Complimentary With VOLUME I

PHYSICS ACTIVITIES VOLUME II: VIVA VOCE

CLASS

XII

Comprehensive PHYSICS ACTIVITIES Volume II : Viva Voce FOR CLASS XII Strictly according to the latest curriculum prescribed by

Central Board of Secondary Education (CBSE) and State Boards of Chhattisgarh, Haryana, Bihar, Jharkhand, Kerala, Mizoram, Meghalaya, Uttarakhand and other States following NCERT Curriculum By J.N. JAISWAL Formerly, Head of Deptt. of Physics Hindu College, Sonepat Haryana

Dr. RAJENDRA SINGH M.Sc., B.Ed., Ph.D., Delhi University J.D. Tytler School New Rajinder Nagar Delhi

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CONTENTS Current Electricity—Sources and Accessories • Electric Current • Cell and Battery • Electric Circuit and Key • Resistors, Resistance Box and Rheostat • Galvanometers, Ammeters and Voltmeters

... ... ... ... ... ...

Page 1 1 1 2 2 3

... ... ... ... ... ... ... ... ... ... ...

4 5 7 10 10 11 11 11 11 12 13

... ... ... ... ... ... ...

15 18 20 21 22 23 24

Experiments • • • •

• • • • • • • •

SECTION A Experiment 1 Experiments 2, 3, 4 Experiments 5, 6 Experiments 7, 8, 9 • Galvanometer • Resistance of a Galvanometer • Ammeter • Voltmeter • Conversion of a Galvanometer into an Ammeter • Conversion of a Galvanometer into a Voltmeter Experiment 10 SECTION B Experiments 1, 2 Experiments 3, 4, 7 Experiment 5 Experiment 6 Experiment 8 Experiment 9 Experiment 10

( iii )

VIVA VOCE Current Electricity—Sources and Accessories Electric Current 1.

2.

3.

4.

Cell and Battery

What is static electricity? Ans. Electric charges at rest, present on the surface of a body, constitute static electricity. What is current electricity? Ans. Electric charges flowing in a conductor, constitute current electricity. Define electric current. Ans. The time rate of flow of electric charge i.e., electric charge flowing per second at a cross-section of conductor, is called electric current. Its symbol is I. Give expression for electric current. Ans. The electric current

8.

9.

10.

11.

q electric charge i.e., I = . t time Give unit of electric current. Ans. The unit of electric current is ampere (A). Define ampere as unit of electric current. Ans. Current through a cross-section of conductor is one ampere, if one coulomb charge flows through that cross-section in one second. =

5.

6.

i.e., or 7.

1 ampere = 1A=

12.

13.

1 coulomb 1 sec

14.

1C 1s

Give direction of electric current. Ans. The current is a scalar quantity but its conventional direction of electric current is the direction of flow of a positive charge. Conventional current is opposite to the electronic current (current due to flow of electrons).

15.

1

What is cell? Ans. A cell is a device by which electric energy is generated due to chemical action taking place inside it. What is a battery? Ans. A battery is a combination of cells in series. The e.m.f. of the battery is the sum of the e.m.fs. of the cells. What is a primary cell? Ans. Cell which directly gives electrical energy from chemical energy, is called a primary cell. This cannot be recharged. What is a secondary cell? Ans. Cell which stores electrical energy as chemical energy and returns it back as electrical energy, is called a secondary cell. This can be recharged. Name common primary cells. Ans. The common primary cells are : (i) Leclanche cell. (ii) Daniel cell (iii) Dry cell. Name common secondary cells. Ans. The common secondary cells are : (i) Lead (Acid) Accumulator (ii) Ni-Fe (Alkali) Accumulator. What is partial polarisation of a Leclanche cell? Ans. In Leclanche cell, polarisation is fast but depolarisation is slow. It makes the cell polarised after sometime. This is called partial polarisation. What happens in partial polarisation? Ans. Hydrogen gas bubbles accumulate on positive electrode and forms an insulating layer over it.

