VIVA QUESTIONS WITH ANSWERS III-SEM B.SC

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1 VIVA QUESTIONS WITH ANSWERS III-SEM B.SC Dept.of Physics Dept.of Physics, HPPC Govt. First Grade College Challaker

2 AIR WEDGE 1. Are the fringes equispaced? Yes, the fringes equispaced. 2. What type of air film do you get when thin plate is placed between two plane parallel glass plates? A wedge shaped air film is formed. That thickness of the film increases from the end where plates are in contact to the other end. 3. What is shape of interference fringes? Fringes are straight. 4. How interference does take place in case of air wedge? Interference occurs between the rays reflected at the upper and lower surface of the air film. VOLTAGE VARIATION DURING CHARGING AND DISCHARGING OF CAPACITOR 1. Define Capacitor. Capacitor is a device for storing charges. It consists if two metal plates placed parallel with a dielectric or an insulating medium in between them. 2. Define Capacitance of a Capacitor. The capacity of a capacitor is numerically equal to the quantity of charge which must be given to the condenser in order to maintain the unit potential difference between the plates. 3. On what factors the capacity of a capacitor depends? It depends on a) area of the plate b) distance between the plates is called dielectric constant of the material used. 4. Define time constant of the RC circuit. It is defined as the time required for the capacitor to charge to 63% of its maximum value. It is equal to the product of R & C. 5. Through which component does the discharge takes place? The discharge takes place through the resistance. 6. What is the maximum voltage attained by the capacitor? Equal to the applied voltage. 1

3 DIFFRACTION GRATING 1. What is diffraction grating? A plane diffraction grating consists of an optically plane glass plate on which number of equidistant, parallel straight lines are ruled. 2. What is grating element? It is sum of which of transparent and opaque portion is called grating element or granting constant. 3. How is grating element related to the number of ruling per cm? Grating element is the reciprocal of number of rulings per cm. 4. What is difference between spectra of grating? Prism gives only one spectrum on both sides of the center. The order of the color is reverse in one another. Prism spectrum depends of material of the prism. Grating spectra are independent of material of grating. Prism spectra are due to dispersion of light. The grating spectra are formed by diffraction of light. 5. Are the spectra of different orders of the same intensity? No, they are of different intensities. The intensity decreases with the increase of the order number. CAUCHY S CONSTANTS: 1. What is a prism? Prism is an optical medium bounded by three rectangular faces and two triangular faces. 2. What is refracting angle of prism? The angle between two refracting surfaces is called refracting angle of prism. 3. What types of eye piece is used in spectrometer telescope? Ramsden s eye piece because it has cross wires so the measurements can be made. 4. Why do the telescope and collimator are adjusted for parallel rays? When the incident rays are parallel then image is formed at the principal focus of the eyepiece. After dispersion same coloured rays are parallel to one another, different colors should be focused at the different points. 5. What is monochromatic source? Is sodium light is monochromatic? A source emitting a particular wavelength is called monochromatic. Sodium light is not strictly monochromatic. It consists of two wavelengths differing by small value 6A. Wave lengths are 5890 A and 5896 A. 6. Is Laser beam is monochromatic? Yes, Laser beam is monochromatic source. 2

4 7. What is dispersion of light? Splitting up of composite light into its constituent colors when it is passed through some dispersive medium like prism is called dispersion, discovered by Newton. 8. Why should there be no parallax between the image of the slit and that of the cross wires? When there is no parallax the image of the slit will be coincide with the cross wires. Otherwise there will be relative movement between the two images with the movement of the eyes which in turn introduce error in the readings. 9. On what a factor does the angle of deviation depends? It depends on the R I of the material of the prism. Wavelength of the incident light, refracting angle of the prism, angle of incidence. 10. Why deviation produced by the prism does is vary with the colour of the light? The deviation produced depends R I of the material of the prism which is different for different wavelengths. I.e. for different colors. 11. Write the Cauchy s dispersion formula and explain the terms? Cauchy s formula is, n = A + B λ 2 Where, n= RI of the material of the prism λ = Wavelength of light used A and B are Cauchy s constants. 12. Does the Cauchy s constant depend on the dispersive medium? Yes, A and B are different for different media. 13. Does the Cauchy s constants depends of the dispersive medium depends upon the wavelength of the light? Differentiating the Cauchy s formula dn = 2B dλ λ 3 dn dλ is the dispersive power of the medium which is inversely proportional to the cube of the wavelength of light. 14. What is angle of minimum deviation? It is an angle of deviation for which for which the ray passes through the prism symmetrically. 15. Define angular dispersion? The difference between deviations for any two colors produced by a prism. 16. Define R I of a Medium? The dispersive power of a material is the ratio of angular dispersion to the mean deviation. 17. What is a spectrum? Band of colours (wavelengths) obtained after dispersion. 3

