Kichoff's Law
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Transcript of Kichoff's Law
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8/2/2019 Kichoff's Law
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1Dunblane High SchoolRG
What you need to know
Derive the expression for the total
resistance of any number of
resistors in series, by consideration ofthe conservation of energy.
Derive the expression for the totalresistance of any number of
resistors in parallel by consideration ofthe conservation of charge.
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2Dunblane High SchoolRG
Resistors in Series
R1 R3R2RT
EMF EMF
These two circuits have the same effective resistance
The emf = to the sum of the p.d.s around the circuit
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3Dunblane High SchoolRG
Resistors in Series
R1 R3R2RT
VT EMF
IRT = IR1 + IR2 + IR3But in a series circuit the current is the same at all
points so we can divide by I so.RT = R1 + R2 + R3
For resistors in
series
I
VT = V1 + V2 + V3
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4Dunblane High SchoolRG
Resistors in Parallel
R1
R3
R2RT
EMF
EMFI
I1
I2
I3I
These two circuits have the
same effective resistance.
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6Dunblane High SchoolRG
Resistors in Parallel
R1
R3
R2RT
VT
EMFI
I1
I2
I3IT
VT/RT = V1/R1 + V2/R2 + V3/R3 BUT
In a parallel circuit
VT = V1 = V2 = V3 (divide by V)
1/RT = 1/R1 + 1/R2 + 1/R3
VT
I = V/R
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