Bí quyết ôn luyện thi đại học theo chủ đề môn vật lý điện xoay chiều

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J= = = ? 6;  $"  /3  8 6 thu Q H .Q cm t H .I Rt= ⇔ ∆ = táa 6 6 8 Y 8 8 R 5 > 78 *8 5 6 78 7 6 76 H .I Rt , . . . . t C cm , . . , ⇒ ∆ = = = Ví dụ 4: +, & !"  /01&,21345 6 788π9:;",%<=4>Ω? 7;$0@&A%"<%<=%&1&"BCD? 6;ED&=FG,HI2"47J6K&0L?MN"1&"789CD; ,0L%&HIO&H"P,?Q"%I 0LGM4R8SFG-&%P&"0LG45J69KTUK&?V 8 ;? Hướng dẫn 7;WC-X&&23 ( ) 6 6 5 > B *8 YY*88Q I Rt . . . J= = = ? 6;  $"  /3  8 6 thu Q H .Q cm t H .I Rt= ⇔ ∆ = táa 6 6 8 Y 8 8 R 5 > 78 *8 5 6 78 7 6 *8 B H .I Rt , . . . . t C cm , . . , ⇒ ∆ = = = 75 Ví dụ 2: C"Z/H[24 6889788\]>πU*;9H^&FJH^&&_;? $%&KA&1&"`88J87C21 /&%aH^&788FGb&1[ :?YU688FG>U*88? ?7U588FG6U588? V?7U>88FGYU>88? c?7U688FGBU*88? Ví dụ 2: C"Z/H[24 6889788\]>πU*;9H^&FJH^&&_;? $%&KA&1&"`88J86C21/ &%aH^&788FGb&1[ :?YU688FG>U*88? ?7U588FG6U588? V?7U>88FGYU>88? c?7U688FGBU*88? 6* Ví dụ 1: c.&!"%&-_-d/ H[469788πeπU*;9:;9H^&&_;?$ 0@&[ " -  f&" -_-d %& 7UY889;K[`D48? :?*J***V??>J>7YV?V?*JY**V?c?*J8R6V? Hướng dẫn Cách 1: ( ) 6 7 7 Y88 8 7 Y88 Y 8 6 788 Y 6 788 * Y** 78 788 Y t / t / Q idt cos t dt sin t , . C π π π π π − = = − = − ≈ ⇒    ÷      ÷   ∫ ∫ Chän C. Ví dụ 1: c.&!"%&-_-d/ H[469788πeπU*;9:;9H^&&_;?$ 0@&["-f&"-_-d%&7UY88 9;K[`D48? :?*J***V??>J>7YV?V?*JY**V?c?*J8R6V? Hướng dẫn Cách 1: ( ) 6 7 7 Y88 8 7 Y88 Y 8 6 788 * 6 788 * Y** 78 788 * t / t / Q idt cos t dt sin t , . C π π π π π − = = − = − ≈ ⇒    ÷      ÷   ∫ ∫ Chän C. 6g Ví dụ 6: V &!"/&%a-X& 6:"HIC_2"-&-aM 6 (h 5 FL i%j?$[K<KPk %"%&1&"7*CD<li? :?8J7*g??8J86Y>?V?8J85B?c?8J8>*? Ví dụ 6: V &!"/&%a-X&6 :"HIC_2"-&-aM 6 (h 5 FL i%j?$[K<KPk %"%&1&">CD<li? :?8J7*g??8J86Y>?V?8J85B?c?8J8>*? >g Ví dụ 9: mC!"FG"Z: 2i&n,-_ZA/AK&788 Y oJ %<=4788oFGXV/-&K&688 Y opq CJ+G[&b"FG=JEG[&b""=FGV?rA G"_khôngD&s :?C"Z :E L C" jC"Z :G6\UY ?V01&, &%tC"\UYFLC"Z :? V?C"Z:ELC"jC"Z :G6\UY? c?C"Z:LC"jC"ZX G\U*? Hướng dẫn Ví dụ 9: mC!"FG"Z: 2i&n,-_ZA/AK&788 Y oJ %<=4788oFGXV/-&K&688 Y opqCJ +G[&b"FG=JEG[&b""=FGV?rAG" _khôngD&s :?C"Z   :E L  C" jC"Z :G6\UY ?V01&, &%tC"\UYFLC"Z :? V?C"Z:ELC"jC"Z :G6\UY? c?C"Z:LC"jC"ZX G\U*? Hướng dẫn 788 Y 688 Y " Y 788 Y L C AB AB Z Z R π ϕ ϕ − − = = = − ⇒ = − 788 Y " Y 788 Y L AN AN Z R π ϕ ϕ = = = ⇒ = 8 3 Y 6 * * AB C AB C u u π π π π ϕ ϕ − = − − − = >    ÷   sím h¬n lµ ?