燈泡Ll標(biāo)有“6V 3W”的字樣,燈泡L2標(biāo)有“6V 6W”的字樣,將它們串聯(lián)在電路中,要使每個燈泡都不損壞,這段電路兩端所加最大電壓為 V;燈L3標(biāo)有“220V 25W”的字樣,燈L4標(biāo)有“220V 60W”的字樣,將L3、L4兩燈串聯(lián)接到220V的電路中,燈 更亮些.
【答案】
分析:根據(jù)R=

分別求出四個燈泡的電阻,
燈泡L
l“6V 3W”和燈泡L
2“6V 6W”串聯(lián)在電路中,要使每個燈泡都不損壞,串聯(lián)電路的電流必須等于小的額定電流.根據(jù)U=IR求出電壓.
燈泡實際功率越大,燈泡越亮,燈L
3“220V 25W”和L
4“220V 60W”串聯(lián)接到220V的電路中,根據(jù)P=I
2R知,電流相等,電阻越大,功率越大,燈越亮.
解答:解:R
1=

=

=12Ω,R
2=

=

=6Ω,
I
1額=

=

=0.5A,I
2額=

=

=1A,
燈泡L
l和燈泡L
2串聯(lián)在電路中,要使每個燈泡都不損壞,串聯(lián)電路的電流為I=0.5A,
所以電源電壓為U=IR=I(R
1+R
2)=0.5A×(12Ω+6Ω)=9V.
R
3=

=

=1936Ω,R
4=

=

=806.7Ω,
燈L
3和L
4串聯(lián)接到220V的電路中,根據(jù)P=I
2R知,電流相等,R
3>R
4,燈L
3更亮.
故答案為:9;L
3.
點評:知道額定電壓和額定功率求得電阻的非常有用的.
兩個燈泡串聯(lián)時,保證一個燈泡正常發(fā)光,讓額定電流小的正常工作,兩個燈泡并聯(lián)時,保證一個燈泡正常工作,讓額定電壓小的正常工作.這是一種習(xí)題類型,一定要掌握.