2

COMPREHENSIVE PHYSICS ACTIVITIES VOL. II—XII

16.

What is the effect of partial polarisation on the working of the cell?

26.

Ans. The cell gives a strong current only for a short time. Then current becomes weak. 17.

How the partial polarisation of the cell is removed?

27.

Ans. The cell is given rest by keeping it in open circuit for few minutes. 18.

How does the ‘resting’ help in removal of partial polarisation? Ans. The accumulated hydrogen bubbles escape from the positive electrode. The cell regains its original e.m.f.

19.

For what applications, is the Leclanche cell suitable?

Resistors, Resistance Box and Rheostat 28.

29.

Ans. The Leclanche cell is suitable where large electric current is required for a short time only e.g., electric bells, telephones, etc. 20.

Which primary cell can be used if a standard cell is not available? Ans. A Daniel cell can be used if a standard cell is not available.

21.

22.

What makes a Daniel cell a substitute for a standard cell?

30.

Ans. Daniel cell gives small but steady current for sufficiently long time. Hence, it becomes a cheap substitute for a standard cell.

31.

Where is a dry cell used?

32.

Ans. A dry cell is used in portable devices like torches and transistor radios.

Electric Circuit and Key 23.

33.

Define electric circuit. Ans. Continuous path along which an electric current flows, is called electric circuit.

24.

When does a current flow in an electric circuit? Ans. Current flows in an electric circuit only when circuit is closed or switched on.

25.

What is short circuit of a cell? How does it harm the cell? Ans. A cell becomes short-circuited, when its terminals are directly connected. A large current flowing from the cell may damage it.

What is a key? Ans. A key is an electrical accessory used in electric circuits to start and stop current in the circuit. What is an open circuit? Ans. A circuit becomes an open circuit, when the key is out. No current flows in an open circuit.

34.

What is a resistor? Ans. It is an electrical component used in electric circuits to control the current. In what different forms, resistors are used in electric circuits? Ans. 1. With fixed known value of their resistance, they are used as resistance coils or in resistance boxes. 2. With variable unknown value of their resistance, they are used as rheostats. Why is wire in a resistance coil or a resistance box double folded? Ans. It is done to avoid induction effect. How are the resistances of different values obtained in a resistance box? Ans. The resistance values are increased by taking longer and thinner wire. What are dimensions of the wire used for providing infinite resistance? Ans. No wire is used for this purpose. Open air gap provides infinite resistance. Which material is suitable for construction of standard resistances for resistance box? Ans. Materials used are : (i) Constantan (or Eureka) (Cu 60%, Ni 40%) (ii) Manganin (Cu 83%, Mn 13%, Ni 4%). What properties make a material suitable for standard resistances? Ans. The suitable properties are : (i) High specific resistance—lesser length of wire gives required resistance. (ii) Low temperature coefficient of resistance—the resistance does not change appreciably with change in temperature.

3

VIVA VOCE

35.

36.

37.

What is a rheostat? Ans. It is a variable resistance device and is commonly used for adjusting the strength of electric current in an electric circuit. How does the slider make contact with the insulated wire? Ans. Due to repeated motion of the slider, insulation from the wire becomes removed. Slider makes contact with the wire. To what other application, a rheostat can be used? Ans. A rheostat can be used as a potential divider.

Galvanometers, Ammeters and Voltmeters 38.

39.

What is a galvanometer? Ans. It is an instrument, used for detecting feeble currents in an electric circuit. What is an ammeter? Ans. It is a low resistance device, used for measuring large electric current in a circuit. It is used in series in the circuit.

40. 41.

42.

43.

44.

45.

What is full name of an ammeter? Ans. The full name is ampere-meter. What is a voltmeter? Ans. It is a high resistance device, used for measuring potential difference between two points in a circuit. It is used in parallel in the circuit. How a galvanometer is converted into ammeter? Ans. A galvanometer is converted into ammeter by connecting a shunt in parallel to it. How a galvanometer is converted into voltmeter? Ans. By connecting a high resistance in series with it. What is ideal ammeter? Ans. It is an ammeter whose resistance is zero. What is ideal voltmeter? Ans. It is a voltmeter whose resistance is infinity.

