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MCQs: Fuel

The Sankey diagram below represents a petrol engine.

If the energy put into the engine is 200 MJ, what is the efficiency of the engine?

Efficiency = (useful work out/energy in) x 100% = (120/200) x 100

The Sankey diagram below represents a petrol engine.

If the amount of energy going into the engine is 200 MJ, how much will be converted to useful work?

The width of the arrow represents the amount of energy.
The width of the petrol arrow is 10 units and it represents 200MJ so 1 unit = 20 MJ
The width of the mechanical energy arrow is 6 units so represents 120 MJ

A Mars bar contains 1000 kJ of energy.

The number of times you could carry a 25 kg suitcase up the stairs of a 200 m high building for the energy in one Mars bar is:

PE gained = mgh = 25 x 10 x 200 = 50 kJ
Number of times for one Mars bar = 1000/50 = 20 times.

Don't worry though. Our bodies are not that efficient, so we would actually use a lot more energy than calculated.
We would also have to move our own bodies up the building so that would make the total mass more like 100 kg.

Which of the following forms of electricity production does not use energy that originates from the sun?

Hydro is based on the water cycle which is driven by the sun, as are winds and waves. The energy released when a nucleus splits was put into the nucleus when it was made at the edge of a supernova or something like that, not the sun.

Which of the following is not a consequence of the second law of thermodynamics?

Although energy cannot be created or destroyed, this is not a consequence of the second law.

Which of the following fuels has the highest energy density?

 

Not really a fair comparison since diesel is refined, but it has 46 MJ kg-1 compared to 33 MJ kg-1 for its nearest rival (coal).

Which of the following types of wood has the highest energy density?

This was a special question for Norwegians who should know that birch is the best wood for burning on the "Peis".

Which of the following is not a fuel?

Although you could argue that the sun does contain fuel, we don't burn bits of the sun to produce heat. So the sun isn't a fuel.

Some of the IB questions really are this easy (but not all of them).

Which of the following statements is not true about energy?

The first three are obviously true but the fourth one is dubious. If there is friction then some of the work done on B will be converted to thermal energy, so B will not gain as much energy as A loses.

Which of the following statements is the best definition of work?

The normal definition is force x distance moved in the direction of the force but this is the same as distance x component of force in direction of motion.

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