An electric bulb is marked “60 watts”. What does that mean?
That is actually a very good question and will take a few sentences to answer. It means that when the correct voltage is applied, the bulb will draw electrical current and heat up, after a few seconds it will stabilise at a certain temperature and then draw a current I which when multiplied by the correct voltage will consume 60 watts. It is a measure of the power going into the bulb, not a measure of any light power coming out... what is good about the question is that it leads to the following. When the bulb is cold, just before it's switched on, the filiment in the bulb has a certain resistance and is at room temperature. When the current flows, the current heats up the filiment, first red hot, then orange hot and eventually white hot. Now metals increase their electrical resistance when hot, and the resistance of a hot filiment might be 10 times more than at room temperature!!!.
Calculate the current through a 60 W Lamp rated at 250 V,if line of voltage falls to 200 V how is the power consumed by the bulb affected?
Two simple equations needed for this.Power = volts x currentVolts = current x resistance60w = 250v x current(a)Current = 0.24aThis is the answer to the first question.You use that in the second equation to find the resistance of the lamp.250v = 0.24a x resistance(ohm)resistance = 1041.67 ohmIf you use the second equation again, but with 200v and that resistance, you can find the new current.200v = current x 1041.67 ohmcurrent = 0.1919aUsing the first equation with 200v and the new current, you can find the power of the lamp at 200v.Power = 200v x 0.1919a = 38.38wThat is the answer to the second question.