4

COMPREHENSIVE PHYSICS ACTIVITIES VOL. II—XII

Experiments Section—A Experiment 1 1.

2.

3.

State Ohm’s Law. Ans. Ohm’s law states that the electric current I flowing through a conductor is directly proportional to the potential difference (voltage) V across its ends (provided that the physical conditionstemperature, pressure, of the conductor remain same). Give mathematical form of Ohm’s law. Ans. Mathematical form of Ohm’s law is, V = RI. Define resistance. Ans. The constant ratio of potential difference V across the ends of a conductor to the current I flowing through it, is called resistance of the conductor. It is represented by the symbol R.

V I What are Ohmic and non-ohmic resistances? Ans. Resistances which obey Ohm’s law, are called ohmic resistances. e.g., metals, like (Ag, Cu, etc.) Resistances which do not obey Ohm’s law, are called non-ohmic resistances. e.g., Diodes and Transistors. Give common examples of non-ohmic resistances. Ans. Vacuum tube diodes and semiconductor diodes are non-ohmic resistances. What is effect of temperature on the resistance of a conductor? Ans. The resistance of most of the conductors increases with increase in temperature. Name some substances whose resistance decreases with increase in temperature. Ans. Resistance of semiconductors (Si, Ge) decreases with increase in temperature. How do you conclude that the conductor used in experiment obeyed Ohm’s law? i.e.,

4.

5.

6.

7.

8.

9.

10.

11.

R=

12.

13.

14.

Ans. It is done by two results : (i) The ratio of voltmeter reading (V) and the corresponding ammeter reading (I) comes to be constant. (ii) A graph between V and I comes to be a straight line. Why a large current is not allowed to be passed through the conductor during the experiment? Ans. If a large current is passed (or even if a small current is passed unnecessarily for a long time), the conductor will become hot and its resistance will increase. Then the graph will not a straight line. Why do we use thick connecting wires? Ans. Thick connecting wires offer negligible resistance. What is ohm? Define it. Ans. Ohm is the S.I. unit of resistance. One ohm is the resistance offered by a conductor when one ampere current is flowing through it, when one volt P.D. is maintained across its ends. What is a battery eliminator? Ans. It is a rectifier. It converts high A.C. voltage (220 V) into low desired D.C. voltage such as 2 V, 4 V, 6 V, 8 V, 10 V, 12 V. It is a good substitute for a battery or a cell. Why, unknown wire whose resistance is to be determined, is made of alloys such as manganin, Eureka? Ans. Unknown resistance wire is made of alloy, but not of metals, because (i) The resistivity of alloys is greater than that of metals. (ii) The temperature coefficient of resistance of alloys is negligible than that of metals. What is the effect of rheostat, range of voltmeter, ammeter on resistance of unknown wire? Ans. No effect because resistance does not depend upon them.

5

VIVA VOCE

15.

16.

17.

18.

19.

20.

What material is chosen for rheostat wire and why? Ans. It is constantan alloy. Because its resistivity is high and temperature coefficient of resistance is quite small. What is the material of the connecting wires used in the experiment? Ans. Copper. Because of least resistance. Is there any advantage of battery eliminator over usual source of e.m.f.? Ans. Main advantage of battery eliminator is that current can be drawn at desired voltages and it does not need any charging. It is easy to handle and maintain. What are the factors affecting the resistance? Ans. The resistance depends upon length, Area of cross-section, nature of material and temperature of the conductors. What is electric current? Define its S.I. unit. Ans. The flow of electric charge per unit time through a conductor is called electric current. S.I. of current is ampere (A). The one ampere is the amount of current flowing in a conductor which offered resistance 1 Ohm when one volts potential difference is maintained across the conductor. Define S.I. unit of electric potential. Ans. Volt is the S.I. unit of electric potential. One volt is said to be the potential difference between two points if one Joule of work is done in bringing one coulomb of charge from one point to the other.

