INSTRUMENT CONSTRUCTION AND WORKING

Chapter 7. Wave optics

Q. 1. Describe the use of a biprism for obtaining two coherent sources of light.

(2 marks)

Ans. Fresnel's biprism is a single prism having an obtuse angle of about 178° and the other two angles of about 1º each. The biprism acts as a combination of two thin prisms of refracting angle of about 1° when a monochromatic source in the form of an illuminated narrow slit is aligned parallel to the refracting edge of the biprism. Cylindrical wavefronts of light from the slit falls on the biprism. The two halves of the biprism split every wavefront such that the light appears to come from two sources S₁ and S₁. These two virtual sources act as two coherent sources as they are derived from the same wavefront. The two split wavefronts, appearing to come

from S, and S₂ interfere under appropriate conditions and form 44 fringes on a screen, far away from the biprism.

Chapter 8. Electrostatics

Q. 2. State the principle of working of the Van de Graaff generator. Describe its construction with a neat labelled diagram. (3 marks)

Ans. Principle of working: The Van de Graaff generator works on the principles of corona or point discharge, that the charge on hollow

continuously driven by an electric motor from the ground up to the inside of the conductor (See figure). Near the ground, the belt passes close to a spraycomb A which is connected to a high-voltage source. The spraycomb consists of a set of sharp needle points. Another spraycomb collector B is connected inside conductor C at the top. The entire apparatus is usually enclosed in a pressurized vessel containing a gas such as nitrogen or Freon. Housed inside the assembly is an evacuated tube through which positively charged particles may be accelerated from a source at the same potential as the conductor C to a target at the ground potential.

Chapter 9. Current electricity

Q. 3. Explain the basic construction of a pivoted-type moving-coil galvanometer with a neat labelled diagram.

(2 marks)

Ans. A table galvanometer consists of a coil of a large number of turns of fine insulated copper wire wound on a light rectangular aluminium frame. The coil has a pointer attached and is pivoted between cylindrically concave pole pieces of a strong horseshoe permanent magnet. The coil swings freely around a cylindrical soft iron core fitted between the pole pieces. The

deflection of the coil depends on the current passing through the galvanometer (or the potential difference across it). The deflection of the coil is arrested

by a spiral spring and is read with the pointer on a scale.

A galvanometer can be used as an ammeter or a voltmeter with a suitable modification.

Chapter 10. Magnetic fields due to electric current

Q. 4. State the principle of working of a cyclotron. Describe the construction with a neat labelled diagram.

(3 marks)

Ans. Principle of working: The cyclotron uses the principle of synchronous acceleration to accelerate charged particles which describe a spiral path at right angles to a constant magnetic field and make multiple passes through the same alternating p.d., whose frequency is the same as the frequency of revolution of the particles.

Construction of the cyclotron: Two hollow D-shaped ct  are open at their straight edges form the electrodes. They are called the dees. The dees are separated by a small gap, as shown in the figure, and a

high-frequency (10° Hz to 107 Hz) alternating p.d. (of the order of 10 V to 105 V) is applied between them. The whole system is placed in an evacuated chamber between the poles of a large and strong electromagnet(B1Tto2T).

The ions to be accelerated are produced in an ion source; a hydrogen tube gives protons, heavy hydrogen or deuterium gives deuterons while helium gives x-particles, etc. The positive ions are injected near the centre and are accelerated each time they cross the gap between the dees. At the edge of one of the dees, an electrostatic deflector at negative potential deflects the spiralling particles out of the system to strike a target.

Q. 5. With the help of neat diagram, describe the working of a moving-coil galvanometer.

(3 marks)

Ans. Consider a rectangular coil-of length 1, breadth b and N turns-

carrying a current / suspended in a uniform magnetic field of induction B.

The magnetic forces on the horizontal sides of the coil have the same line of action and do not exert any torque. The magnetic forces on the vertical sides constitute a couple and exert a deflecting torque. If the plane of the coil is parallel to B, the magnitude of the deflecting torque maximum equal to

ta = NIAB

where A = Ib is the area of each turn of the coil. This torque rotates the coil. 

