Detailed Instructions: Given a context passage, generate a question from the passage such that its answer is shortest continous span from the passage.
Problem:Net energy is the amount of useable energy available from a resource after subtracting the amount of energy needed to make the energy from that resource available. For example, every 5 barrels of oil that are made available for use require 1 barrel for extracting and refining the petroleum. What is the net energy from this process? About 4 barrels (5 barrels minus 1 barrel). What happens if the energy needed to extract and refine oil increases? Why might that happen? The energy cost of an energy resource increases when the easy deposits of that resource have already been consumed. For example, if all the nearshore petroleum in a region has been extracted, more costly drilling must take place further offshore (Figure 1.1). If the energy cost of obtaining energy increases, the resource will be used even faster. Offshore drilling is taking place in deeper water than before. It takes a lot of energy to build a deep drilling platform and to run it. The net-energy ratio demonstrates the difference between the amount of energy available in a resource and the amount of energy used to get it. If it takes 8 units of energy to make available 10 units of energy, then the net-energy ratio is 10/8 or 1.25. What does a net-energy ratio larger than 1 mean? What if the net-energy ratio is less than 1? A net-energy ratio larger than 1 means that there is a net gain in usable energy; a net-energy ratio smaller than one means there is an overall energy loss. Table 1.1 shows the net-energy ratios for some common energy sources. Energy Source Solar Energy Natural Gas Petroleum Coal-fired Electricity Net-energy Ratio 5.8 4.9 4.5 2.5-5.1 Notice from the table that solar energy yields much more net energy than other sources. This is because it takes very little energy to get usable solar energy. Sunshine is abundant and does not need to be found, extracted, or transported very far. The range for coal-fired electricity is because of the differing costs of transporting the coal. What does this suggest about using coal to generate electricity? The efficiency is greater in areas where the coal is locally mined and does not have to be transported great distances (Figure 1.2). Obtaining coal for energy takes a lot of energy. The coal must be located, extracted, refined, and transported. Because so much of the energy we use is from fossil fuels, we need to be especially concerned about using them efficiently. Sometimes our choices affect energy efficiency. For example, transportation by cars and airplanes is less energy-efficient than transportation by boats and trains.
Solution:
what is the amount of useable energy available from a resource after subtracting the amount of energy needed to make that energy available?