Question

UWE
27. (a) With the help of a suitable circuit diagram prove that the reciprocal of the equivalent resistance of a
group of a resistances joined in parallel is equal to the sum of the reciprocals of the individual resistances.
(b) What would be the readings of the ammeter and the voltmeter when key is closed? Give reason to
justify your answers.
852
52
125
6V​

Answer 1

Consider 3 resistors having resistances R1,R2,R3 connected in parallel to a cell,key and ammeter.A voltmeter is connected in parallel to the parallel combination of resistors.

In parallel combination,P.D across each resistor remains constant.

and,the current across the combination of resistors is equal to sum of current across each resistor.

ie, I(effective)= I1 +I2 +I3———-———(1)

But from ohm’s law,V=IR

so,I=V/R

substituting in (1),

we get, V/R(effective)= V/R1 + V/R2 + V/R3

dividing the whole equation by ”V”

we get, 1/R(effective)= 1/R1 + 1/R2 + 1/R3

..............

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