Resistance of Tungsten Filament in Tungsten Filament Lamp

The power number of the bulb is usually marked on the bulb, because we usually use 110V AC. Therefore, it is easy to calculate the resistance of the tungsten wire by Ohm's law from the power consumption and the power supply voltage. Electric power P=V2/R so R=V2/P so the resistance of a 100 watt light bulb should be _____ ohms. But if you actually take an electricity meter and measure the resistance of a light bulb. you will be surprised to find out.

The actual measured resistance is not only different from the calculated resistance value, but also very different. why? If you actually measure the resistance of the tungsten wire (bulb), use the resistivity of tungsten at room temperature of 20°C ρ=5.8×10-8Ω-m, resistance R=ρι/A, ι length A cross-sectional area, first estimate its diameter After that, you can calculate the straight length of the tungsten wire, try it out, and verify that the length I mentioned earlier is correct!

Back to the question of why the actual measured resistance is not the same as the calculated resistance value above: When you measure the resistance of the light bulb, the light bulb is not working. When the power is applied, the tungsten wire will be heated to a high temperature to generate thermal radiation. It is known that the temperature coefficient of tungsten is 0.0045/°C (percentage increase of resistance value for every 1°C increase) This hint should be obvious! Then I'm going to ask the next question: the calculated resistance is the resistance value of the light bulb when it is working, and you have measured the resistance value at room temperature, (you should measure it!) Can you estimate the tungsten value? What about the temperature at which the wire works? Verify again that the temperature I said earlier is correct!

If you have measured the resistance of the light bulb when it is not yet operating, think about how the current flowing through the light bulb when it is first plugged in compares to the current when it is operating normally. Can you figure out why the lightbulb always burns out the moment it is turned on! The newly purchased light bulb can withstand such changes, but after many times of switching, heating-cooling will make it expand and contract and reduce its mechanical strength, and it will also become thinner due to the sublimation of tungsten after long-term work. Coupled with the change at the moment when the power was turned on, it was cut off!

Sometimes the bulb will still light up even if the filament is broken. If the position of the bulb doesn't change because of the burnout, it's so close. The current can still be conducted by the discharge of the space, but this situation lasts for a short time. When the light is turned off, the relative movement of the filament will cause the filament to separate further, or even break.