⇒ Chä 788 Y 688 Y " Y 788 Y L C AB AB Z Z R π ϕ ϕ − − = = = − ⇒ = − 788 Y " Y 788 Y L AN AN Z R π ϕ ϕ = = = ⇒ = 8 3 Y 6 * * AB C AB C u u π π π π ϕ ϕ − = − − − = >    ÷   sím h¬n lµ V?⇒ Chä g8 Bài 79: :pqC2i&n%<=4>8Ω ,-_/%<%J/,iAFGXV48J6UπuJ+G[ q&b"VFG,-_?+,C!"va0@pFG :+JK/ &%& 7 469788πeπUY;9:;?CG pFG:I &" 6 49788π]πU*;9:;?,iA ",-_H^&3 Bài 79: :pqC2i&n%<=4>8Ω ,-_/%<%J/,iAFGXV48J86UπuJ+G[ q&b"VFG,-_?+,C!"va0@pFG:+J K/ &%& 7 469788πeπUY;9:;?CGp FG:I &" 6 49788π]πU*;9:;?,iA" ,-_H^&3 gg Ví dụ 3: +,2""%&H"CZ)3X J%<ZJ,AZpqC? C&b""ZFG01&, & "/Z0@/H[234*89788πeπU6;9;J 469788π]πU*;9:;?MN%&/ CZ)Gs$-&K&JAK&m%< 0j&2&FLlCZ)/?$&QPX "? :?=47> Y Ωwx  47>ΩFGy4Y89z;? ?=47>Ωwx V 47> Y ΩFGy4Y8 Y 9z;? V?=47> Y Ωwx V 47>ΩFGy4Y8 Y 9z;? c?=47>Ωwx  47> Y ΩFGy4Y89z;? Hướng dẫn     H[  23    4  69788π  ] πU*;  9:;  4 69788πeπUY;9:;? ( ) *8 6 7> Y 7> 6 Y L C u Z R i Z Z i i π π ∠ − = + − = = = − ∠ − ( ) ( ) ( ) 7> Y 7> 7> 8 C L C L R Z Z Z Z = Ω ⇒ = Ω − = − Ω ⇒ =         ⇒ ( ) 6 Y8 YP I R W= = ⇒ Chän B. Ví dụ 3: +,2""%&H"CZ)3X J%<ZJ,AZpqC? C&b""ZFG01&, & "/Z0@/H[234*89788πeπU6;9;J 469788π] πU*;9:;?MN%&/ CZ)Gs$-&K&JAK&m%< 0j&2&FLlCZ)/?$&QPX "? :?=47> Y Ωwx  47>ΩFGy4Y89z;? ?=47>Ωwx V 47> Y ΩFGy4Y8 Y 9z;? V?=47> Y Ωwx V 47>ΩFGy4Y8 Y 9z;? c?=47>Ωwx  47> Y ΩFGy4Y89z;? Hướng dẫn H[23469788π]πU*;9:;469788π eπUY;9:;? ( ) *8 6 7> Y 7> 6 Y L C u Z R i Z Z i i π π ∠ − = + − = = = − ∠ − ( ) ( ) ( ) 7> Y 7> 7> 8 C L C L R Z Z Z Z = Ω ⇒ = Ω − = − Ω ⇒ =         ⇒ ( ) 6 Y8 YP I R W= = ⇒ Chän C. 786 Ví dụ 7: +,!"qC:&n %<Z=J,AZFGX/ -&V?C"Z:G4{ 8 6 ω 9;IC%PG  4{ 8 6 9ω]πUY;9;? +q!A%",&0<&I-&"XH^& Ví dụ 7: +,!"qC:&n %<Z=J,AZFGX/ -&V?C"Z:G4{ 8 ω9;I C%PG   4{ 8 6 9ω] πUY;9;?+q !A%",&0<&I-&"XH^& 776 Bài 8: +,!"qC:&n %<Z=J,AZFGX/-& V?C"Z:G4{ 8 ω9;I C%PVG V 4{ 8 9ωe6πUY;9;?+q!A %",&0<&I-&"XH^& Bài 8: +,!"qC:&n %<Z=J,AZFGX/-&V? C"Z:G46{ 8 ω9;IC %PVG V 4{ 8 9ωe 6πUY;9;?+q!A%" ,&0<&I-&"XH^& 776 Bài 9: +,!"&n,XVq Bài 9: +,!"&n,XVq CFL,,-_?mFG"Z, C4{ g ω9;IC"ZXV G  4{9ω]YπU5;9;?$|q&b"-&K&FG AK&H^& CFL,,-_?mFG"Z, C4{ g ω9;IC"ZXV G V 4{9ω]YπU5;9;?