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Questions received at Facebook Page

  1. An energy saver light bulb of 11 W gives the same illumination as a conventional 60W light bulb. (a) If a typical light bulb is used on average for five hours a day, how much energy is saved per year by one light-bulb? (b) If each household used eight such light-bulbs, ad there are 20 million households in a country, how much energy would that country save annually?
  2. A system consists of a gas inside a cylinder. Inside the cylinder there is a 10ohm resistor connected to an electrical circuit through which a current can pass. A mass of 0.1 kg is placed on the piston which drops by 0.2m and comes to rest. A current of 0.1 A is passed through the 10 ohm resistor for 2s. What is the change in the internal energy of the system when it has reached equilibrium?
  3. An explosive liquid at temperature 300 K contains a spherical bubbles of radius 5 mm, full of its vapour. When a mechanical shock to the liquid causes adiabatic compression of the bubble, what radius of the bubble is required for combustion of the vapour, given that the vapour ignites spontaneously at 1100 C? The ratio Cv/nR is 3 for the vapour.

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