题目

一矩形线圈在匀强磁场中绕垂直于磁场轴线匀速转动时产生的正弦式交变电流，其电动势的变化规律如图线a所示。当调整线圈转速后，电动势的变化规律如图线b所示。以下关于这两个正弦式电流的说法正确的是（ ）A. 图线a电动势的有效值为VB. 线圈先后两次转速之比为2：3C. 在图中t＝0时刻穿过线圈的磁通量均为零D. 图线b电动势的瞬时值表达式为e＝100sinπt（V）Against the supposition that forest fires in Alaska, Canada and Siberia warm the climate, scientists have discovered that cooling may occur in areas where burnt trees allow more snow to mirror more sunlight into space. This finding suggests that taking steps to prevent northern forest fires to limit the release of greenhouse gases may warm the climate in northern regions. Usually large fires destroyed forests in these areas over the past decade. Scientists predict that with climate warming, fires may occur more frequently over the next several centuries as a result of a longer fire season. Sunlight taken in by the earth tends to cause warming, while heat mirrored back into space tends to cause cooling. This is the first study to analyze all aspects of how northern fires influence climate. Earlier studies by other scientists have suggested that fire in northern regions speed up climate warming because greenhouse gases from burning trees and plants are released into the atmosphere and thus trap heat. Scientists found that right after the fire, large amounts of greenhouse gases entered the atmosphere and caused warming. Ozone（臭氧） levels increased, and ash from the fire fell on far-off sea ice, darkening the surface and causing more radiation from the sun to be taken in. The following spring, however, the land within the area of the fire was brighter than before the fire, because fewer trees covered the ground. Snow on the ground mirrored more sunlight back into space, leading to cooling. “We need to find out all possible ways to reduce the growth of greenhouse gases in the atmosphere.” Scientists tracked the change in the amount of radiation entering and leaving the climate system as a result of the fire, and found a measurement closely related to the global air temperature. Typically, fire in northern regions occurs in the same area every 80 to 150 years. Scientists, however, found that when fire occurs more frequently, more radiation is lost from the earth and cooling results. Specifically, they determined when fire returns 20 years earlier than predicted, 0.5 watts per square meter of area burned are soaked up by the earth from greenhouse gases, but 0.9 watts per square meter will be sent back into space. The net effect is cooling. Watts are used to measure the rate at which energy is gained or lost from the earth.【小题1】According to the new findings, taking steps to prevent northern forest fires may __________.A．result in a warming climateB．cause the forest fires to occur more frequentlyC．lead to a longer fire seasonD．protect the forests and the environment there【小题2】The following are all the immediate effects after a forest fire EXCEPT __________.A．large amounts of greenhouse gases enter the atmosphereB．the levels of ozone which is a type of oxygen increase C．snow on the ground mirrors more sunlight back into spaceD．ashes from the fire fall on the ice surface and take in more radiation from the sun【小题3】Earlier studies about northern forest fires __________.A．analyze all aspects of how northern fires influence climateB．indicate that forest fires will pollute the atmosphereC．suggest that people should take measures to protect environmentD．suggest that the fires will speed up climate warming【小题4】The underlined phrase “soak up” in the last paragraph most probably means __________.A．releasedB．absorbedC．createdD．distributed【小题5】From the passage we can draw a conclusion that forest fires in Alaska, Canada and Siberia may __________.A．warm the climate as the supposition goesB．allow more snow to reflect more sunlight into space and thus cool the climateC．destroy large areas of forests and pollute the far-off sea iceD．help to gain more energy rather than release more energy