Sources of Energy — Questions & Answers

Sample questions from Sources of Energy

1. Q1. Solar cells are not yet widely used for domestic power supply mainly because

   • A.they cause serious air pollution while working
   • B.they need a continuous supply of fuel
   • C.they cannot be used in remote or inaccessible areas
   • D.special grade silicon is scarce and the silver used for interconnection makes them costly✓
   Solution

   Solar cells work silently without pollution or fuel, but the limited availability of special-grade silicon and the costly silver interconnections make the panels expensive.

2. Q2. A good source of energy is one that:

   • A.Gives a large amount of work per unit mass or volume, is easy to store and transport, and is safe and economical to use✓
   • B.Is the cheapest, whatever else may be true of it
   • C.Burns with the brightest flame
   • D.Is available only in one part of the country
   Solution

   A is correct: those four conditions together are what make a source worth using, and a source that fails any one of them is troublesome however plentiful it is. B takes one condition alone, and a cheap fuel that cannot be stored safely is of little use. C confuses the appearance of a flame with the energy released. D is a disadvantage rather than a merit.

3. Q3. In the given figure of a box-type solar cooker, the plane mirror lid is tilted at an angle instead of lying flat over the box. The temperature inside then

   • A.remains exactly the same, since the mirror does not absorb heat
   • B.falls, because the mirror blocks the sunlight
   • C.rises, because the mirror reflects additional sunlight into the box✓
   • D.rises, because the mirror itself becomes very hot and conducts heat inside
   Solution

   A correctly tilted plane mirror reflects extra solar radiation through the glass sheet into the box, so more energy enters and the inside temperature rises.

4. Q4. Among the following fuels, the one having the highest calorific value per kilogram is

   • A.Wood
   • B.Coal
   • C.Hydrogen✓
   • D.Kerosene
   Solution

   Hydrogen has the highest calorific value of all fuels, but its storage and transport are difficult and costly.

5. Q5. A plane mirror is fitted to a box solar cooker in order to:

   • A.Let the cook see inside
   • B.Reduce the temperature to a safe level
   • C.Reflect additional sunlight in the box, raising the temperature✓
   • D.Make the cooker look attractive
   Solution

   C is correct: the mirror is tilted so that it throws extra sunlight through the glass, which raises the temperature inside by several tens of degrees. A is not its function. B states the opposite of its effect. D is not a physical reason.

Other Class 10 Physics chapters