In a moving-coil galvanometer, the coil swings in a radiar magnetu field produced by the combination of the cylindrically concave pole pieces and the soft-iron core. Hence, the plane of the coil is always parallel to the field lines, as shown in the figure. Therefore, the deflecting torque is constant and maximum as given by Eq. (1)

In a pivoted-type galvanometer, the rotation of the coil twists the helical springs which exert a restoring torque on the coil. The restoring torque is proportional to the angle of twist 0. (2)

where C is the torque constant of the springs. C depends on the dimensions and the elasticity of the springs.

The coil eventually comes to rest in the position where the restoring torque equals the deflecting torque in magnitude. Therefore, in the equilibrium position, . CONIAB 0=(NAB)1 (3)

C since N, A, B and C are constant. Thus, the deflection of the coil is directly proportional to the current in it.

Chapter 12. Electromagnetic induction

Q. 6. Briefly describe the construction of a simple ac generator. Obtain an expression for the emf induced in a coil rotating with a uniform angular velocity in a uniform magnetic field. Show graphically the variation of the emf with time (t). OR est

Describe the construction of a simple ac generator and explain its working. (4 marks)

Ans. AC generator construction: A simplified diagram of an ac generator is shown in Fig. 1. It consists of many loops of wire wound on an armature that can rotate in a magnetic field. When the armature is turned by some mechanical means, an emf is generated in the rotating coil.

Consider the coil to have N turns, each of area A, and rotated with a constant angular speed about an axis in the plane of the coil and perpendicular to a uniform magnetic field B, as shown in the figure. The frequency of rotation of the coil is f = ο/2π.

Working: The angle between the magnetic field B and the area of the coil 4 at any instant is cot (assuming 0-0° at 10). At this position, the magnetic flux through the coil is

NBA NBA cos 0=NBA cos ot

The sinusoidal variation of emf with time t is shown in Fig. 2. The emf changes direction at the end of every half rotation of the coil. The frequency of the alternating emf is equal to the frequency of rotation of the coil. The period I of the alternating emf is T= 1 f

Q. 7. Describe the construction and working of a transformer with a neat labelled diagram.

(3 marks)

Ans. Construction: A transformer consists of two coils, primary and secondary, wound on two arms of a rectangular frame called the core as shown in the figure.

(1) Primary coil: It consists of an insulated copper wire wound on one arm of the core. Input voltage is applied at the ends of this coil.

In a step-up transformer, thick copper wire is used for primary coil. In a step-down transformer, thin copper wire is used for primary coil.

(2) Secondary coil: It consists of an insulated coppe 315/4 on the other arm of the core. The output voltage is obtained this coil.

44

In a step-up transformer, thin copper wire is used for secondary coil. In a step-down transformer, thick copper wire is used for secondary coil.

(3) Core: It consists of thin rectangular frames of soft iron stacked together, but insulated from each other. A core prepared by stacking thin sheets rather than using a single thick block helps reduce eddy currents.

Working: When the terminals of the primary coil are connected to a source of an alternating emf (input voltage), there is an alternating current

through it. The alternating current produces a time-varying magnetic field in the core of the transformer. The magnetic flux associated with the secondary coil thus varies periodically with time according to the current in the primary coil. Therefore, an alternating emf (output voltage) is induced in the secondary coil.

Chapter 16. Semiconductor devices

Q. 8. Draw a neat block diagram of a de power supply and state the function of each part. OR

With the help of a block diagram, explain the scheme of a power supply for obtaining de output voltage from ac line voltage. (3 marks) Ans. A consumer electronic system called a de power supply produces a fairly constant de voltage from ac supply voltage. The figure shows a functional block diagram of the circuits within a power supply.

