Explore topic-wise InterviewSolutions in .

Correct option is (a) 450 OHMS

To elaborate: POTENTIAL gradient along the potentiometer wire = \(\FRAC {potential \, difference \, along \, wire}{length \, of \, wire}\).

0.5 × 10^-3 = I × \(\frac {50}{1000}\)

I = 0.5 × 10^-3 × \(\frac {1000}{50}\)

I = \(\frac {1}{100}\)

So, \(\frac {5}{(50 + R)} = \frac {1}{100}\)

R + 50 = 500

R = 450 ohms

Therefore, the resistance unplugged in the box is 450 ohms.

Correct option is (c) 0.5 × 10^-1 V/m

The explanation: RESISTANCE of a wire = \(\frac {Ꝭl}{A}\), where Ꝭ is the SPECIFIC resistance of the MATERIAL of the wire.

Potential gradient = \(\frac {V}{L} \rightarrow \frac {IR}{l}\) → \(\frac {I(\frac {Ꝭl}{A})}{l}\)

= \(\frac {Ꝭl}{A}\)

= 0.5 × \(\frac {10^{-9}}{10^{-8}}\)

= 0.5 × 10^-1 V/m

Therefore, the potential gradient is equal to 0.5 × 10^-1 V/m.

Correct choice is (d) 4 ohms

To ELABORATE: l1= 120 CM; l2 = 60 cm; R = 4 ohms

Internal resistance (r) = \([ \frac {(l2 – l1)}{I_2} ]\) × R

= \([\frac {(120 – 60)}{60} ]\) × 4

= \(\frac {60}{60}\) × 4

= 4 ohms

Therefore, the internal resistance is 4 ohms.

Correct option is (B) 10 OHMS

The explanation is: LET × be the unknown resistance.

First case→\(\frac {5}{x} = \frac {l}{(150 – l)}\)

750 – 5l = xl ……….. 1

Second case→\(\frac {x}{5} = \frac {(l + 50)}{(100 – l)}\)

100 – xl = 5l + 250

xl = 100x – 5l – 250 ………… 2

Comparing 1 and 2

750 – 5l = 100x – 5l – 250

100x = 750 + 250

100x = 1000

x = 10 ohms

Therefore, the unknown resistance is 10 ohms.

Right answer is (a) No change

Best explanation: For a balanced Meter BRIDGE \(\frac {P}{Q} = \frac {x}{(100-x)}\). From this, we can UNDERSTAND that the null deflection of GALVANOMETER does not DEPEND on the radius of the wire. So, even if the radius of the wire is TRIPLED, the null deflection of the galvanometer undergoes no change.

Right choice is (d) Wheatstone bridge is susceptible to high dc CURRENT

To explain: The false statement is Wheatstone bridge is susceptible to high dc current. It is actually not susceptible to high dc current and can give inaccurate READINGS if not BALANCED. The Wheatstone bridge measures resistance from few OHMS tomega ohms.All the other statements are valid.

Right answer is (d) While traversing in a closed loop, if POSITIVE POLE of cell is encountered first, then the emf is positive

To EXPLAIN: According to sign CONVENTION while traversing a closed loop (in clockwise or anti-clockwise direction), if positive pole of the cell is encountered first then its emf is NEGATIVE or else, it will be positive.

Right option is (d) 36

To EXPLAIN: Let P be the SMALLER resistance and Q be the bigger resistance.

First case→ \(\frac {P}{Q} = \frac {20}{80} = \frac {1}{4}\)

Second case→\(\frac {(P+15)}{Q} = \frac {40}{60} = \frac {2}{3}\)

Comparing both→\(\frac {P}{(P + 15)} = \frac {1}{4} \, \TIMES \, \frac {3}{2} = \frac {3}{8}\)

8P = 3P + 45→5P = 45→P = 9 ohms

Therefore, substituting in \(\frac {P}{Q} = \frac {1}{4}\) → \(\frac {9}{Q} = \frac {1}{4}\) →Q = 36 ohms.

The correct option is (C) KIRCHHOFF’s 2^nd law is applied at nodes

For explanation I would say: Kirchhoff’s 2^nd year is applied in a closed loop. Kirchhoff’s 2^nd law supports the law of conservation of ENERGY. This MEANS that energy is NEITHER created nor destroyed in the closed loop. Whatever energy enters the loop, same amount leaves the loop.

