dicky821 2012-7-12 04:02 PM

[img]http://i1126.photobucket.com/albums/l617/dicky821/ph5.jpg[/img]

[img]http://i1126.photobucket.com/albums/l617/dicky821/PH9.jpg[/img]

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).

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

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

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