Question

calculate the electric current in the given circuit if switch is open

Answer 1

When key open

Calculate the resistance

Given R₁ and R₂ are in series .

Also R₃ and R₄ are in series .

Add the resistances in series :

R₁ + R₂

==> 4 Ω + 4 Ω

==> 8 Ω

R₃ + R₄

==> 4 Ω + 4 Ω

==> 8 Ω

Now 8 Ω and 8 Ω are in parallel .

Hence let R be the equivalent resistance.

1 / R = 1 / 8 Ω + 1 / 8 Ω

==> 1 / R = 2 × 1 / 8 Ω

==> 1 / R = 1 / 4 Ω

==> R = 4 Ω

But when key is open :

current will flow only from A to B

Hence it will come across 2 resistors

Resistance will be 4 Ω + 4 Ω because the current will still pass through those two  4 Ω resistors

R = 4 Ω + 4 Ω

==> R = 8 Ω

Calculate the current

by Ohm's law :

V = I R

==> I = V / R

==> I = 12 v / 8 Ω

==> I = 3 /2 A

==> I = 1.5 A

The current in the circuit is 1.5 A

Hope it helps :-)

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Answer 2

1/Rp= 1/ R1+1/R2
= 1/ 4 +1/4 =2/4 ohm
Rp = 4/2 = 2 ohm
=2×2ohm= 4 ohm ( resistance )
R= V/I
4= 12 /I
I = 12/4
I =3 ampere