The correct OPTION is (b) False

The explanation: No. A Wheatstone bridge is an electrical device used to measure unknown resistance by placing it in one of the branches and BALANCING the TWO legs of the bridge. If R is the unknown resistance, S is varied until the galvanometer shows null deflection, whereas P and Q are fixed. This is the balanced condition. So the EQUATION is \(\frac {P}{Q} = \frac {R}{S}\)→PS = QR. ALSO, in the balanced condition, no current passes through the galvanometer.

The correct ANSWER is (b) It measures internal resistance

The explanation is: POTENTIOMETER is an electric INSTRUMENT used to measure internal resistance, electromotive FORCE (emf), and it is also used for comparing the EMFS of different cells. Potentiometer can be used as a variable resistor as well.

The correct answer is (a) True

Easy EXPLANATION: If we have to shift the BALANCE point of a potentiometer to a HIGHER length, the potential gradient of the wire is to be DECREASED. This can also be achieved by increasing the resistance in series with the potentiometer wire, and this is POSSIBLE by decreasing the current of the main circuit. So, this is a true statement.

The correct option is (c) Galvanometer

For explanation: A galvanometer is used as the null detector in a WHEATSTONE bridge. The null point means the situation in which no current flows through the CIRCUIT. The galvanometer is used for measuring the current and also to determine the voltage between any TWO points of the circuit. Galvanometer is used DUE to its sensitivity and therefore, even SMALL currents can be measured.

Right choice is (b) 4

The BEST explanation: Wheatstone bridge is an arrangement of four resistors P, Q, R, and S, such that if we know the value of the RESISTANCES of any three of them, we can obtain the value of fourth unknown RESISTANCE. Therefore, there are 4 resistances in a Wheatstone bridge.

Correct answer is (b) False

The BEST I can explain: Kirchhoff’s law are not applicable in the presence of a magnetic FIELD. This is because, Kirchhoff’s law is based on the ASSUMPTION that magnetic fields do not exist in closed loops. Therefore, this law cannot be used when there are time VARYING magnetic fields.

The correct answer is (c) End error

The best I can explain: End error is removed when the KNOWN and unknown resistances are interchanged in a METER BRIDGE. This ADDITIONAL length has a resistance known as end resistance. So, when an end error arises, it can be removed by interchanging the known and unknown resistances and taking the MEAN of the resistances determined.

The correct ANSWER is (a) True

The best explanation: Yes, the sensitivity of the meter bridge is at the PEAK when all resistors have the same order. The sensitivity can be increased by keeping the CURRENT in the GALVANOMETER HIGH and this can be achieved by reducing the values of the resistors used in the Meter Bridge.

Correct choice is (a) KIRCHHOFF’s second LAW

Explanation: The equation → ∑e = ∑IR is applicable to Kirchhoff’s second law. This law is also KNOWN as Kirchhoff’s loop rule. This expression TELLS us that in a closed loop, the algebraic SUM of the emfs is equal to the algebraic sum of the products of the resistance and currents flowing through them.

The correct choice is (a) Kirchhoff’s Law

Easy EXPLANATION: Kirchhoff’s Law is used to analyze circuits. This law is important because they represent CONNECTIONS of a circuit. Kirchhoff’s Law PROVIDE the constraints that let US find the current FLOWING and voltage across every circuit element.

Correct OPTION is (a) True

Explanation: Kirchhoff’s first law or Kirchhoff’s junction law supports law of conservation of charge. According to this law, whatever charges are PASSING towards the junction is EQUAL to the charges leaving the junction. As a result, the charges are CONSERVED, thereby, supporting the law of conservation of charge.

The correct choice is (a) 18 hp

Easiest explanation: CURRENT used = 30 A; Resistance = 15 ohms

The required EQUATION is: POWER = current^2 × resistance

= 30 × 30 × 15

= 13,500 Watts

We know that, 1 Watt = 746 horse power (hp). So, 13,500 Watts = 18.096 hp, which can APPROXIMATELY be equal to 18 hp.

Therefore, the engine consumes 18 hp power.

