Question

Which form of energy listed below is the most useful?(a) 1000 J of thermal energy in very hot gas in a cylinder closed by a moveable piston (b) 1000 J of thermal energy in cold gas in a cylinder closed by a moveable piston (c) 1000 J of thermal energy in water at $15^{\circ} \mathrm{C}$ in the rocks at the bottom of a waterfall (d) 1000 J of thermal energy in the water in a boiler at $700^{\circ} \mathrm{C}$ (e) 1000 J of chemical energy stored in the chemical bonds of a piece of wood

Solution

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$\text{\color{#4257b2}Multiple choice questions}$

$\text{\color{#4257b2}Heat engine:}$

Taking each statement in turn;

• (a) If not insulated from the environment, this system has the capacity to do some negative work on its surroundings at the expense of some of its thermal energy. The gas volume will contract at constant pressure. Since we have hot gas that is assumed to be at a higher temperature than the environment, a quantity of heat greater than the quantity of work, will transfer out the system;
• (b) System has the same capacity as in (a) to do work. Here the gas does positive work on its surroundings and again the greatest transfer of energy is by heating rather than doing work;
• (c) Since the water is at the base of the waterfall it has no gravitational potential energy, which is a useful form of energy. It only possesses thermal energy, a less useful form for doing work on another system;

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