# Two 60W-lightbulbs are connected on 220 V AC-voltage - how much electric power is spent by each lightbulb?

Two 60W-lightbulbs are connected on 220 V AC-voltage - how much electric power is spent by each lightbulb?

There are two scenarios:

In one, these two lighbulbs are connected serially, and in the other parallelly. The trouble is, no resistance value is given, so I don't know how electric power can be calculated for each bulb. No amperage given either.

How do I proceed?

Edit: Thanks to the answer I did figure out the power consumption for each connection.

Serially:

$P=\dfrac{U^2}{R} => R=\dfrac{U^2}{P}$

At 220 Volt and 60 W the resistance is $806 \dfrac{2}{3}$ ohm for each bulb. In a serial connection the resistances add up, meaning, the two bulbs together have $1613 \dfrac{1}{3}$ ohm. Repeating the above formula, we get a total power consumption of $30$ watt. So each bulb consumes $15$ watt.

Parallelly:

The total resistance in a parallel connection can be calculated as

$R_T=\dfrac{R_1*R_2}{R_1+R_2}$

which is $403 \dfrac{1}{3}$ ohm. Thus, again repeating the formula for power consumption $\big(P=\dfrac{U^2}{R}\big)$, the total power consumption is $120$ watt. Which is $60$ watt for each bulb.

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I'm going to take a complete guess and say that each is consuming 60W of power. –  Ignacio Vazquez-Abrams Jun 16 '12 at 16:47
When ordered parallelly, they do, as I found out. I guess, lightbulbs are nowadays manufactured with parallel connections in mind. –  Miroslav Cetojevic Jun 17 '12 at 9:47

Use this formula:
Watt = (Voltage x Voltage)/Resistance
and hence the restistace of the bulb would be (220x220)/60 = 806.67 ohm

Hope it helps and you can proceed now very easily.

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Yes, it helped. Can't believe I forgot this formula. Thanks. –  Miroslav Cetojevic Jun 17 '12 at 9:46