Question

Three bulbs, each with a resistance of 180 Ω, are connected in parallel to an ideal battery of emf 60 V.
Find the current delivered by the battery when (a) all the bulbs are switched on, (b) two of the bulbs are switched on and (c) only one bulb is switched on.

Answer 1

(a) When all the bulbs are switched on, we have a combination of three bulbs in parallel.
The effective resistance of the circuit,
$R_{\mathrm{eff}}=\frac{180}{3}=60\mathrm{\Omega }$
The current delivered by the battery,
$i=\frac{60}{60}=1\mathrm{A}$

(b) When two of the bulbs are switched on, they are parallel to each other.
The effective resistance of the circuit,
$R_{\mathrm{eff}}=\frac{180}{2}=90\mathrm{\Omega }$
The current delivered by the battery,
$i=\frac{60}{90}=0.67\mathrm{A}$

(c) When only one bulb is switched on, the effective resistance of the circuit,
$R_{\mathrm{eff}}=180\mathrm{\Omega }$
The current delivered by the battery,
$i=\frac{60}{180}=0.33\mathrm{A}$