Parabolic surfaces are used when it is required to focus the sound at a particular point. Hence, many halls are designed with parabolic reflecting surfaces.

(i) Power is defined as the rate of doing work.
(ii) Power = Voltage x current
(iii) P = VI

Nuclear fusion can take place only under certain conditions. Nuclear fusion is possible only at extreme high temperature of the order of 10⁷ to 10⁹ K and a high pressure to push the hydrogen nuclei closer to fuse with each other. Hence, It is named as "Thermonuclear reaction".

The expansion of liquid apparently observed without considering the expansion of the container is called the apparent expansion of liquid.

Some substances when exposed to the atmospheric air at ordinary temperature, absorb moisture without changing their physical state. Such substances are called hygroscopic substances.

(i) To see distant objects, the ciliary muscle relaxes and makes the eye lens thinner. This increases the focal length of eye lens.
(ii) Hence, the distant object can be clearly seen. When we look at a closer object, the focal length of the eye lens is decreased by the contraction of ciliary muscle.
(iii) Thus, the image of the closer object is clearly formed on the retina.

True. The size of the anion is larger than the neutral atom and size of the cation is smaller than the neutral atom.

Dynamics: It is the study of moving bodies under the action of forces. Dynamics is further divided as follows.
(i) Kinematics deals with the motion of bodies without considering the cause of motion.
(ii) Kinetics deals with the motion of bodies considering the cause of motion.

(i) Resistor is used to fix the magnitude of the current through the circuit
(ii) Rheostat is used to fix the magnitude of the current through the circuit
(iii) Ammeter is used to measure the current.
(iv) Voltmeter is used to measure the potential difference.

(a) Atoms of different elements are very small in size and their actual mass are extremely small. For example, the mass of an atom of hydrogen is 1.67 x 10^-27 kg. To solve this problem, we consider the relative atomic masses of the elements. The relative atomic mass of hydrogen is 1 u and its corresponding gram atomic mass is 1g.
(b) If the atomic mass of an element is in fraction, this means that it exists in the form of isotopes. The atomic mass is the average atomic mass and is generally fractional.
(c) It represents the number of atoms in one gram atom of an element or the number of molecules in one gram mole of a compound. If we divide the atomic mass of an element by actual mass of its atom, the value is 6.022x10²³. Similarly, by dividing the molecular mass of a compound by the actual mass of its molecule, the same result is obtained.
(d) No, one gram mole of a gas occupies a volume of 22.4 L only under N.T.P conditions i.e. at 273 K temperature and under 760 mm pressure.