Deir an chéad dlí de chuid Kirchhoff, ar a dtugtar dlí an phointe brainse freisin, gurb ionann an sruth leictreach a théann isteach i bpointe brainse agus an sruth leictreach a fhágann an pointe brainse sin. Is pointe é pointe brainse i gciorcad leictreach ina mbuaileann dhá sheoltóir nó níos mó le chéile. cosúil le pointe a sa phictiúr ar an taobh.
Is é I an sruth leictreach atá ag dul isteach sa phointe brainse, agus is iad I1 agus I2 na sruthanna leictreacha atá ag fágáil an phointe brainse, I = I1 + I2 . Sampla eile, féach ar an íomhá thíos.
Is é I an sruth leictreach atá ag teacht isteach, agus I1, Mé2 agus mise3 an sruth leictreach a thagann amach, I = I1 + I2 + I3.
Tá Céad Dlí Kirchhoff bunaithe ar dhlí caomhnaithe an mhuirir leictrigh, a deir go gcaomhnaítear an muirear, ionas go gcaithfidh an muirear leictreach atá ag teacht isteach a bheith cothrom leis an muirear leictreach atá ag dul amach; ní chailltear aon mhuirear leictreach. Is é an sruth leictreach an muirear leictreach a shreabhann thar eatramh ama áirithe. Toisc go gcaomhnaítear an muirear leictreach, is ionann an sruth leictreach a théann isteach i bpointe brainse agus an sruth leictreach a fhágann an pointe brainse sin.
Cuireadh an chéad dlí de chuid Kirchhoff chun cinn ag Gustav Kirchhoff (1824-1887), fisicí Gearmánach.
Ceist shamplach 1:
I1 = 2 Aimpéar, I2 = 3 Aimpéar, I3 = 5 Aimpéar, I4 = 3 Aimpéar. Faigh amach I5!
Plé
Mé isteach = Mé amach
I1 + I3 = I2 + I4 + I5
2 A + 5 A = 3 A + 3 A + I 5
7 A = 6 A + I 5
I5 = 7A – 6A
I5 = 1 Aimpéar
Ceist shamplach 2:
I = 10 Aimpéar, I1 = 2 Aimpéar, I2 = 4 Aimpéar. Faigh amach I3!
Plé
Mé isteach = Mé amach
I = I1 + I2 + I3
10 A = 2 A + 4 A + I 3
10 A = 6 A + I 3
I3 = 10A – 6A
I3 = 4 Aimpéar