3.

FG H

4.

5.

2.

How does resistance depend upon the length of a conductor? Ans. The resistance is directly proportional to the length of a conductor (provided its area of cross-section remains constant) i.e., R ∝ l. How does resistance depend upon the area of cross-section of a conductor? Ans. The resistance is inversely proportional to the area of cross-section of a conductor (provided its length remains constant) i.e.,

1 R∝ . A

IJ K

because it makes current zero R = V . I What is cause of resistance? Ans. The collisions of free electrons with the ions or atoms of conductor during their drift. What is the conductance? Ans. The reciprocal of resistance is called conductance. It is denoted by G.

1 . S.I. of G is mho, or Siemen. R What is the length of resistance wire used between the gap of resistance box marked INFINITE? Explain it. Ans. Infinite marked plug has no wire. G=

6.

ρl . We have A made R infinite by making A = 0 (rather than by making l infinite). Define resistivity or specific resistance of the material of conductor. Explanation. We have R =

7.

RA . l If l = 1 m, A = 1 m2, then ρ = R. Hence, resistivity is defined as the resistance of a conductor of unit length and unit crosssectional area. The unit of resistivity is ohmmetre (Ω-m). Define electrical conductivity. Ans. It is reciprocal of resistivity. It is

Ans. Resistivity ρ =

8.

represented by the symbol σ =

Experiment 2, 3, 4 1.

What is the resistance of an open key? Explain it. Ans. An open key has infinite resistance

9.

10.

1 . The unit ρ

of electrical conductivity is Siemen per metre (Sm–1) What is the order of magnitude of resistivity of conductors? Ans. Resistivity of the conductors is of the order of 10–8 Ω-m. What is effect of temperature on the resistance of a conductor? Ans. Resistance of all conductors increases with increase in temperature of the conductor and vice versa.

11.

Define temperature coefficient of resistance.

6

COMPREHENSIVE PHYSICS ACTIVITIES VOL. II—XII

Ans. Temperature coefficient, α=

R2 − R1 R1 (t2 − t1)

21.

If R1 = 1, (t2 – t1) = 1, then α = (R2 – R1).

12.

13.

14.

15.

16.

Hence, temperature coefficient is defined as the increase in resistance of a conductor of unit resistance due to unit increase in temperature. Give unit of temperature coefficient of resistance. Ans. The unit of temperature coefficient is per °C (or K) (°C–1 or K–1). How does resistance change in series combination? Ans. Resistance increases in series combination. Explain increase of resistance in series combination. Ans. In series combination, the effective length of resistor increases. As R ∝ l, resistance increases in series combination. How does resistance change in parallel combination? Ans. Resistance decreases in parallel combination. Explain decrease of resistance in parallel combination. Ans. In parallel combination, the effective

1 , A resistance decreases in parallel combination. What is Wheatstone bridge? Ans. It is the arrangement of four resistance in quadrilateral form to determine one unknown resistance in term of other three resistances. What is a metre bridge? Ans. It is the practical form of Wheatstone bridge to determine the unknown resistance and resistivity of a given alloy wire. Why is constantan used in the bridge wire? Ans. 1. The resistivity (49 × 10–8 Ωm) of the constantan is high. 2. The temperature coefficient of resistance (α) is very small (0.40 × 10–4) (°C–1). How are the coils wound in a post office box or resistance box? area of cross-section increases. As R ∝

17.

18.

19.

20.

22.

Ans. The resistance coil is doubly wound to avoid electromagnetic induction. When is a Wheatstone’s bridge said to be balanced? Ans. A Wheatstone’s bridge is said to be balanced, when no current flows through the galvanometer and it gives no deflection. What is the condition for a Wheatstone’s bridge to become balanced? Ans. In a balanced Wheatstone’s bridge, we have, P R = . Q S

23.

