When k₁ is closed : only Current passing through 5Ω resistor and lamp of resistance R ( Let). There is zero current passing through 10Ω.
Also we can see 5Ω and RΩ are in series
So, Req = 5Ω + RΩ
Now, use Ohm's law,
V = IR
⇒ 6 = 0.6 × ( 5 + R) [ ∵ i = 0.6A is given ]
⇒ 10 = 5 + R
⇒ R = 5Ω
Hence, resistance of lamp is 5Ω
when K₂ is also closed :- Let i current flows through the circuit.
Req = 10Ω × 10Ω/(10Ω + 10Ω) = 5Ω [ because two resistors each of resistance 5Ω are joined in series and and both are joined with 10Ω resistor in parallel ]
Now, Req = 5Ω
so, current through the circuit is i = V/R = 6/5 = 1.4A
So, current through the resistor 5Ω is half of 1.4A [ ∵ 10Ω resistor and (5Ω + 5Ω) resistors are connected in parallel . And we know , potential in parallel combination is same . So, Current will be half due to each of resistors have same resistance e.g. 10Ω ]
so, current through 5Ω resistor is 0.7A
And current through lamp is also 0.7A , so potential difference across lamp is 0.7 × 5 = 3.5Volt