The correct ANSWER is (c) ACCORDING to Kirchhoff’s law, the current flowing towards a junction is equal to the current LEAVING the junction

The explanation: According to Kirchhoff’s first law, the current flowing towards the junction is equal to the current leaving the junction. Mathematically, this law can be expressed as ∑^nK – 1 IK = 0 (where n is the number of branches CARRYING current towards or away from the junction)

The CORRECT choice is (b) 256^oC

Easy EXPLANATION: α=\(\frac {(R_2-R_1)}{R_2(T_2-T_1)}\).

α=\(\frac {(2R_0-R_0)}{R_0(T-0)} = \frac {1}{T}\).

T=\(\frac {1}{\alpha}\)

= \(\frac {1}{3.9}\) X 10^-3 ^oC^-1

= 256^oC.

Therefore, the REQUIRED temperature is 256^oC.

The correct answer is (d) Drift velocity decreases with increasing TEMPERATURE

For explanation I would say: On increasing the temperature of a CONDUCTOR, the value of resistivity of its material increases. Resistivity is indirectly PROPORTIONAL to drift velocity. THEREFORE, the drift velocity of electrons decreases with the increasing temperature of the conductor.

The CORRECT answer is (c) The total emf increases by 2e

Explanation: When one CELL is wrongly connected in series of n IDENTICAL cells, each of emf e, it will reduce the total emf by 2e. So, effective emf is CALCULATED as eEFF = ne – 2e. This happens in case of mixed grouping.

The correct option is (a) 47 × 10^1Ω ± 5%

The EXPLANATION is: A color CODE is used to indicate the resistance value of a carbon RESISTOR and its PERCENTAGE accuracy. The corresponding value of resistance for the given color code is 47 × 10^1Ω ± 5% (Yellow-4; Violet-7; Brown-1; Gold-5%).

Correct OPTION is (a) Am^-2, [A^1L^-2]

The best EXPLANATION: Current density is the amount of charge flowing per second through a unit area.

The SI unit of current density is AMPERE per SQUARE METRE (Am^-2) and its dimensions are [A^1L^-2].

The CORRECT OPTION is (C) Silver

To explain: Silver is one of the most electrically conductive ELEMENT. This is because electrons can MOVE more freely in silver than any other elements. Silver also has the lowest contact resistance of any metal.

Correct answer is (b) Conductors

The EXPLANATION: When an electric FIELD is APPLIED ACROSS the ends of conductors, the electrons begin to MOVE under the action of the electric field. The flow of electrons constitutes electric current through the conductor.

Correct answer is (c) 32

The explanation is: The given ratio of radii = 5:3; \(\frac {R2}{R1} = \frac {5}{3}\) → R2 = \((\frac {5}{3})\) R1

R1 = 12 ohms (given); R2 = \(( \frac {5}{3} )\) × 12 = 20 ohms. So, R1 = 12 ohms and R2 = 20 ohms

Therefore, total resistance (R) = R1 + R2 (since they are COMBINED in series)

= 12 + 20

= 32 ohms

Thus, the total resistance of the combination is 32 ohms.

The correct choice is (b) \(\frac {10}{3}\)

EASY explanation: Equivalent resistance = 5 + 5 + \(\frac {1}{5}\) (since first two are in series, and they are in parallel to the third in case of a triangular arrangement)

 \(\frac {1}{R}\) = 10 + \(\frac {1}{5} = \frac {(5 + 10)}{5 \times 10} = \frac {15}{50} = \frac {3}{10}\)

THUS, R = \(\frac {1}{\frac {3}{10}} = \frac {10}{3}\) ohms

Therefore, the equivalent resistance is \(\frac {10}{3}\) ohms.

The correct answer is (a) Conductance, OHM^-1

Easy explanation: The conductance of a MATERIAL is the ease with which electric charges flow through it.

Conductance=\(\frac {1}{Resistance}\).

SI UNIT of conductance=\(\frac {1}{ohm}\)

=ohm^-1.

The correct choice is (a) CURRENT that flows from lower POTENTIAL to higher potential

Explanation: The current that flows from a point at the lower (NEGATIVE) potential to a point at higher (positive) potential is called electronic current. Electronic current is produced by the movement of NEGATIVELY charged ELECTRONS.

