The resistance of the incandescent bulb in working condition is 240 ohms. The voltage in the network is 120 V.

The resistance of the incandescent bulb in working condition is 240 ohms. The voltage in the network is 120 V. How many lamps are connected in parallel to the network, if the power consumed by all the lamps is 600 W?

The power of the electric current is determined by the following expression:
P = U * I
According to Ohm’s law, the amount of current is directly proportional to the applied voltage and inversely proportional to its resistance:
I = U / R
If there are several resistances in the circuit, I = U / Req.
Equivalent resistance with parallel resistance:
Substitute the expression for the current in the expression for the power:
P = U * I = U * U / R = U² / Req.
From here we find Req:
Req = U² / P = 120² / 600 = 24 Ohm.
Equivalent resistance with parallel resistance:
1 / Req = (1 / R1) + (1 / R2) + … + (1 / Rn)
If the resistances are the same:
Req = R / n, we express the number of resistances from this expression:
n = R / Req = 240/24 = 10
Answer: 10 bulbs are included in the network.