$|q&b"-&K&FG AK&H^& 7*7 Ví dụ 19: 9}~•E€=eVe%;mC!" 4768 * ω9;FG"Z:&n" :+FG+pqCI01&, -X&"G8J>:?:+&n%<Z= pqCFLXVJ+&n,A? C&b""Z+FGC&b" "Z:C""πU6?C -X&%P=H^&,)"C-X&%P :+?V&QPX"G Ví dụ 19: 9}~•E€=eVe%;mC!" 4768 * ω9;FG"Z:&n" :+FG+pqCI01&, -X&"G8J>:?:+&n%<Z= pqCFLXVJ+&n,A? C&b""Z:+FGC&b"" Z:C""πU6?C-X& %P=H^&,)"C-X&%P:+? V&QPX"G 68Y I • i%tj % F1&"G$U592GF C"  G 6πU6;  P3  6 * X Lr π π ϕ ϕ = − = − ?  $"  Q3 8 6 X π ϕ − < < P•/[G%<pqCFL X⇒V? I • i%tj % F1&"G$U592GF C"  G πU6;  P3  6 * X Lr π π ϕ ϕ = − = − ?  $"  Q3 8 6 X π ϕ − < < P•/[G%<pqCFLX ⇒V? 6gR Ví dụ 12: :qC&n" :+FG+?:+G,,-_/%< Z=458 Y  ΩFG,iA 48J5UπMJ +G,X/-&V"v0@J V/&%abFGKK&?mFG:, C3 :  4768 6 788π9;?‚V[ v&C-X&9{ :+  ]{ + ;&%ai? $I&%ai"v&qG? :?658? ?768 Y ? V?768? Hướng dẫn •ƒ∆:„+3 … " Y R L L U R AME U Z = = =  8 … *8AME⇒ =  8 8 … 7g8 768AME α β ⇒ + = − = WC-X&aGq∆:+3 8 *8 AB AM MB AM MB sin sin sin sin sin α β α β + = = = + 6 6 6 AM MB U U sin cos α β α β + = + − Ví dụ 12: :qC&n" :+FG+?:+G,,-_/%< Z=458 Y ΩFG,iA48J5UπMJ +G,X/-&V"v0@JV/& %abFGKK&?mFG:,C3 : 4768 6 788π9;?‚V[v&C -X&9{ :+ ]{ + ;&%ai?$I&%ai "v&qG? :?658? ?768 Y ? V?768? Hướng dẫn •ƒ∆:„+3 … " Y R L L U R AME U Z = = =  8 … *8AME⇒ =  8 8 … 7g8 768AME α β ⇒ + = − = WC-X&aGq∆:+3 8 *8 AB AM MB AM MB sin sin sin sin sin α β α β + = = = + 6 6 6 AM MB U U sin cos α β α β + = + − ( ) 8 6 *8 6 AM MB U U AB sin cos α β − ⇒ + = max= α β ⇔ = r/3 ( ) ( ) 768 Y AM MB max VU U =+ ⇒V? ( ) 8 6 6 *8 6 AM MB U U AB cos sin sin α β α β + − ⇒ + = max= α β ⇔ = r/3 ( ) ( ) 6 658 AM MB AB max U VU U = =+ ⇒V:? Y8B Chú ý: Chúng ta nhớ lại các công thức giải nhanh sau đây: ♣ Khi R thay đổi hai giá trị L 1 và L 2 mà có cùng P thì P max khi: 8 7 6 R R R= . Chú ý: Chúng ta nhớ lại các công thức giải nhanh sau đây: ♣ Khi R thay đổi hai giá trị R 1 và R 2 mà có cùng P thì P max khi: 8 7 6 R R R= . Y8g *có cùng U C thì U Cmax khi: 7 6 8 7 6 6C C C C C = + . *có cùng U C thì U Cmax khi: 7 6 8 6 C C C = + . Y66 G5RC G5RCc? Y6g _&1PK3 6 6 L n R C = I"/3 _&1PK3 6 L nR C = I"/3 Y6R Chú ý: Điều kiện 6 6 L n R C = có thể trá hình dưới dạng điều kiện vuông pha. Chú ý: Điều kiện 6 L nR C = có thể trá hình dưới dạng điều kiện vuông pha. Y5B G7*CJGY8C: G7*CcJGY8C Y*6 Ví dụ 20: +=V p FG  C!" ?rq,""%G9F.