Exercise 7.4
For each of the following situations, identify the energy transformation that takes place: (i) a truck moving uphill, (ii) unwinding of a watch spring, (iii) photosynthesis in green leaves, (iv) water flowing from a dam, (v) burning of a matchstick, (vi) explosion of a fire cracker, (vii) speaking into a microphone, (viii) a glowing electric bulb, and (ix) a solar panel.
Not cross-checked
NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.
(i) Truck moving uphill: chemical energy of the fuel → kinetic energy of the truck + gravitational potential energy as it gains height (with some thermal energy lost to friction).
(ii) Unwinding of a watch spring: potential energy stored in the deformed spring → kinetic (mechanical) energy of the gears and hands.
(iii) Photosynthesis in green leaves: light energy from the Sun → chemical energy stored in food.
(iv) Water flowing from a dam: potential energy of the stored water → kinetic energy of the falling water (which is then used to generate electrical energy).
(v) Burning of a matchstick: chemical energy → thermal energy + light energy.
(vi) Explosion of a fire cracker: chemical energy → sound energy + light energy + thermal energy + kinetic energy of the flying pieces.
(vii) Speaking into a microphone: sound energy → electrical energy.
(viii) Glowing electric bulb: electrical energy → light energy + thermal energy.
(ix) Solar panel: light energy (solar) → electrical energy.