21.

The fundamental frequency in an open organ pipe is equal to the third harmonic of a closed organ pipe. If the length of the closed organ pipe is 20 cm, the length of the open organ pipe is

• 13.2 cm

• 8 cm

• 16 cm

• 12.5 cm

The efficiency of an ideal heat engine working between the freezing point and boiling point of water, is

• 26.8%

• 20%

• 12.5%

• 6.25%

An electron falls from rest through a vertical distance h in a uniform and vertically upward directed electric field E. The direction of electric field is now reversed, keeping its magnitude the same. A proton is allowed to fall from rest in it through the same vertical distance h. The time of fall of the electron, in comparison to the time of fall of the proton is

• Smaller

• 5 times greater

• Equal

• 10 times greater

The electrostatic force between the metal plates of an isolated parallel plate capacitor C having a charge Q and area A, is

• Independent of the distance between the plates

• Linearly proportional to the distance between the plates

• Inversely proportional to the distance between the plates

• Proportional to the square root of the distance between the plates

Current sensitivity of a moving coil galvanometer is 5 div/mA and its voltage sensitivity (angular deflection per unit voltage applied) is 20 div/V. The resistance of the galvanometer is

• 40 Ω

• 25 Ω

• 500 Ω

• 250 Ω

A thin diamagnetic rod is placed vertically between the poles of an electromagnet. When the current in the electromagnet is switched on, then the diamagnetic rod is pushed up, out of the horizontal magnetic field. Hence the rod gains gravitational potential energy. The work required to do this comes from

• The current source

• The magnetic field

• The induced electric field due to the changing magnetic field

• The lattice structure of the material of the rod

An inductor 20 mH, a capacitor 100 µF and a resistor 50 Ω are connected in series across a source of emf, V = 10 sin 314 t. The power loss in the circuit is

• 0.79 W

• 0.43

• 1.13

• 2.74 W

A metallic rod of mass per unit length 0.5 kg m^–1 is lying horizontally on a smooth inclined plane which makes an angle of 30° with the horizontal. The rod is not allowed to slide down by flowing a current through it when a magnetic field of induction 0.25 T is acting on it in the vertical direction. The current flowing in the rod to keep it stationary is

• 7.14 A

• 5.98 A

• 11.32 A

• 14.76 A

A carbon resistor of (47 ± 4.7) kΩ is to be marked with rings of different colours for its identification. The colour code sequence will be

• Violet – Yellow – Orange – Silver

• Yellow – Violet – Orange – Silver

• Green – Orange – Violet – Gold

• Yellow – Green – Violet – Gold

A set of 'n' equal resistors, of value 'R' each, are connected in series to a battery of emf 'E' and internal resistance 'R'. The current drawn is I. Now, the 'n' resistors are connected in parallel to the same battery. Then the current drawn from battery becomes 10 I. The value of 'n' is

• 10

• 11

• 9

• 20