&UCD;I& QGyq&Q8J> Y ?rq,"" %G69F.&UCD;I&QG5y?rq, ""%G 6  9F.&UCD;I&QH^& H"Ps :?7*yUB??y Y ?V?Ry?c?65yU7Y? Hướng dẫn † ( ) ( ) 6 6 6 6 Y 6 Y L C L C R R cos Z Z R Z Z ϕ = = ⇒ − = + − 97; † ( ) 6 6 6 6 6 6 L C C L R Z Z P' I' k P I Z R kZ k + − = = ⇒ + −    ÷      ÷   6 6 6 6 Y 5 5 6 6 C L R R . Z R Z + = + −    ÷   6 6 6 6 Y C L Z R Z⇒ − =    ÷   96;?$`97;J96;%"3  6 Y Y L C R R Z ;Z= = ? † ( ) 6 6 6 6 6 6 6 6 6 6 7* Y 6 B 6 Y 6 Y L C C L R Z Z P'' I'' k' P I Z R k' Z k' R R . R R R + − = = + − + = = + −    ÷      ÷      ÷   7* ‡‡ B P P⇒ = ⇒ Chän A. Ví dụ 20: +=VpFGC!"? rq,""%G9F.&UCD;I&QG yq&Q8J> Y ?rq,""%G6 9F.&UCD;I&QG5y?rq,""% G 6 9F.&UCD;I&QH^&H"Ps :?7*yUB??y Y ?V?gyUY?c?65yU7Y? Hướng dẫn † ( ) ( ) 6 6 6 6 Y 6 Y L C L C R R cos Z Z R Z Z ϕ = = ⇒ − = + − 97; † ( ) 6 6 6 6 6 6 L C C L R Z Z P' I' k P I Z R kZ k + − = = ⇒ + −    ÷      ÷   6 6 6 6 Y 5 5 6 6 C L R R . Z R Z + = + −    ÷   6 6 6 6 Y C L Z R Z⇒ − =    ÷   96;?$`97;J96;%"3  6 Y Y L C R R Z ;Z= = ? † ( ) 6 6 6 6 6 6 6 6 6 6 g Y 6 Y 6 6 Y 6 Y L C C L R Z Z P'' I'' k' P I Z R k' Z k' R R . R R R + − = = + − + = = + −    ÷      ÷      ÷   g ‡‡ Y P P⇒ = ⇒ Chän C. YgR Ví dụ 21: Trong mt gi thc hành , hc sinh mun mt qut i n loi 180V–220W hot   ng bình Ví dụ 21: Trong mt gi thc hành , hc sinh mun mt qut i  n loi 180V–120W hot   ng bình th ng d i mt i  n áp xoay chiu có giá tr hiu dng 220V, nên mc ni tip vi qut mt bin tr. Ban   u hc sinh ó   bin tr có giá tr B8Ωthì o thy c ng   hiu dng trong mch là 0,75A và công sut ca qut i n   t 92,8%. Mun qut hot   ng bình th ng thì phi i u chnh bin tr nh th nào? A. Gim i  68 Ω?B. Tng thêm 76Ω? V? Gim i  76 Ω?c? Tng thêm 68Ω? Hướng dẫn †D  ZJ  ,&  j ,&-0La2J&QPX"/3 R6Jg ‡  ?768 ?8JB> 788 P UI U ϕ ϕ = ⇒ = ( )  75gJ5gU V ϕ ⇒ = $`C0j&%IFƒ j3  AB R U U U= + r r r  P%X GFG%X&"0@3    8  AB AB R AB AB U U U U U ϕ ϕ ϕ ϕ = + = +    668 668   B8?8JB> 75gJ5g  8 AB AB U ϕ ϕ ϕ = + = +     gRJ5g6U ϕ ⇒ =  K @CFL   75gJ5gU ϕ = J %"3 ϕ ≈ 8J>565%"-"  8Jg>*> ϕ = ? *r,&j,&HI01&3 ( )  768 7g8? ?8Jg>*> 8JBBg5P UI I I A ϕ = ⇒ = ⇒ = $`C0j&%IFƒ j3  AB R U U U= + r r r  P%X GFG%X&"0@3    8  AB AB R AB AB U U U U U ϕ ϕ ϕ ϕ = + = +    