The correct answer is (d) [ML^2T^-3A^-1]

The best I can explain: Electromotive FORCE (emf) is defined as the POTENTIAL difference between the TWO terminals of a cell in an OPEN circuit. The SI unit of emf is \(\frac {joule}{coulomb}\) or volt. So, its dimensional formula is [ML^2T^-3A^-1].

The correct option is (a) True

To elaborate: Yes, electric energy is dependent on TIME.

Electric energy = electric power × time = P × t. Electric energy is BASICALLY the energy derived from electric charge. It can be calculated by multiplying the electric power of the body with the time TAKEN for the power emission.

Right option is (c) Ohmic conductors

Easy explanation: The conductors which obey Ohm’s LAW are called Ohmic conductors. The linear RELATIONSHIP between voltage and CURRENT for these conductors hold GOOD.

The correct OPTION is (d) 7

To explain I would say: Number of cells (n) = 36; Emf of each CELL (e) = 4V; Total emf (E) = 88V;

Let the number of REVERSED cells be ‘y’

The required equation: EEFF = n × e – 2y × e

88 = 36 × 4 – 2y × 4

88 = 144 – 8y

8y = 56

y = 7

Therefore, there are 7 reversed cells.

Right answer is (B) 1V

Easiest explanation: Current = 3 A; TIME TAKEN = 5 minutes = 300 seconds

Work DONE = 900 J; Power = \(\frac {Work \, done}{Time \, taken} = \frac {900}{300}\) = 3 W

Power = Voltage (emf) x Current→Emf = \(\frac {Power}{Current} = \frac {3}{3}\) = 1V

Therefore, the emf of the source is 1 volt.

Right choice is (d) Potentiometer

Easy explanation: Potentiometer is the more accurate device to measure emf than the other ones such as MULTIMETER or voltmeter. Potentiometer is HIGHLY sensitive and thus, EVEN small emfs can be measured using this device. Moreover, potentiometers do not DRAW current from the CIRCUIT during measurements, like voltmeters.

Correct choice is (d) Silicon

Easy explanation: The resistivities of SEMICONDUCTORS lie between 10^-6 Ωm and 10^4 Ωm. A SEMICONDUCTOR material has an electrical CONDUCTIVITY VALUE falling between that of a conductor, such as METALLIC aluminum, and an insulator, such as wood. Silicon and germanium are typical semiconductors.

The correct ANSWER is (C) 57 × 10^2Ω ± 20%

For explanation I WOULD say: A color CODE is used to indicate the resistance value of a carbon resistor and its percentage accuracy. The corresponding value of resistance for the given color code is 57 × 10^2Ω ± 20% (Green-5; Violet-7; Red-2; No fourth band-20%).

Right ANSWER is (d) Electromotive FORCE

For EXPLANATION I would say: To maintain steady current through the conductor, electromotive force must be provided by the external device to take positive charge from lower potential to higher potential i.e. in the DIRECTION OPPOSITE to that of the electric field.

Correct choice is (c) The third band indicates the third significant figure

To explain: The third band indicates the power of ten with which the first and second significant FIGURES MUST be MULTIPLIED to get the resistance value in OHMS. All the other statements are valid.

The correct choice is (c) 1:6

For EXPLANATION I would say: When the resistors are connected in series:

RS = 4 + 4 + 4 + 4 = 16 OHMS

They are connected to a cell of internal resistance of 2 ohms, so current (I1) = \(\frac {e}{R_S}\) + r= \(\frac {e}{6}\) + 2= \(\frac {e}{18}\)

When the resistors are connected in parallel:

\(\frac {1}{R_P} = \frac {1}{4} + \frac {1}{4} + \frac {1}{4} + \frac {1}{4}\) = 1→RP = 1

The current through the CIRCUIT (I2) = \(\frac {e}{1+2}\)

= \(\frac {e}{3}\)

Ratio of both the currents = \(\frac {I_1}{I_2} = \frac {\frac {e}{18}}{\frac {e}{3}} = \frac {1}{6}\)→1:6

The CORRECT OPTION is (b) Silicon

To explain I would say: The COEFFICIENT of resistivity is negative for semiconductors and their resistivity decreases with temperature. The relaxation time does not change with temperature but the NUMBER density of free electrons increases exponentially with the increase in temperature. CONSEQUENTLY, the resistivity decreases exponentially with the increase in temperature.