21.

If we study the vibration of a pipe open at both ends, then the following statements is not true

• open end will be anti-node

• Odd harmonics of the fundamental frequency will be generated

• All harmonics of the fundamental frequency will be generated

• All harmonics of the fundamental frequency will be generated

A source of unknown frequency gives 4 beats/s when sounded with a source of known frequency 250 Hz. The second harmonic of the source of unknown frequency gives five beats per second when sounded with a source of frequency 5.13 Hz. The unknown frequency is

• 250 Hz

• 246 Hz

• 240 Hz

• 240 Hz

Two pith balls carrying equal charges are suspended from a common point by strings of equal length, the equilibrium separation between them is r, Now the strings are rigidly clamped at half the height. The equilibrium separation between the balls now become.

A wire loop is rotated in a magnetic field. The frequency of change of direction of the induced emf is

• one per revolution

• twice per revolution

• four times per revolution

• four times per revolution

A coil of self -inductance L is connected in series with a bulb b and an AC source. Brightness of the bulb decreases when

• frequency of the AC source is decreases

• number of turns in the coil is reduced

• a capacitance of reactance X_c=X_L is  included in the same circuit

• a capacitance of reactance X_c=X_L is  included in the same circuit

The condition under which a microwave oven heats up a food item containing water molecules most efficiently is

• the frequency of the microwave must match the resonant frequency of the water molecules

• the frequency of the microwave has no relation  with natural frequency of water molecules

• microwave are heat waves, so always produce heating 

• microwave are heat waves, so always produce heating 

A, B and C are three points in a uniform electric field. The electric potential is 

 E

• Maximum at A

• Maximum at B

• Maximum at C

• Maximum at C

A wire of resistance 4Ω is stretched to twice its original length. The resistance of stretched wire would be

• 2Ω

• 4Ω

• 8Ω

• 8Ω

The internal resistance of a 2.1 V cell which gives a current of 0.2 A through of 0.2A through a resistance of 10Ω is 

• 0.2Ω

• 0.5Ω

• 0.8Ω

• 0.8Ω

The resistance of the four arms P, Q, R and S a Wheatstone's bridge are 10Ω, 30Ω and 90Ω respectively. The emf and internal resistance of the cell are 7V and 5Ω respectively. If the galvanometer resistance is 50Ω, the current drawn from the cell will be

• 1.0 A

• 0.2 A

• 0.1 A

• 0.1 A