Question

A 100W,250V lamp is connected in series with a 100W,200V lamp across a 250V supply mains. Calculate the current drawn from the supply mains and voltage across each lamp. Assume lamp resistance.

Answer 1

Using nominal power and voltage of lamps find lamps' resintance:

$P=I\times U=\frac{U^2}{R}$

$R=\frac{U^2}{P}$

$R_{1}=\frac{250^2}{100}=625Ohm$

$R_{2}=\frac{200^2}{100}=400Ohm$

Current in circuit:

$I=\frac{U}{R_{1}+R_{2}}=\frac{250}{625+400}=0.244A$

Lamps' voltage:

$U=I\times R$

$U_{1}=I\times R_{1}=0.244\times 625=152.5V$

$U_{2}=I\times R_{2}=0.244\times 400=97.6V$