Will the interchange of positions of cell and galvanometer effect the balance condition? Ans. No.

24.

When is a Wheatstone’s bridge most sensitive? Ans. The bridge is most sensitive when all the four resistances P, Q, R and S are of same order of magnitude.

25.

What are applied Wheatstone’s bridge?

forms

of

a

Ans. The applied forms of a Wheatstone’s bridge are : (i) Metre Bridge or Slide Wire Bridge. (ii) Post Office Box. 26.

Why is a metre bridge so called? Ans. Since the bridge uses one metre long wire, it is called a metre bridge.

27.

Why is a metre bridge also called a slide wire bridge? Ans. Since a jockey is slided over the wire (during the experiment), it is also called a slide-wire bridge.

28.

Why the jockey should not be pressed too hard on the wire when sliding over it ? Ans. Sliding the jockey with a hard press, will scratch the wire and make its thickness non-uniform. Then the resistance per unit length of the wire will not remain constant.

29.

What is null point? Ans. It is a point on the wire, keeping jockey at which, the galvanometer gives no deflection.

7

VIVA VOCE

30.

31.

32.

33.

34.

35.

36.

37.

38.

39.

Why is it advised to keep null point between 45 cm and 55 cm? Ans. It is done to minimise the effect of neglecting of end resistances in calculation and Wheatstone is most sensitive when all the four arms have same order of resistances. What are end resistances? Ans. The resistances of thick copper strips which keep the two ends of the wire pressed, are called end resistances. What is an ideal value of null point and why? Ans. Null point at 50 cm is an ideal null point. It makes P/Q = 1. This ratio is not affected by neglecting end resistance of equal values at the two ends. How can a null point be obtained near 50 cm? Ans. It can be done by keeping value of R very near the value of X. Why copper strips, used to pressed the ends of wire, are thick? Ans. Thick Cu strips have less resistance and minimise effect of end resistances. Why should the bridge wire have uniform thickness and material density throughout? Ans. Because only then, the resistance per unit length (σ) will be same throughout. Then P = σ l and Q = σ (100 – l) will be correct. Why the bridge method for resistance measurement is better than Ohm’s Law? Ans. It is so because the bridge method is a null method and more sensitive. Under what conditions, the metre bridge is most sensitive (and hence result most accurate)? Ans. The bridge is most sensitive when all the four resistances are of equal value. It brings null point automatically at 50 cm. Why the metre bridge is suitable for measuring moderate resistances? Ans. Because, Wheatstone bridge is suitable for moderate values of resistances. Therefore, meter bridge is more sensitive for moderate values. When the sensitivity of the bridge becomes less? Ans. Bridge has poor sensitivity when resistances P, Q, R and S (or X) are of different order.

40.

Why should current be passed for a short time? Ans. Continuous current will cause unnecessary heating effecting values of resistances used.

Experiments 5, 6 1.

2.

3.

4.

5.

6.

7.

8.

9.

What do you understand by the e.m.f. of a cell? Ans. Electromotive force i.e., e.m.f. of a cell is the potential difference across the terminals of the cell when the cell is in an open circuit i.e., when no current is drawn from the cell. What is a potentiometer? Ans. It is an instrument used to measure potential difference or e.m.f. of a cell. Why is it called a potentiometer? Ans. Because it measures potential difference between any two points of electric current. What is the principle of a potentiometer? Ans. It works on the principle that for a constant current, fall of potential along a uniform wire is directly proportional to its length. What is potential gradient? Ans. It is the fall of potential per unit length of the potentiometer wire. How does the potential gradient vary along the length of the wire from end P to end Q? Ans. Potential gradient is same throughout because the wire has uniform cross-section material density and constant current. What kind of source of e.m.f. should be used as auxiliary battery? Ans. The e.m.f. of the source must be steady. A freshly charged accumulator or battery eliminator should be used for this purpose. What should be the order of magnitude of the e.m.f. of the auxiliary battery? Ans. The e.m.f. of the auxiliary battery should be slightly greater than the e.m.f. of the individual cells. (With battery of lesser e.m.f., null point will not be obtained on the potentiometer wire). Why do we use a rheostat in the battery circuit? Ans. To vary the potential gradient.

