12th Class Physics Chapter 5 Alternating Current Short Questions Answer

(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.

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.

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.

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.

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.

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.

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.

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.

No the ions inside the electrolyte will continue vibrating about its mean position due to continuous reversal of polarity of A.C.

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.

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.

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.

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.

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.

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.

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.

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.

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.

The highest or maximum value attained by the voltage or current in one cycle is known as peak value.

The combined effect of resistance and reactants in the circuit is called impedance.Its unit is ohm.

The combined effect of resistance and reactances in the circuit is called impedence.Its unit is ohm.

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.

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.

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.