coils inclined at an angle 120ᵒ to each other and each coil connected to its own pair of slip rings.This creates a phase difference of 120ᵒ with respect to one another.
19.Describe some advantages of 3- phase A.C.supply?
The main advantage of 3- phase A.C.supply is that it can bear large load since it can generate voltage of about 400 V.Due to this fact the total load of a house or factory can be divided into three parts. As it can bear large load so it can be used to operate some special appliances needing 400V for their working.
20.What is resonant frequency of R – L C series circuit? 21.Describe some important properties of a series resonant circuit? 22.What is the condition of resonance in a parallel resonance circuit? 23.Describe some important properties of parallel resonant circuit? 24.What do you understand by electromagnetic waves?
Electromagnetic waves are those waves which require no material medium for their propagation.As a matter of fact electromagnetic waves consist of oscillating electric and magnetic field in such a way that they can easily travel through space.The electric field magnetic field and the directing of propagation of electromagnetic waves are always mutually orthogonal.
25.Describe the principle upon which the electromagnetic waves can be generated?
An accelerated charge is essential for generating electromagnetic waves.In this way electromagnetic waves can be produced by the change of magnetic flux which creats an electric field while change of electric flux creates magnetic energy field.Thus each field generates the other field and the electrical energy travels in the form of whole packets of EMW perpendicular to changing electric and magnetic fields.
26.What is the source of transmitting electromagnetic waves?
Electromagnetic waves can be transmitted by a radio transmitting antenna.Such antenna can be used to generate EMW through an oscillating accelerating charge.An antenna is a long loop of wire charged by an A.C. source of frequency ‘f’ and time period ‘t’.The electromagnetic waves supplied by a transmitting antenna are called radio waves.
The radio waves can be received by the use of antenna of a L- C parallel circuit.Moreover inductor coil ‘L’ is connected parallel to a variable capacitor C in such a way that the frequency of capacitor ‘C’ is so adjusted that it becomes equal to the frequency of incoming electromagnetic waves.In such position the broadcast frequency of radio programmes or TV.can be easily picked up by adjusting the value of ‘C’ in an L-C circuit.
28.What do you understand by modulation and modulated carrier wave?
The process of combining a low frequency signal with a high frequency radio waves is called modulation and the wave is called a carrier wave.The resultant wave so produced is called as modulated carrier wave.Te low frequency signal is known as modulation signal.Modulation is achieved by changing the amplitude or the frequency of the carrier wave in accordance with the modulating signal.
29.Describe the types of modulation?
A low frequency signal can be modulated by two ways. i)Amplitude modulated (A.M). ii)Frequency modulated (F.M). i) Amplitude modulated:-Such type of modulation in which the amplitude of the carrier wave is increased or decreased as the amplitude of superposing modulating signal increases or decreases but the frequency remains the same is called amplitude modulation. ii) Frequency modulation:- Such type of modulation in which the frequency of the carrier wave is increased or decreased as the modulating signal amplitude constant is called frequency modulation.
30.What is peak value?
The highest or maximum value attained by the voltage or current in one cycle is known as peak value.
31.Define impedence of a circuit and give its unit?
The combined effect of resistance and reactants in the circuit is called impedance.Its unit is ohm.
32.What is impedance?Write its unit?
The combined effect of resistance and reactances in the circuit is called impedence.Its unit is ohm.
33.What is a choke?Why it is used in A.C circuits?
Choke is a coil which is made of thick copper wire wound over a soft iron laminated cores.This makes the inductance L of the coil quite  large while its resistance R is very small.It consumes very small power as compared to a resistor.It is used  in A.C circuit to limit current with very small wastage of energy as compared to a resistance or rheostat.
34.Show that resonance at the resonance frequency is given as fr= 1/2πLC? 35.What is meant by impedence?Explain briefly. 36.What are conditions for representing A.C voltage or current by vector diagram?
A sinusoidally alter nating current or voltage can be graphically represented by a anti- clockwise rotating vector if it satisfies the following conditions. i) Its length represents the peak or rms value of the alternating quantity. ii) It is in the horizontal position at the instant when the alternating quantity is zero and is increasing positively. iii) The angular frequency of the rotating vector is the same as the angular frequency ɷ of the alternating quantity.
37.Define power factor in A.C?
The relation for power dissipation in an A.C. circuit is given by. P= VI cosθ……………………….. (1) Where θ is the phase difference between the applied voltage V and current I. In equation (1) the factor cosθ is known as power factor. Def.of power Factor:- It is defined as the ratio of the power consumed in an A.C. to the power applied to the circuit.
38.Define Root Mean square value of A.C. Voltage? You Can Learn and Gain more Knowledge through our Online Quiz and Testing system Just Search your desired Preparation subject at Gotest.

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