8

COMPREHENSIVE PHYSICS ACTIVITIES VOL. II—XII

10.

11.

12.

13.

14.

15.

16.

What purpose is served by varying the potential gradient? Ans. A lower potential gradient gives more length of wire upto null point. Accuracy becomes more. On what factor does the potential gradient depend? Ans. Potential gradient depends directly on the strength of the current and resistance per cm of the wire. What is the preferred material used for making potentiometer wires? Ans. Manganin. It is characterised by a low temperature coefficient of resistance and a high resistivity. Why do we want the material of the potentiometer wire to have a low temperature coefficient of resistance? Ans. There is invariably some heating of the potentiometer wire when a current flows through it. A material with a low temperature coefficient ensures that its resistance does not change much because of this heating. Why don’t we use a copper wire as a potentiometer wire? Ans. Copper has a low resistivity and hence a copper wire will have a low resistance. There would then be no appreciable potential drop across the ends of the potentiometer wire. Copper, also, has high temperature coefficient of resistance. Which materials can be used for making potentiometer wire? Ans. The alloys such as manganin, constantan nichrome, Eureka are used. What do you mean by sensitivity of a potentio-meter? Ans. Sensitivity of a potentiometer is the smallest potential difference that it can measure.

1 K where K = potential gradient S = sensitivity. Why is a ten-wire potentiometer more sensitive than a four-wire one? Ans. The potential gradient, under same conditions, decreases with an increase in the length of the potentiometer wire. Hence, a S=

17.

10-wire potentiometer (having a smaller potential gradient) is more sensitive than a

l . V How will you know that the apparatus can give a null point? Ans. The jockey is put at the two ends of the potentiometer wire. The deflection in the galvanometer must be in the opposite directions. What will you conclude if the deflection of the galvanometer is in the same direction at both the ends? Ans. The reasons may be (i) the positive terminals of all the cells are not connected at one point. (ii) the potential difference between the ends of the wire is less than the e.m.f. of the cell which is to be measured. How are above situations corrected? Ans. (i) Connections of positive terminals are checked. (ii) Current in potentiometer wire is increased. Under what conditions galvanometer will give no deflection when jockey is put on the wire? Ans. The reason may be (i) the cell whose e.m.f. is being measured, is totally damaged to have infinite internal resistance. (ii) connecting wire in the galvanometer circuit may be broken. Under what conditions deflection in the galvanometer is shaky? Ans. The reason may be (i) the e.m.f. of the battery or the cells may be fluctuating. (ii) the circuit has a loose contact somewhere. Why should we use a sensitive galvanometer? Ans. A sensitive galvanometer will respond to even a small departure from the exact balance point and will hence enable us to locate the balance point with greater precision. Why do we need a protective series resistance/shunt along with a sensitive galvanometer? 4-wire one. S =

18.

19.

20.

21.

22.

23.

24.

9

VIVA VOCE

25.

26.

27.

28.

Ans. To prevent it from damage from the flow of excessive currents that may exist when the jockey is far from the balance point. Does the use of a series protective resistance/shunt effect the location of the balance point? Ans. No; however, it makes the galvanometer less sensitive. We therefore, remove it once we are near the balance point. Why do we not want the balance point to be on the first wire, say? Ans. The smaller is the balancing length, the greater is the relative uncertainty in its location. What is the merit of a potentiometer over a voltmeter in measurement of e.m.f. of a cell? Ans. E.M.F. measured by potentiometer is more accurate because the cell is in open circuit, giving no current. How will you determine specific resistance of potentiometer wire material? Ans. We measure V across a known length l of the wire. We measure diameter D of wire and current I through it. Then R = ρl

2

πD /4

29.

30.

31.

