Rad i snaga električne struje - seminarskirad.biz

Rad i snaga elektrine struje Rad W = UQ = UIt W = UIt = IRIt 2 U W = I2Rt 2 Rt R Elektrina energija Snaga P P = UI W t P = I2 R U2 P R U2

W t R Primjer: Grijalo prikljueno na napon 220 V ima snagu 800 W. a) Koliki je otpor grijala? b) Kolika je jakost struje kroz grijalo? c) Kolika je korisnost grijala ako se na njemu ugrije 3 litre vode od 10 oC do 100 oC za 45 minuta. Specifini toplinski kapacitet vode je 4,19103 J kg-1(oC)-1. Rjeenje: U = 220 V P = 800 V U 2 (220V) 2 U2 R a) R = ? P 800W P R P 800 W b) I = ? P = UI I U 220 V R = 60,5

I = 3,64 A c) V = 3 l , m = 3 kg t1 = 10 oC t2 = 100 oC t = 45 min = 2700 s =? mc(t 2 t1 ) 3kg 4190J kg -1 ( o C) -1 (100o C - 10o C) = 0,52 800W 2700s Pt = 52 % Zadatak 1: Elektrina lokomotiva vozi brzinom 36 km h-1 i pritom razvija silu 4 500 N. Kolika je jakost struje kroz motor, ako je prikljuen na napon 500 V, a korisnost je 90 %? Rjeenje: v = 36 km h-1 = 10 m s-1 F = 4500 N U = 500 V = 90 % = 0,90 % I=?

-1 Pi Fv Fv 4500 N 10 m s I Pu UI 0,90 500 V U I = 100 A Zadatak 2: Struja jakosti 10 A prolazi bakrenom icom duljine 1 m i povrine presjeka 0,1mm2. Koliko se topline oslobodi u ici svake sekunde? Elektrina otpornost bakra je 0,017210-6 m. Rjeenje: I = 10 A l=1m S = 0,1 mm2= 0,110-6 m2 t=1s

= 0,017210-6 m Q=? Q = W = I Rt 2 Q I 2 R l S l 1m t (10 A) 2 0,0172 10 6 m 1 s -6 0,110 m S Q = 17,2 J Zadatak 3: Na akumulatoru automobila napona 12 V stoji i podatak 50 A h. a) Koja je fizika veliina iskazana A h i koliko ona iznosi kada se iskae SI jedinicom? Koliina naboja.

Q = 50 A h = 50 A3600 s = 180000 C Q = 180 kC b) Kolika je energija akumulirana u akumulatoru? W = UQ= 12 V180000 C = 2160000 J W = 2,16 MJ c) Kada se akumulator isprazni punimo ga ispravljaem snage 300 W. Koliko najmanje vremena se puni akumulator uz pretpostavku da je struja punjenja stalna? P W W 2160000 J t = 7200 s t P 300 W t=2h

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