# 查看完整版本 : 幾題關於current electricity 既MC!!!

dicky821 2012-7-12 04:02 PM

## 幾題關於current electricity 既MC!!!

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Ron~ 2012-7-12 05:03 PM

1994/II/28
A
When S is closed, current doesn't flow though L1, brightness of L1 decreases to 0.
All current passes though L2 causing its increase in brightness.
Brightness of L3 remains unchanged because its voltage doesn't change(since L3 is in parallel, voltage across L3 still equals to emf of the cell).

hk56740 2012-7-12 05:04 PM

1. average current=(800/1000) x 60 x60/(3x24x60x60)=1/90 A
P=VI=40mW

2. Short circuit occurs after closing the switch, no current flows through bulb L1 which causes the brightness to decrease, options D and E are therefore not considered. Let the voltage across the cells be Vo and resistance of each bulb be R.
Initially,
P3=(    (Vo) ^2  )/R
Vo=(I2) x (R+R)
I2 = Vo /(2R)
P2=((I2)^2) x R= ( (Vo)^2/(4R^2) ) x R=0.25 ( (Vo)^2 )/R

Finally, P3=( (Vo) ^2 ) /R  (unchanged)
P2=(  (Vo)^2  )/R  > initial P2
so ans = A

3. P=VI=1.2 x 225/1000=0.27W
Energy stored=Pt=0.27 x 10 x 60 x 60=9720J
ans = D