=

V . From which ρ can be calculated. I

What do you mean by internal resistance of a cell? Ans. It is the resistance offered by the electrolyte and electrodes to the flow of current. Is there any change in the internal resistance of cell in open and closed circuit ? Ans. It remains the same. On what factors does the internal resistance of a cell depend? Ans. Internal resistance of a cell depends upon : (i) distance between electrodes in electrolyte and is directly proportional to it (ii) facing surface area of the electrodes in electrolyte and is inversely proportional to it (iii) nature of electrolyte and is inversely proportional to its specific conductivity. (iv) nature of electrodes materials and decreases with increase of conductivity.

32.

33.

34.

35.

36.

37.

38.

39.

Does the internal resistance depend on the current drawn from the cell? Ans. Yes, the internal resistance usually increases as more current is drawn from the cell. Can we find the internal resistance of an accumulator or secondary cell? Ans. No, the internal resistance of an accumulator is so small (≈ 0.01 Ω) that this method cannot be used. Why a cell should not be disturbed during experiment? Ans. Disturbing of the cell may change the factors (Q. 31 above) on which the internal resistance of the cell depends. What other measurements can be made by a potentiometer? Ans. A potentiometer can be used for measuring small thermo e.m.f. It can also be used for calibrating voltmeter and ammeter. Can you measure e.m.f. by a voltmeter? Ans. No. The voltmeter measure the terminal potential difference of a cell because it draw some current V = E – Ir, when I ≠ 0, then V < E. Which voltmeters can be used to measure the e.m.f. of the cells. Ans. Electric voltmeter. Vaccum tube volt meter (VTVM) afters nearly infinite resistance. So the current drawn is minimum, nearly zero. These two voltmeter are act as ideal voltmeter. Is the terminal potential difference (V) and e.m.f. (E) of a cell different? Explain : Ans. Yes. (i) When the Cell is discharged (current is taken) V = E – Ir, (V < E) (ii) When the cell is charged V = E + Ir, (V > E) (iii) When the I = 0, or r = 0 or R → ∞, then V = E. Does the position of balance at null point mean no current through the potentiometer? Ans. No. the current always flow in potentiometer wire. These is no current in galvanometer because there is no current

10

40.

COMPREHENSIVE PHYSICS ACTIVITIES VOL. II—XII

drawn from the cell whose e.m.f. is to be measured or compared. Does the potentiometer is used to determine the internal resistance of (i) primary cell (ii) secondary cell? Ans. The potentiometer is used to determine the internal resistance of primary cell only but not secondary cell because of very small resistance (0.02 Ω).

41.

1.

2.

What are the factors on which the e.m.f. of a cell depends? Ans. (i) Nature of electrodes, (ii) Nature of electrolyte, (iii) concentration of electrolyte, (iv) Temperature of electrolyte.

42.

Experiments 7, 8, 9 Galvanometer

Why is a potentiometer preferred over a voltmeter for measuring the e.m.f. of cell? Ans. A potentiometer draws no current from the cell whose e.m.f. is to be measured. On the other hand, the voltmeter always some current. Thus e.m.f. measured by voltmeter will be slightly less than the e.m.f. measured by potentiometer.

3.

4.

5.

V = E – Ir 43.

Why do we prefer a potentiometer with a longer bridge wire? Ans. When the bridge wire is longer, the potential gradient is smaller. Smaller the potential gradient, more is the sensitivity of potentiometer wire.

44.

What are the factors on which internal resistance of a cell depends? Ans. (i) Nature of electrodes (ii) Nature of electrolyte (iii) Concentration of electrolyte (iv) Temperature of electrolyte (v) Distance between the electrodes (vi) The area of electrodes immersed in electrolyte.

45.

6.

Can we consider the potentiometer as an ideal voltmeter? Ans. Yes. At null point, the potentiometer does not draw any current. Hence it measure the e.m.f. The potentiometer is equivalent to an ideal voltmeter. V = E – Ir Let

I = 0, then V = E.

7.

8.

