JEE physics Assertion & Reason

(b) The free surface of liquid tries to acquire a minimum area due to surface tension, hence liquid drop is spherical because sphere has minimum area than other shape.

(c) With increase in temperature of liquid its surface tension decreases so that it tends to acquire larger area. Hence hot soup having low value of surface tension spread properly on our tongue & provides better taste than cold soup.

(b) When a drop of liquid is poured on a glass plate, the shape of the drop is governed by two forces, the force of gravity. For very small drops, the potential energy due to gravity is insignificant compared to that due to surface tension. Hence, in this case the shape of the drop is determined by surface tension alone and drop becomes spherical.

(a) Since the excess pressure due to surface tension is inversely proportional to its radius, it follows that smaller the bubble, greater is the excess pressure. Thus, when the larger and the smaller bubbles are put in communication, air starts passing from the smaller into the large bubble because excess pressure inside the former is greater than inside the latter. As a result, the smaller bubble shrinks and the larger one swells.

(a) Zero surface tension means no opposition to expansion.

(b) We know that the intermolecular distance between the gas molecules is large as compared to that of liquid. Due to it the forces of cohesion in the gas molecules are very small and these are quite large for liquids. Therefore, the concept of surface tension is applicable to liquid but not to gases.

(c) With increase in temperature surface tension of the liquid decreases and angle of contact also decreases.

(e) The presence of impurities either on the liquid surface or dissolved in it, considerably affect the force of surface tension, depending upon the degree of contamination. A highly soluble substance like sodium chloride when dissolved in water increase the surface tension. But the sparing soluble or substance like phenol when dissolved in water reduces the surface tension of water.

(c) The two glass plates stick together due to surface tension.

(e) The soap solution, has less surface tension as compared to ordinary water and its surface tension decreases further on heating. The hot soap solution can, therefore spread over large surface area and also it has more wetting power. It is on account of this property that hot soap solution can penetrate and clean the clothes better than the ordinary water.

(c) When a liquid is sprayed, the surface area of the liquid increases. Therefore, work has to be done in spraying the liquid, which is directly proportional to the surface tension.Because on adding soap, surface tension of water decreases, the spraying of water becomes easy.

(a) The rise in temperature of the soft steel is an example of transferring energy into a system by work and having it appear as an increase in the internal energy of the system. This works well for the soft steel because it is soft. This softness results in a deformation of the steel under blow of the hammer. Thus the point of application of the force is displaced by the hammer and positive work is done on the steel. With the hard steel, less deformation occur, thus, there is less displacement of point of application of the force and less work done on the steel. The soft steel is therefore better in absorbing energy from the hammer by means of work and its temperature rises more rapidly.

(d) Work done in the motion of a body over a closed loop is zero only when the body is moving under the action of conservative forces (like gravitational or electrostatic forces). i.e. work done depends upon the nature of force.

(b) If two protons are brought near one another, work has to be done against electrostatic force because same charge repel each other. This work done is stored as potential energy in the system.

(c) When two bodies of same mass undergo an elastic collision, theirvelocitiesgetinterchanged after collision. Water and heavy water are hydrogenic materials containing protons having approximately the same mass as that of a neutron. When fast moving neutrons collide with protons, the neutrons come to rest and protons move with the velocity of that of neutrons.

(c) When we supply current through the cell, chemical reactions takes place, sochemicalenergy of cell is converted into electrical energy. If a large amount of current is drawn from wire for a long time only then wire get heated.

(a) From, definition, work done in moving a body against a conservative force is independent of the path followed.

(e) Work done and power developed is zero in uniform circular motion only.

(d) A body can have energy without having momentum if it possess potential energy but if body possess momentum then it must posses kinetic energy.Momentumandenergyhavedifferentdimensions.

(d) In an elastic collision both the momentum and kinetic energy remains conserved. But this rule is not for individual bodies, but for the systemofbodiesbeforeandafterthecollision.While collision in which there occurs some loss of kinetic energy is called inelastic collision. Collision in daily life are generally inelastic. The collision is saidtobeperfectlyinelastic, if two bodies stick to each other.

(e) Rate of change of momentum is proportional to external forces acting on the system. The total momentum of whole system remain constant when no external force is acteduponit.Internal forces can change the kinetic energy of the system.

(c) The gravitational force on the comet due to the sun is a conservative force. Since the work done by a conservative force over a closed path is always zero (irrespective ofthenature of path), the work done by the gravitational forces over every complete orbit of the comet is zero.

(a) The work done on the spring against the restoring force is stored as potential energy in both conditions when it is compressed or stretched.

(c) Change in kinetic energy = work done by net force.This relationship is valid for particle as well as system of particles.

(d) In a round trip work done is zero only when the force is conservative in nature.Force is always required to move a body in a conservative or non-conservative field