668 668 8J>565  7g8?8J>565  8 7g8? AB AB R U ϕ ϕ = + = +  ⇒   ( ) ( ) 5>J6>8J5Y* >g R AB R U U Rrad I ϕ ⇒ = ⇒ == ⇒ ≈ Ω  }AB8 >g476Ω⇒VV? th ng d i mt i n áp xoay chiu có giá tr hiu dng 220V, nên mc ni tip vi qut mt bin tr. Ban   u hc sinh ó   bin tr có giá tr B8Ωthì o thy c ng   hiu dng trong mch là 0,75A và công sut ca qut i n   t 92,8%. Mun qut hot   ng bình th ng thì phi i u chnh bin tr nh th nào? A. Gim i  68 Ω?B. Tng thêm 76Ω? V? Gim i  76 Ω?c? Tng thêm 68Ω? Hướng dẫn †DZJ,&j ,&-0La2J&QPX"/3 R6Jg ‡  ?768 ?8JB> 788 P UI U ϕ ϕ = ⇒ = ( )  75gJ5gU V ϕ ⇒ = $`C0j& %I  Fƒ j3  AB R U U U= + r r r  P  %X GFG%X&"0@3    8  AB AB R AB AB U U U U U ϕ ϕ ϕ ϕ = + = +    668 668   B8?8JB> 75gJ5g  8 AB AB U ϕ ϕ ϕ = + = +     gRJ5g6U ϕ ⇒ =  K @CFL   75gJ5gU ϕ = J  %"3 ϕ ≈ 8J>565%"-"  8Jg>*> ϕ = ? *r,&j,&HI01&3 ( )  768 7g8? ?8Jg>*> 8JBBg5P UI I I A ϕ = ⇒ = ⇒ = $`C0j& %I  Fƒ j3  AB R U U U= + r r r  P  %X GFG%X&"0@3    8  AB AB R AB AB U U U U U ϕ ϕ ϕ ϕ = + = +    668 668 8J>565  7g8?8J>565  8 7g8? AB AB R U ϕ ϕ = + = +  ⇒   ( ) ( ) 5>J6>8J5Y* >g R AB R U U Rrad I ϕ ⇒ = ⇒ == ⇒ ≈ Ω  }AB8 >g476Ω⇒VV? 585 Ví dụ 19: +,HC/ˆq!2&&nHq 0&‚/"0@Q",-_? rp,,-_FGC!"I 01&2`-/%"K&Ha%"&GFG 0@"".?rp,79/ 7888F.&;FGC-X&*8I<,6K [</C-X&G58?(qF.&-_" ,6G Ví dụ 20: +,HC/ˆq!2&&n> 0&‚/"0@Q",-_?r p,,-_FGC!"I01& 2`-/%"K&Ha%"&GFG0@" ".?rp,7FGC -X&768I<,6K[</C -X&{ 6 ?r2,6FLC-X&Y{ 6 I C-X&<,7K[<G Ví dụ 19: +,HC/ˆq!2&&nHq 0&‚/"0@Q",-_? rp,,-_FGC!"I 01&2`-/%"K&Ha%"&GFG 0@".?rp,79/ 7888F.&;FGC-X&*8I<,6K [</C-X&G58?(qF.&-_", 6G Ví dụ 20: +,HC/ˆq!2&&n> 0&‚/"0@Q",-_?rp ,,-_FGC!"I01&2` -/%"K&Ha%"&GFG0@" .?rp,7FGC -X&768I<,6K[</C-X& { 6 ?r2,6FLC-X&Y{ 6 IC -X&<,7K[<G . Chúng ta nhớ lại các công thức giải nhanh sau đây: ♣ Khi R thay đổi hai giá trị L 1 và L 2 mà có cùng P thì P max khi: 8 7 6 R R R= . Chú ý: Chúng ta nhớ lại các công thức giải nhanh sau. kiện 6 6 L n R C = có thể trá hình dưới dạng điều kiện vuông pha. Chú ý: Điều kiện 6 L nR C = có thể trá hình dưới dạng điều kiện vuông pha. Y5B G7*CJGY8C: G7*CcJGY8C Y*6 Ví. áp xoay chiu có giá tr hiu dng 220V, nên mc ni tip vi qut mt bin tr. Ban   u hc sinh ó   bin tr có giá tr B8Ωthì o thy c ng   hiu dng trong mch là 0,75A và công