What is a galvanometer? Ans. It is a device (instrument) used for detecting feeble electric currents in circuits. What type of galvanometer is available in laboratories? Ans. The laboratory galvanometer is a Weston Galvanometer. It has a pivoted coil. Why the scale of galvanometer has zero in the middle? Ans. A needle at zero in the middle can deflect on both sides. Do we have positive and negative terminal in a galvanometer? Ans. No, a galvanometer has no positive and negative terminals. The pointer can deflect on either side. Why the divisions of a galvanometer scale are equally spaced? Ans. Deflection in pointer is proportional to the current passed. The number of divisions in deflection will be proportional to the current passed. I ∝ θ. Define figure of merit of a galvanometer. Ans. The quantity of current required to produce a deflection of one division in the galvanometer, is called figure of merit of the galvanometer. It is represented by the symbol k. Its unit is ampere per division. Define current sensitivity of a galvanometer. Ans. The deflection produced in the galvanometer when a unit current is passed through it, is called current sensitivity of the galvanometer. How are ‘figure of merit’ and ‘current sensitivity’ related to each other?

1 k Why is the galvanometer called a moving coil galvanometer? Ans. Because in this galvanometer, the coil moves (deflects), while the magnet remains fixed. Ans. They are reciprocal, S1 ∝

9.

11

VIVA VOCE

10.

11.

Is there any moving magnet galvanometer? Ans. Yes, the tangent galvanometer is called a moving magnet galvanometer. Why is tangent galvanometer, called a moving magnet galvanometer? Ans. Because in tangent galvanometer, the magnet (a small pivoted magnetic needle) moves (deflects), while the coil remains fixed.

21.

Voltmeter 22.

Resistance of a Galvanometer 12.

13.

14.

15.

What do you mean by resistance of a galvanometer? Ans. The resistance of the coil of a galvanometer, is called the resistance of the galvanometer. It is represented by the symbol G. How do you determine the resistance of a galvanometer? Ans. The resistance of a galvanometer is determined by half-deflection method. Why is this method called half deflection method ? Ans. It is so because the deflection is made half by using a shunt resistance S. Under what conditions, G = S? Ans. G = S, only when series resistance R is very high.

23.

24.

25.

26.

Ammeter 16.

17.

18.

19.

20.

What is an ammeter? Ans. An ammeter is a device (instrument) for measuring large electric currents in circuits. How is an ammeter used in a circuit? Ans. An ammeter is used in series in a circuit. Why is an ammeter used in series in a circuit? Ans. The whole current to be measured is passed through it. What are the required properties of an ammeter? Ans. An ammeter must have a very small resistance (zero, if possible) and a large current carrying capacity. Why should an ammeter have a very small resistance?

Ans. So that when put in series in circuit, it should not reduce much the original current to be measured. Why should an ammeter have a large current carrying capacity? Ans. So that it may measure large currents.

27.

What is a voltmeter? Ans. A voltmeter is a device (instrument) for measuring electric potential difference between two points in a circuit. How is a voltmeter used in a circuit? Ans. A voltmeter is used in parallel with that branch of circuit at the ends of which the potential difference is to be measured. Why is a voltmeter used in parallel in a circuit? Ans. The potential difference to be measured is maintained at the terminals of the voltmeter. What are the required properties of a voltmeter? Ans. A voltmeter must have a very large resistance (infinite, if possible) and a very small current carrying capacity. Why should a voltmeter have a very large resistance? Ans. So that when put in parallel in circuit, it should not divert much current from parallel branch. Why should a voltmeter have a very small current carrying capacity? Ans. So that it may not withdraw much current from parallel branch of the circuit.

Conversion of a Galvanometer into an Ammeter 28.

29.

Why is a galvanometer not suitable to work as ammeter? Ans. A galvanometer has more resistance and less current-carrying capacity from those required by an ammeter. For large current, the galvanometer will damage. How is a galvanometer converted into an ammeter? Ans. A galvanometer is converted into an ammeter by connecting a low resistance in

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