如图所示，某小型水电站发电机的输出功率为10kW，输出电压为400V，向距离较远的用户供电，为了减少电能损失，使用2kV高压输电，最后用户得到220V、9.5kW的电力，求：
（1）水电站升压变压器原、副线圈匝数比
（2）输电线路导线电阻R。
（3）用户降压变压器原、副线圈匝数比
。
（1）1：5；（2）20Ω；（3）95：11。
试题解析：（1）由于升压变压器的原线圈的电压为400V，副线圈的电压为2000V，故水电站升压变压器原、副线圈匝数比
。
（2）由于升压变压器中原线圈的电流为I1=
=25A，
故根据公式
，副线圈上的电流为I2=
=5A，
输电线上损失的电功率为△P=10kW－9.5kW=500W，故输电线路导线电阻R=
=20Ω；
（3）降压变压器原线圈电压U3=U2－I2R=（2000V－5A×20Ω）=1900V，降压变压器副线圈电压U4=220V，
故降压变压器原、副线圈匝数比
。
| Electronic mail has become an extremely important and popular means of communication. The convenience and efficiency of electronic mail are threatened by the extremely rapid growth in the volume of unsolicited commercial electronic mail. Unsolicited commercial electronic mail is currently estimated to account for over half of all electronic mail traffic, up from an estimated 7 percent in 2001, and the volume continues to rise. Most of these messages are fraudulent or deceptive in one or more respects. The receipt of unsolicited commercial electronic mail may result in costs to recipients who cannot refuse to accept such mail and who incur costs for the storage of such mail, or for the time spent accessing, reviewing, and discarding such mail, or for both. The receipt of a large number of unwanted messages also decreases the convenience of electronic mail and creates a risk that wanted electronic mail messages, both commercial and noncommercial, will be lost, overlooked, or discarded amidst the larger volume of unwanted messages, thus reducing the reliability and usefulness of electronic mail to the recipient. Some commercial electronic mail contains material that many recipients may consider vulgar or pornographic in nature. The growth in unsolicited commercial electronic mail imposes significant monetary costs on providers of Internet access services, businesses, and educational and nonprofit institutions that carry and receive such mail, as there is a finite volume of mail that such providers, businesses, and institutions can handle without further investment in infrastructure. Many senders of unsolicited commercial electronic mail purposefully disguise the source of such mail. Many senders of unsolicited commercial electronic mail purposefully include misleading information in the messages’ subject lines in order to induce the recipients to view the messages. While some senders of commercial electronic mail messages provide simple and reliable ways for recipients to reject (or ’opt-out’ o0 receipt of commercial electronic mail from such senders in the future, other senders provide no such ’opt-out’ mechanism, or refuse to honor the requests of recipients not to receive electronic mail from such senders in the future, or both. Many senders of bulk unsolicited commercial electronic mail use computer programs to gather large numbers of electronic mail addresses on an automated basis from Internet websites or online services where users must post their addresses in order to make full use of the website or service. The problems associated with the rapid growth and abuse of unsolicited commercial electronic mail cannot be solved by the government alone. The development and adoption of technological approaches and the pursuit of cooperative efforts with other countries will be necessary as well. |
It can be inferred from Paragraph 6 that bulk unsolicited commercial e-mail will probably spread ______.
A．harmful virus
B．unpleasant news
C．advertisements
D．adult jokes