5 MAXIMUM POWER TRANSFER THEOREM 1. State Maximum power transfer theorem. Maximum power is delivered to the load resistance from the network when its resistance is equal to the Thevenin s resistance of the network or internal of the source with all voltage and current source are replaced by their internal resistances. 2. Mention the applications of the maximum power transform theorem. A) It is used in communication network. B) In radio communication, it is required to transfer maximum power from the circuit to the load like antenna or loud speaker. C) Motor cars, telephone lines and T V arial loads. 3. What is meant by resistance matching? When R = RL maximum power is transferred to RL. This is called resistance matching. 4. Maximum power if PMax = V th 2 4R L THEVENIN S THEOREM 1. State Thevenin s theorem. Any linear two terminal networks consisting of resistances and sources of emf can be replaced by a single source of emf v th in serial with a single resistance R th Vth= Open circuit voltage at the two specified terminals Rth =Resistance looking back into the network with all the source of emf shorted and replaced by their internal resistance. 2. How do you Thevinise the given network? A) Calculate the open circuit voltage v th (RL is disconnected) B) Replace voltage source by a short circuit in series with the internal resistance of the voltage source or replace each current source by an open circuit in parallel with the internal resistance of the current source. C) Calculate Rth D) Vth is in series with Rth and RL 3. What is a Voltage source? The source which has very low internal resistance. 4. What is current source? The source which has very high internal resistance. 5. What is a circuit element? A Circuit component with two terminals by which it can be connected to other electric components is called circuit element. Ex: - Resistor, Capacitor and Inductor. 4

6 6. What are active elements? Circuit elements which supply energy to a network are called active elements. Ex:- Batteries, Generators, Transistors.etc. 7. What are passive elements? Circuit elements which either absorb or store energy are called passive elements. Ex:- Resistors, Inductors and Capacitors. 8. What is a node (Junction) It is point is a network where two or more elements (Currents) meet. 9. What is mesh or loop? A closed path for the current in a given network is called the mesh or loop. FILTERS 1. What is a frequency filter? Circuit which allows a certain band of frequency is called a filter. 2. What is high pass filter? Circuit which allows high frequency above the cut off frequency. 3. What is low pass filter? Circuit which allows only low frequency below the cut off frequency. 4. What is cut of frequency? It is a frequency of applied AC below or above which the circuit allow the frequency 5. What is the expression for cut off frequency of line pass or high pass filter? f= 1 2π RC 6. What is cut off frequency? It is a frequency at which stop band changes to pass band or vice versa. or It is a frequency at which the power level reduced to -3db. or It is the frequency at which the voltage gain reduced to 70.7% of max value. 7. What is frequency response curve? It is the graph of voltage gain against frequency. 8. When does the cut of frequency occur? When Xc =R SERIES AND PARALLEL RESONANCE 1. What is meant by electrical resonance? When impedance in the circuit is minimum current is maximum this is known as electrical resonance. 2. What is the condition for resonance? XL = XC Inductive reactance= capacitive reactance,z becomes minimum, When Z=R 5

7 3. What is the value of Z in LCR series circuit at resonance? Z =R 4. What is the resonance frequency? The frequency of applied AC at which resonance takes place or current is maximum. 5. Expression for resonance frequency? f o = 1 2π LC, f o is independent of circuit resistance. 6. Define band width. In series LCR circuit, bandwidth is defined as the range of frequency for which the power dissipated in the resistance is equal to half the power dissipated at resonance. 7. Expression for bandwidth. Bandwidth=(f2 f1) Hz 8. What are f1, f2? f1, f2 are half power frequencies. The values of frequencies at which the power input is half its maximum value are called half power frequencies. 9. What are characteristics of series resonance? a) Z is minimum when Z=R. b) I is maximum, I m = V Z min = V R c) I is in phase with V i.e. Φ=0. d) Reactance=0, i.e. X L = X C. 10. Why do you call LCR series resonance circuit as an acceptor circuit? It offers a low impedance to the current at a resonance frequency, it is called acceptor circuit 1. What is parallel resonance? PARALLEL RESONANCE FREQUENCY: IL and IC are such that the resultant current I is in phase with the supply voltage, then the network is said to be in resonance. 2. What is the current at resonance? Current is minimum because impedance is maximum. 3. What is the meant by rejector circuit? As the parallel resonate circuit offers high impedance to the current at the frequency, the circuit is called a rejector circuit. 4. What is a tank circuit? A parallel resonate circuit is called tank circuit. This produces oscillations in the network. 5. What is anti-resonance? The condition of parallel resonance is called anti-resonance. 6

8 6. What is resonance curve? A variation of line current with frequency is called resonance curve. 7. What are the characteristics of parallel resonance? A) The circuit impedance is maximum B) The circuit current is minimum C) I is out of phase with V. 8. What is impedance? Combined effect of reactance and resistance Z= V rms I rms 9. Resonant frequency is given by f o = 1 2π LC 10. Define self-inductance of a coil It is the property of the coil due to which it opposes the change of current in the circuit. 11. Quality factor(q) The ability of a reactive device to store energy as compared to the dissipation of the energy within the device is called Q-factor. It is given by Q = X L = f o R (f 2 f 1 ) FIELD ALONG THE AXIS OF A CIRCULA COIL 1. What is magnetic field of electric current? When current flows through a wire magnetic field is produced around it. 2. Where does the field become maximum when current flows through the circular coil? At the centre of the circular coil the field is maximum. 3. What happens to the magnetic field if you go away from the centre of the coil? Field decreases on either side of the coil. 4. What is tangent law? When a magnetic needle is placed between two mutually perpendicular uniform magnetic fields. Needle is deflected making an angle Ѳ with the deflecting field. B=BH tan Ѳ 5. Why do you set the plane of the coil in the direction of BH? To satisfy tangent law. 6. What are the initial adjustment in the experiment? A) Plane of the coil must be set parallel to BH B) Aluminium pointer should read Why do you take two deflections Ѳ1 and Ѳ2? To eliminate eccentric error i.e. centre of the magnetic needle may not be exactly at the centre of the circular degree scale. 7