问题 填空题

如图,三个电阻R1、R2、R3依次标有“12Ω,3W”、“8Ω,5W”、“6Ω,6W”,则允许接入电路的最大电压为______V,此时三个电阻消耗的总功率为______W.

答案

p=

u2
R
可求得R1的额定电压U1=6V,R2的额定电压U2=2
10
V,R3的额定电压U3=6V,

     R1与R2的并联阻值为R′=

R1R2
R1+R2
=4.8Ω,则因R3大于其并联值,则当R3达到6V时,并联电压小于6V,

   则以R3达到额定电压为安全值计算:

U1
U3
=
R′
R3
  即
U1
6
=
4.8
6
  求得U1=4.8V

   则接入电路的最大的电压为Um=U1+U3=10.8V 功率为:P=

U2m
R′+R3
=
10.82
4.8+6
=10.8w

故答案为:10.8,10.8

判断题
阅读理解

Federal regulators(调整者) Wednesday approved a plan to create a nationwide emergence alert (警报) system using text messages delivered to cell phones.

Text messages have exploded in popularity in recent years, particularly among young people. The wireless industry’s trade association, CTIA, estimates(估计) more than 48 billion text messages are sent each month.

The plan comes from the Warning Alert and Response Network Act, a 2006 federal law that requires improvements to the nation’s emergency alert system. The act tasked the Federal Communications Commission (FCC) with coming up with new ways to alert the public about emergencies.

"The ability to deliver accurate and timely warnings and alerts through cell phones and other mobile services is an important next step in our efforts to help ensure that the American public has the information they need to take action to protect themselves and their families before, and during, disasters and other emergencies," FCC Chairman Kevin Martin said following approval of the plan.

Participation in the alert system by carriers –telecommunications companies is voluntary, but it has received solid support from the wireless industry.

The program would be optional for cell phone users. They also may not be charged for receiving alerts.

There would be three different types of messages, according to the rules.

The first would be a national alert from the president, likely involving a terrorist attack or natural disaster. The second would involve "approaching threats," which could include natural disasters like hurricanes or storms or even university shootings. The third would be reserved for child abduction (绑架) emergencies, or so-called Amber Alerts.

The service could be in place by 2010.

小题1:What is the purpose of the approved plan?  

A.To warn people of emergencies via messages.

B.To popularize the use of cell phones.

C.To estimate the monthly number of messages.

D.To promote the wireless industry.小题2:The improvement to the present system is in the charge of ____. 

A.CTIA

B.the Warning Alert and Response Network

C.FCC

D.federal regulators小题3:The carriers’ participation in the system is determined by _____. 

A.the US federal government

B.mobile phone users

C.the carriers themselves

D.the law of the United States小题4:Which of the following would be the best title for the text?  

A.Cell Phone Alerts Protecting Students

B.Cell Phone Alerts by Wireless Industry

C.Cell Phone Alerts of National Disasters

D.Cell Phone Alerts Coming Soon