21.

If n₁, n₂ and n₃ are the fundamental frequencies of three segments into which a string is divided, then the original fundamental frequency n of the string is given by,

• 

• 

• 

• 

The number of possible natural oscillations of air column in a pipe closed at one end of length 85 cm whose frequencies lie below 1250 Hz are (velocity of sound = 340 m/s)

• 4

• 5

• 7

• 7

A speeding motorcyclist sees traffic jam ahead of him. He slows down to 36 km/hr. He finds that traffic has eased and a car moving ahead of him at 18 km/hr is honking at a frequency of 1392 Hz. If the speed of sound is 343 m/s, the frequency of the honk as heard by him will be,

• 1332 Hz

• 1372 Hz

• 1412 Hz

• 1412 Hz

Two thin dielectric slabs of dielectric constants K₁ and K₂ (K₁ < K₂) are inserted between plates of a parallel plate capacitor, as shown in the figure. The variation of electric field E between the plates with distance d as measured from plate P is correctly shown by:

A conducting sphere of radius R is given a charge Q. The electric potential and the electric field at the centre of the sphere respectively are:

• zero and 

• 

• 

• 

In a region, the potential is represented by V (x,y,z) = 6x - 8 xy - 8y + 6yz, where V is in volts and x, y, z are in metres. The electric force experienced by a charge of 2 coulomb situated at point (1,1,1) is,

• 6√5 N

• 30 N

• 24 N

• 24 N

The resistances in the two arms of the meter bridge are 5 ohm and R ohm, respectively. When the resistance  R is shunted with an equal resistance, the new balance point is at 1.6 l₁. The resistance R is,

• 10 ohm

• 15 ohm

• 20 ohm

• 20 ohm

A potentiometer circuit has been set up for finding the internal resistance of a given cell. The main battery, used across the potentiometer wire, has an emf of 2.0 V and a negligible internal resistance. The potentiometer wire itself is 4 m long. When the resistance R, connected across the given cell, has values of i) infinity ii) 9.5 ohm the balancing lengths on the potentiometer wire are found to be 3 m and 2.85 m, respectively. The value of internal resistance of the cell is,

• 0.25 ohm

• 0.95 ohm

• 0.5 ohm

• 0.5 ohm

Following figures show the arrangement of bar magnets in different configurations. Each magnet has magnetic dipole moments m. Which configuration has highest net magnetic dipole moment? 

In an ammeter 0.2 % of the main current passes through the galvanometer. If resistance of galvanometer is G, the resistance of ammeter will be,

• 1/499 G

• 499/500 G

• 1/500 G

• 1/500 G