请教几个问题，关于electrical 的。

小鸡快跑

原帖由 vincent2000 于 3-2-2009 12:34 AM 发表
1=D
2=A
3=B

谢谢

what is the condition of an AC circuit when the capacitive reactance and inductive reactance are the same value.

a,equilibrium   b, resonance   c, voltage devider
d,inductive state

[ 本帖最后由 小鸡快跑 于 4-2-2009 01:55 PM 编辑 ]

小鸡快跑

1。 what happens to current flow in a capacitive circuit when the DC voltage across the capacitor is approximately equal to the source voltage.
a.current flow is optimized   b.current flow is maximum at source   c.current flow is maximum at capacitor.
d.little current flows


2.加温，变高温情况下，电线运送电流能力会怎样？ a带更多电，b带更少电，c没改变

3.一个device的效率降低时，以下哪个东西会增加？
a  热, b 电阻 R+X (impedance)   ,c amplifier gain

4.which is sub-cycle power disturbance event?
a.voltage sag   b.over frequency  c.transient voltae  d.brownout

5.what is the condition of an AC circuit when the capacitive reactance and inductive reactance are the same value.
a,equilibrium   b, resonance   c, voltage devider
d,inductive state

6.What is the relationship between current (I) and Voltage(E) in a circuit consisting of a capacitor in series with a resistor?
a.I & E are in phase across the capacitors.
b.I lead E across the resistor
c.E leads I across the capacitor
d.I & E in phase across the resistor

谢谢

[ 本帖最后由 小鸡快跑 于 4-2-2009 02:05 PM 编辑 ]

vincent2000

1=D
2=A
3=B

kensai

1.D

capacitor 和source有一样的 potential ， so potential difference = 0, no current flows,
问题写approximate equal, 所以就当作 little current flows. 答案 D

2. .加温，变高温情况下，电线运送电流能力会怎样？
resisitivity increases with temperature,
T increases, R increases, current decreases. 答案 B

来源
http://en.wikipedia.org/wiki/Resistivity 
under Temperature dependence, 第一行
”In general, electrical resistivity of metals increases with temperature...“

3.一个device的效率降低时，以下哪个东西会增加
efficiency = useful output/ input
when efficiency decreases, assuming other properties remain, useful output decreases, 
所以我assume total output- useful output = waste heat energy increases, 答案A

小鸡快跑

原帖由 kensai 于 4-2-2009 01:38 AM 发表
1.D

capacitor 和source有一样的 potential ， so potential difference = 0, no current flows,
问题写approximate equal, 所以就当作 little current flows. 答案 D

2. .加温，变高温情况下，电线运送电流能 ...

很好的解析阿， 加了第4和5题 。谢谢。