Question

A coil of inductance 300mH and resistance 2Ω is connected to a source of voltage 2V. The current
reaches half of its steady value in:
(A) 0.05 s
(B) 0.1 s
(C) 0.105 s
(D) 0.3 s

Answer 1

Hint:
A hydrometer is a sealed and hollow glass device. The hydrometer is used to measure specific
gravity. The principle of fluid displacement is used by the device to determine the quantity. This
quantity doesn’t have units and is just a ratio. Think of other names of specific gravity to find out the
correct answer.

Complete step by step answer:
The hydrometer is based on the concept of buoyancy. The specific density of liquids is determined
by a device called a hydrometer.
The hydrometer is gently lowered into a fluid that is poured in a tall container whose relative density
is to be tested until it floats by itself. The point at which the stem of the hydrometer touches the surface of the fluid is indicative of its relative density.
Therefore, a hydrometer is used to measure the relative density of a liquid.

Additional information:
The density of a substance is the mass of substance present in one unit volume of the substance. The
relative density is the ratio of the density of a given substance to the density of the of a given
reference material.
If a substance’s relative density is less than one then the substance is less dense than the reference
and if the relative density is greater than one then the substance is denser than the reference. If the
relative densities are equal then they both have equal densities.

Note:
The hydrometer works on Archimedes’ principle that is a solid placed in a liquid displaces some
liquid, the weight of the displaced liquid is equal to the weight of the submerged part of the solid.
The weight of the displaced liquid also equals the upward buoyant force. We can determine the
relative density of the fluid by observing the extent of the buoyancy.