My question is simple. In Ideal situation, at constant temperature, we know that normal appliances like a filament bulb has straight Voltage vs Current graph, meaning its resistance is constant or voltage is directly proportional to current.
Now, we also have bulbs of desired power available. eg. 10W, 20W, 100W bulbs etc. Since I understand that power = V x Current, the power for a bulb can not be a constant if its resistant is assumed a constant. A normal mathematical thinking can confirm that.
So, what does it means when we say the a certain bulb is a 10W bulb? Does it simply means that it would consume 2 times energy at a given voltage if it replaces a 5W bulb?
Best Answer
Actually, we don't know that "filament bulb has straight Volatage vs Current graph": "The actual resistance of the filament is temperature dependent. The cold resistance of tungsten-filament lamps is about 1/15 the hot-filament resistance when the lamp is operating. For example, a 100-watt, 120-volt lamp has a resistance of 144 ohms when lit, but the cold resistance is much lower (about 9.5 ohms)." (http://en.wikipedia.org/wiki/Incandescent_light_bulb )