The ac supply voltage is usually stepped-down by a transformer and its secondary voltage is converted to a pulsating de by a diode rectifier. By the superposition theorem, this rectifier output can be looked upon as having two different components a de voltage (the average value) and an ac voltage (the fluctuating part). The filter circuit smooths out the pulsating de. It blocks almost all of the ac component and almost all of the de component is passed on to the load resistor. Figure shows the filtered output for a rectified full-wave de. The only deviation from a perfect de voltage is the small ac load voltage called ripple. A well-designed filter circuit minimizes the ripple. In this way, we get an almost perfect de voltage, one that is almost constant, like the voltage out of a battery.

The regulation of a power supply is its ability to hold the output steady under conditions of changing input or changing load. As power supplies are loaded, the output voltage tends to drop to a lower value. Nowadays,

an integrated circuit (IC) voltage regulator is connected between a filter and the load resistor, especially in low-voltage power supplies. This device not only reduces the ripple, it also holds the output voltage constant under varying load and ac input voltage.

Q. 9. With a neat circuit diagram, explain the use of two junction diodes as a full-wave rectifier. Draw the input and output voltage waveforms.

(3 marks)

Ans. Electric circuit: The alternating voltage to be rectified is applied across the primary coil (P_{1}P_{2}) of a transformer with a centre-tapped secondary coil (S_{1}S_{2}) The terminals S_{1} and S_{2} of the secondary are connected to the two p-regions of two junction diodes D_{1} and D_{2} respectively. The centre-tap T is connected to the ground. The load resistance R_{I} is connected across the common n-regions and the ground.

Working: A full-wave rectifier rectifies both halves of each cycle of an alternating voltage.

During one half cycle of the input, terminal S_{1} of the secondary is positive while S_{2} is negative with respect to the ground (the centre-tap T). During this half cycle, diode D_{1} is forward-biased and conducts, while diode D_{2} is reverse-biased and does not conduct. The direction of current through R_{1} is in the sense shown. I_{1}

During the next half cycle of the input voltage, S_{2} becomes positive while S_{1} becomes negative with respect to T. Diode D_{2} now conducts

sending a current I_{L} through R_{1} is the same sense as before while D_{1} does not conduct. Thus, the current through R_{1} is in the same direction, i.e., it is unidirectional, for both halves or the full-wave of the input. This is called full-wave rectification.

The output voltage has a fixed polarity but varies periodically with time between zero and maximum. Fig. 2 shows the input and output voltage waveforms.

Ripple frequency: The pulsating de output voltage of a full-wave rectifier has twice the frequency of the input.

Q. 10. Explain the construction of a photodiode with a neat diagram. (2 marks)

Ans. Construction: A photodiode consists of an n-type silicon substrate with a metal electrode back contact. A thin p-type layer is grown over the n-type substrate by diffusing a suitable acceptor dopant. The area of the p-layer defines the photodiode active area. An ohmic contact pad is deposited on the active area. The rest of the active area is left open with a protective antireflective coating of silicon nitride to minimize the loss of photons. The non-active area is covered with an insulating opaque SiO₂ coating.

Depending on the required spectral sensitivity, i.e., the operating wavelength range, typical photodiode materials are silicon, germanium, indium gallium arsenide phosphide (InGaAsP) and indium gallium arsenide

(InGaAs), of which silicon is the cheapest while the last two are expensive. est

(2 marks)

Q. 11. Explain the principle of operation of a neat diagram. photodiode with a

Ans. Working: In a semiconductor diode, photons or particles with energies greater than or equal to band gap energy E, can transfer electrons from the valence band into the conduction band..

A photodiode is operated in the reverse bias mode which results in a wider depletion region. When operated in the dark (zero illumination), there is a reverse saturation current due solely to the thermally generated minority charge carriers. This is called the dark current.

When exposed to radiation of energy hv EG (in the range near- UV to near-IR), electron-hole pairs are created in the depletion region. The electric field in the depletion layer accelerates these photogenerated electrons and holes towards the n side and p side, respectively, constituting a photocurrent / in the external circuit from the p side to the n side. Due to the photogeneration, more charge carriers are available for conduction and the reverse current is increased. The photocurrent is directly proportional to the intensity of the incident light. It is independent of the reverse bias voltage.

