问题 选择题

甲中的理想变压器的原副线圈匝数比为11 :   1,原线圈接如图乙所示的正弦交流电时电动机正常工作,此时电流表的读数为1A,已知R=20,电机线圈的内阻r=0.5,电流表和电压表都是理想电表,则下列说法不正确的是

A.电压表的读数为20V

B.流过电阻R中的电流为1A

C.流过电动机的电流为40A

D.电动机输出的机械功率为150W

答案

答案:C

题目分析:电压表的读数为电压的有效值,原线圈的电压有效值为220V,根据电压与匝数成正比可知,副线圈的电压有效值为20V,所以电压表的读数为20V,A正确;

电阻R的电压为20V,电阻为20Ω,所以流过电阻中的电流为1A,所以B正确.

原线圈的电流为1A,根据电流与匝数成反比可知,副线圈的电流为11A,由于电阻和电动机并联,流过电阻中的电流为1A,所以流过电动机的电流为10A,所以C错误.

电动机的总功率为,电动机的发热功率为,所以电动机输出的机械功率为150W,所以D正确.

让选错误的,故选C

点评:电压与匝数程正比,电流与匝数成反比,变压器的输入功率和输出功率相等,

填空题

Motivation is "the driving force within individuals that impels them to action." And goals are the sought-after results_____(1) motivated behavior.Motivation can be either positive or negative_____(2) direction. We may feel a driving force toward some object or condition, _____(3) a driving force away from some object or condition. For example, a person may be impelled toward a restaurant to fulfill a need, hunger, and away_____(4) an airplane to fulfill a need of safety. Some psychologists refer to positive drives_____(5) needs, wants or desires,_____(6) negative drives as fears or aversions._____ (7), though negative and positive motivational forces seem to differ dramatically_____(8) terms of physical and sometimes emotional activity, they are basically similar in_____(9) they both serve to initiate and sustain human behavior.____(10) this reason, researchers often refer _____(11) both kinds of drives or motives as needs, wants and desires.Goals, _____(12), can be either positive or negative. A positive goal is one toward_____ (13) behavior is directed and it is often referred to as an approach object. A negative goal is _____(14) from which behavior is directed away and it is sometimes referred to as an avoidance object. Since both approach and avoidance goals can be considered objectives of motivated behavior, most researchers refer to_____(15) types simply as goals. Consider this example. A middle-aged woman may wish to remain_____(16) attractive as possible. Her positive goal is to appear desirable, and_____(17) she may use a perfume advertised to make her irresistible. A negative goal may be to prevent her skin_____(18) aging, and therefore she may buy and use face creams._____(19) the former case, she uses perfume to help her achieve her positive goal — attractiveness; in the____(20) case, she uses face creams to help avoid a negative goal — wrinkled skin.

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