NDA (II) 2017 General Ability Test · Question 112 of 150

Correct — C, Energy can neither be created nor destroyed. This is the law of conservation of energy, one of the most thoroughly tested statements in physics. Energy changes form endlessly — chemical energy in fuel becomes heat and then mechanical work in an engine, electrical energy becomes light and heat in a lamp, potential energy becomes kinetic energy in a falling body — but the total in an isolated system stays the same. When energy appears to vanish, it has been converted into a form that is harder to see, usually heat spread thinly into the surroundings by friction or resistance.

• (a)Energy can be created as well as destroyed. — If energy could be created out of nothing a perpetual-motion machine would work, and none has ever been built. Every claimed case of energy appearing has turned out to be energy arriving from somewhere unnoticed.
• (b)Energy can be created but not destroyed. — This halves the law and keeps the wrong half. Creation is exactly what conservation forbids; a system's total energy can only change if energy crosses its boundary.
• (d)Energy cannot be created but can be destroyed. — This is the intuition that comes from watching a rolling ball stop or a battery run down. Nothing was destroyed in either case — friction turned the kinetic energy into heat, and the chemical energy of the cell went out as electrical energy and warmth.

Conservation of energy states that the total energy of an isolated system is constant; energy can be transferred from one body to another and transformed from one kind into another, but the books always balance. The same principle appears in disguise all over the syllabus. Kirchhoff's loop rule, that the sum of potential differences around a closed circuit is zero, is conservation of energy applied to charge going round a loop. The work-energy theorem and the constancy of mechanical energy in the absence of friction are the same law in mechanics.

Two refinements are worth carrying. First, the law is stated for an isolated system; a system that exchanges energy with its surroundings can gain or lose energy, and the total across system and surroundings is what stays fixed. Second, in nuclear reactions mass and energy are interconvertible under the relation between them, so the modern statement conserves mass-energy together rather than each separately. At school level the parallel law for matter is stated separately as the law of conservation of mass, that matter can neither be created nor destroyed in a chemical reaction.

• The law of conservation of energy holds for an isolated system: the total energy stays constant while its forms change.
• Energy that seems to disappear in friction, resistance or collisions has usually become heat dispersed into the surroundings.
• Kirchhoff's loop rule, that the sum of the potential differences around a closed loop is zero, is a consequence of energy conservation.
• In nuclear processes mass and energy are interconvertible, so the conserved quantity is mass-energy taken together.
• The corresponding chemical law is the law of conservation of mass — matter is neither created nor destroyed in a chemical reaction.

• Reading energy lost to friction as energy destroyed.
• Applying the law to a system that is exchanging energy with its surroundings without accounting for the exchange.
• Mixing this law with the law of conservation of mass, which is the chemistry statement about matter.

NDA asks for the correct statement of the law, or supplies a device and asks which energy conversion it performs.

• practice — not a real PYQ

  In an electric bulb, electrical energy is converted mainly into

  • (a)chemical and sound energy
  • (b)light and heat energy
  • (c)nuclear and light energy
  • (d)mechanical and chemical energy

  Answer(b) light and heat energy — in a filament lamp most of it leaves as heat, which is why such lamps are inefficient.

• practice — not a real PYQ

  A ball rolling on the ground gradually comes to rest. Its kinetic energy has

  • (a)been destroyed
  • (b)been converted into heat by friction
  • (c)been converted into potential energy
  • (d)become mass

  Answer(b) been converted into heat by friction — the energy is dispersed into the surface and the air, not destroyed.