Q. 12. Explain the I-V characteristics of a photodiode. (2 marks) Ans. When a Si photodiode is operated in the dark (zero illumination), the current versus voltage characteristics observed are similar to the curve of a rectifier diode as shown by curve in the figure. This dark current Si photodiodes range from 5 pA to 10 nA.

When light is incident on the photodiode, the curve shifts to 2 and increasing the incident illuminance (light level) shifts this characteristic curve still further to in parallel. The magnitude of the reverse voltage has nearly no influence on the photocurrent and only a weak influence on the dark current. The normal reverse currents are in tens to hundreds of microampere range.

The almost equal spacing between the curves for the same increment in luminous flux reveals that the reverse current and luminous flux are almost linearly related. The photocurrent of the Si photodiode is extremely linear with respect to the illuminance. Since the total reverse current is the sum of the photocurrent and the dark current, the sensitivity of a photodiode is increased by minimizing the dark current.

Q. 13. Describe with a neat labelled diagram the construction and working of a solar cell.

(3 marks)

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Ans. Construction: A simple pn-junction solar cell consists of a p-type semiconductor substrate backed with a metal electrode back contact. A thin n-layer (less than 2.5 µm, for silicon) is grown over the p-type substrate by doping with suitable donor impurity. Metal finger electrodes are prepared on top of the n-layer so that there is enough space between the fingers for sunlight to reach the n-layer and, pn-junction.

Working: When exposed to sunlight, the absorption of incident radiation (in the range near-UV to infrared) creates electron-hole pairs in and near the depletion layer.

Consider light of frequency v incident on the pn-junction such that the incident photon energy hv is greater than the band gap energy E_{G} of the semiconductor. The photons excite electrons from the valence band to the conduction band, leaving vacancies or holes in the valence band, thus generating electron-hole pairs.

The photogenerated electrons and holes move towards then side and p side, respectively. If no external load is connected, these photogenerated charges get collected at the two sides of the junction and give rise to a forward photovoltage. In a closed-circuit, a current / passes through the external load as long as the solar cell is exposed to sunlight.

Q. 14. Explain the working of a junction transistor with a neatly labelled circuit diagram.

(4 marks)

Ans. For normal operation of a junction transistor, the emitter-base junction is always forward biased and the collector-base junction is always reverse biased. Figure shows the biasing of the junctions for an npn transistor connected as an amplifier with the common-base configuration, that is, the base lead is common to the input and output circuits. The emitter-base junction is forward biased by the battery r~V_{B} while the collector-base junction is reverse biased by the battery V_{cc} V_{BB} should be greater than the emitter- base barrier potential (the threshold voltage). The arrows of the various currents indicate the direction of current under normal operating conditions (also called the active mode)

Since the emitter-base junction is forward biased, majority carriers electrons in then emitter are injected into the base and holes (majority carriers in the p-type base) are injected from the base into the emitter. Under the ideal-diode condition, these two current components constitute the total emitter current Ig.

The emitter is a very heavily doped n-type region. Hence, the current between emitter E and base B is almost entirely electron current from E into B across the forward biased emitter junction.

The p-type base is narrow and the hole density in the base is very low. Therefore, virtually all the injected electrons (more than 95%) diffuse right across the base to the collector junction without recombining with holes. Since the collector junction is reverse biased, the electrons on reaching the collector junction are quickly swept by the strong electric field there into the n-type collector region, where they constitute the collector current Ic.

In practice, about 1% to 5% of the holes from the emitter with holes in the base layer and cause a small current /g in the base lead. Therefore, IIB + Ic≈ Ic. recombin est noo base

Therefore, carriers injected from a nearby emitter junction can result in a large current flow in a reverse biased collector junction. This is the transistor action, and it can be realized only when the two junctions are physically close enough to interact as described.

If a pap transistor is used, the battery connections must be reversed to give the correct bias. The conduction process is similar but takes place instead by migration of holes from emitter to collector. A few of these holes recombine with electrons in the base.