问题 问答题

(适合于二期课改教材的考生)某小型实验水电站输出功率是20kW,输电线路总电阻是6Ω.

(1)若采用380V输电,求输电线路损耗的功率.

(2)若改用5000高压输电,用户端利用n1:n2=22:1的变压器降压,求用户得到的电压.

答案

(1)输电线路损耗的功率为

P=(

P
U
)2R=(
20×103
380
)2×6W=16.62KW

(2)改用高压输电后,输电线上的电流强度变为I′=

P
U′
=
20×103
5000
A=4A

用户端在变压器降压前获得的电压U1=U-I′R=(5000-4×6)V=4976V

根据

U1
U2
=
n1
n2

用户得到的电压为U2=

n2
n1
U1=
1
22
×4976V=226.18V

答:输电线上损失的功率为16.62KW;改用高压输电后,用户得到的电压是226.18V.

问答题 简答题
问答题

(46)"Popular art" has a number of meanings, impossible to define with any precision, which range from folklore to junk, with poles being clear enough but the middle tending to blur. The Hollywood Western of the 1930’s, for example, has elements of folklore, but is closer to junk than to high art or folk art. There can be great trash, just as there is bad high art. The musicals of George Gershwin are great popular art, never aspiring to high art. Schubert and Brahms, however, used elements of popular music--folk themes--in works clearly intended as high art. The case of Verdi is a different one : he took a popular genre-- bourgeois melodrama set to music (an accurate definition of nineteenth-century opera)- and, without altering its fundamental nature, transmuted it into high art. (47) This remains one of the greatest achievements in music, and one that cannot be fully appreciated without recognizing the essential trashiness of the genre.

As an example of such a transmutation, consider what Verdi made of the typical political elements of nineteenth-century opera. (48) Generally in the plots of these operas, a hero or heroine--usually portrayed only as an individual, unrestrained by class--is caught between the immoral corruption of the aristocracy and the doctrinaire rigidity of the leaders of the civilians. Verdi transforms this naive and unlike formulation with music of extraordinary energy and rhythmic vitality, music more subtle than it seems at first hearing. There are scenes and arias that still sound like calls to arms and were clearly understood as such when they were first performed. Such pieces lend an immediacy to the otherwise veiled political message of these operas and call up feelings beyond those of the opera itself.

Or consider Verdi’ s treatment of character. (49) Before Verdi, there were rarely any characters at all in musical drama, only a series of situations which allowed the singers to express a series of emotional state. Any attempt to find coherent psychological portrayal in these operas is misplaced ingenuity. The only coherence was the singer’ s vocal technique: when the cast changed, new arias were almost always substituted, generally adapted from other operas. Verdi’ s characters, on the other hand, have genuine consistency and integrity, even if, in many cases, the consistency is that of pasteboard melodrama. The integrity of the character is achieved through the music: (50) once he had become established, Verdi did not rewrite his music for different singers or allow alterations or substitutions of somebody else’s arias in one of his operas, as every eighteenth-century composer had done. When he revised an opera, it was only for dramatic economy and effectiveness.

(48) Generally in the plots of these operas, a hero or heroine--usually portrayed only as an individual, unrestrained by class--is caught between the immoral corruption of the aristocracy and the doctrinaire rigidity of the leaders of the civilians.