题干

如图所示,电源电压保持12V不变电流表A1和电压表A的量程分别为0﹣0.6A和0﹣3A,滑动变阻器上标有“50Ω 2A”字样,开关闭合后,将滑动变阻器的滑片移至图中所示位置时,电流表A1和电流表A的示数分别为0.5A和0.8A.求:

(1)此时通过滑动变阻器的电流是多少?它连入电路中的电阻是多少?

(2)为了确保所有元件都不损坏,滑动变阻器连入电路中的电阻最小为多少?

(3)移动滑动变阻器的滑片,电路消耗的最小总功率是多少?

 

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解:(1)由图可知,R1与R2并联,电流表A测干路电流,A1测R1支路的电流,并联电路干路电流等于各支路电路之和,所以I2=I﹣I1=0.8A﹣0.5A=0.3A,并联电路中各支路两端电压与电源电压都相等,由I=UR

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    Denali National Park, Alaska

    History

    Within Denali National Park, the Toklat River will always have a special significance, as it was the location where naturalist Charles Sheldon built a cabin and was so moved that he fought to preserve the land. He moved back east and spent nine years lobbying (游说)to create Alaska's first national park.

    When to Visit

    To avoid the crowds, visit in June, but keep in mind that there are up to 21 hours of sunlight in Alaska in summer. If it seems like a bit much for your taste, try visiting in late August or September.

    If you visit to climb Mount McKinley, May and early June are the best times to climb.

    Major Attractions

    It is difficult not to see Denali's largest attraction towering 20,320 feet high. Mount McKinley can even be seen up to 70 miles away on a clear day. If you brave the climb to the top, you will be rewarded with charming views of the Alaska Range.

    Sable Pass is a prime spot to view grizzly bears. Closed to off-road foot traffic, the area is popular for bears feeding on berries, roots, and even occasionally on other mammals.

    Beginning at just below the summit of Mount McKinley, Muldrow Glacier flows 35 miles through a gorge and across the tundra. Twice in the last hundred years, the Muldrow has surged (涌动), most recently in the winter of 1956-1957.

Accommodation

    Five campgrounds are located within the park, and many are open from late spring to early fall.Note: reservations are strongly recommended during summer. Riley Creek campground is open all year round, and all but two (Sanctuary and Wonder Lake) offer RV (可供居住的大篷车)sites.

    Also within the park are a few lodges (旅馆)—North Face Lodge, Denali Backcountry Lodge and Kentishna Roadhouse.