求助三道物理电学比例题图片http://hiphotos.baidu.com/%CB%F7%C2%ED%C0%EF%B5%C4%D3%F1%CD%C3/pic/item/de2c0f3333fa828b6d7f6e47fd1f4134960a5acb.jpghttp://hiphotos.baidu.com/%CB%F7%C2%ED%C0%EF%B5%C4%D3%F1%CD%C3/pic/item/d0e9fd106d224f4aa086dca

求助三道物理电学比例题图片http://hiphotos.baidu.com/%CB%F7%C2%ED%C0%EF%B5%C4%D3%F1%CD%C3/pic/item/de2c0f3333fa828b6d7f6e47fd1f4134960a5acb.jpghttp://hiphotos.baidu.com/%CB%F7%C2%ED%C0%EF%B5%C4%D3%F1%CD%C3/pic/item/d0e9fd106d224f4aa086dca
求助三道物理电学比例题
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求助三道物理电学比例题图片http://hiphotos.baidu.com/%CB%F7%C2%ED%C0%EF%B5%C4%D3%F1%CD%C3/pic/item/de2c0f3333fa828b6d7f6e47fd1f4134960a5acb.jpghttp://hiphotos.baidu.com/%CB%F7%C2%ED%C0%EF%B5%C4%D3%F1%CD%C3/pic/item/d0e9fd106d224f4aa086dca
【第5小题】 掷1 （12-4U1)²/R3=1(瓦）---------------------------------------①
掷2 （12-2U1)²/R4=4(瓦)-----------------------------------------②
又I相等 掷1 4U1/4R1=（12－4U1)/R3---------------------------------③
掷2 2U1/R1=(12－2U1)/R4--------------------------------------④
由①②③④易得 U1=2伏 R1=8Ω
∴P1=(2U1)²/R1=2W
【第4小题】设后来示数为U
S闭合 I1=2U/RL
S断开 I2=U/RL ∴I1/I2=2:1
又 S闭合 （10-2U)²/Rpa=2w--------------------------------①
S断开 (10-3u)²/Rpb=0.25w----------------------------②
①/② Rpa/Rpb=(10-2U)²/8(10-3U)²-------------------------------③
又I相等 S闭合 2U/RL=(10-2U)/Rpa------------------------------④
S断开 U/RL=(10-3U)/Rpb--------------------------------⑤
④/⑤ Rpa/Rpb=(10-2U)/2(10-3U)-----------------------------------⑥
③⑥联立 U=3v 代入②得 Rpb=4Ω
【第3小题】只闭S1 I1=U/R1 只闭S2 I2=U/(R21+R2) I1/I2=3/2
∴R1=2R2 ---------------------------------------------------①
只闭S2 [U/(R1+R2)]²×R2=20w----------------------------------------②
①②联立 得 U²/R1=90w=P1
只闭S3 [U/(R1+R2+R3)]²×R3=15w-------------------------------------③
①②③联立 R3=3R2--------------------------------------------------------------④
把 ①④代入② 得 [U/(R1+R2)]²×R1=10w=P′1