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Why did the hot resistance of the bulb filament is higher than its cold resistance?
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What is meant by cold resistance of filament lamp?
It means exactly what it sounds like. The resistance of an incandescent bulb's filament depends on its temperature. A filament has a positive temperature coefficient, which means that its resistance increases as its temperature increases. A typical 40 watt bulb (120 volts) has a cold resistance of about 28 ohms, but its hot, operating resistance is about 360 ohms. If the cold resistance were constant, the bulb would dissipate 379 watts. In fact, cold turn on is the most stressful time for a bulb.
Does the resistance of a light bulb filament change after the light has been turned on?
The electrical resistance of a light bulb increases when it is turned on As a resistor, the tungsten light bulb has a positive resistance coefficient. This means that the electrical resistance goes up when the filament becomes hot. For example, a 100 watt light bulb operated at 120 volts - it does not matter if it is AC or DC for this calculation - will have a resistance of 144 ohms when hot and draw .833 ampere. When cold the filament typically has a resistance of only 10 ohms which increases as the filament heats up.
Why does the resistance of a filament bulb depend on the current in the bulb?
The resistance of a piece of wire changes with temperature. In a filament bulb the wire is heated to about 3000 degrees C so a large change in resistance can be expected. A 240 v 105 w halogen bulb has a cold resistance of 35 ohms, but when running its resistance is 549 ohms.
What is cold resistance?
That is e.g. the resistance of a cold bulb before the bulb is lighted and heats up.
Why do light filament bulbs usually blow when they are first switched on?
because when they are first switched on a large surge of electrical current flow through a cold filament. When you first switch on the bulb the resistance is small, so you get a current surge which breaks the weakened filament. The filament is weakened by repeated heating and cooling from the bulb being turned on and off; which makes it brittle.
What is the resistor inside a light bulb?
In an 'old fashioned' incandescent light bulb, the only electrical part inside is the filament wire itself. When you put a meter across a cold bulb and measure the resistance, you're measuring the resistance of the filament. More modern and more efficient artificial light sources, like fluorescent tubes, LED lights and CFLs, have additional electronic components inside the structure of the bulb.
What is a resistor inside a light bulb?
In an 'old fashioned' incandescent light bulb, the only electrical part inside is the filament wire itself. When you put a meter across a cold bulb and measure the resistance, you're measuring the resistance of the filament. More modern and more efficient artificial light sources, like fluorescent tubes, LED lights and CFLs, have additional electronic components inside the structure of the bulb.
How is the filament of the light bulb a resistor?
Most simple incandescent light bulbs are made of a thin section of tungsten through which the current flows. This section of tungsten is called a "filament". The tungsten filament has electrical resistance and so is a resistor. As a resistor it develops a voltage drop. This voltage drop multiplied by the amperage passing through it equals the wattage of the bulb. The heated tungsten gets to thousands of degrees above room temperature and becomes hot enough to produce yellow-white visible light. As a resistor, the tungsten light bulb has a positive resistance coefficient. This means that the electrical resistance goes up when the filament becomes hot. For example, a 100 watt light bulb operated at 120 volts - it does not matter if it is AC or DC for this calculation - will have a resistance of 144 ohms when hot and draw .833 ampere. When cold the filament typically has a resistance of only 10 ohms which increases as the filament heats up.
Why the slope of voltage-resistance characteristic increases in the case of tungsten lamp and decresing in case of carbon lamp?
If the filament really was made from a material that has a negative temperature coefficient (as temperature increases, resistance decreases) then the decreasing resistance would cause more and more current to be taken as the lamp heated up and the temperature would get higher and higher in a runaway manner until either the power supply's breaker would trip or (more likely) the light bulb's filament would simply burn open. In fact the filament has to be made from a material that has a positive temperature coefficient. (As temperature increases, resistance increases.) Then, as the bulb's temperature rises, its filament's increasing resistance causes less current to be taken than when it was cold. Quite quickly a stable "steady-state" temperature and "running" resistance is reached so that the bulb simply continues to give out a steady amount of light according to the current it is taking from the electricity supply.
Will the current I1 in a light bulb connected to 300 V be more or less than the current I2 when the same bulb is connected to 110 V?
R = V / I (Ohms Law) A greater voltage results in more current. In the particular case of a light bulb, the resistance will change with current because its temperature changes. We assume here that the bulb is rated to take 200V.
Why is a bulb most likely to fail when the current is first switched on?
Light bulb filaments glow rather than burn because of the lack of oxygen within the vacuum of the bulb. Over time, the heating and contracting of the filament and the glass surrounding it creates a leak in the base of the bulb causing the vacuum to be lost and allowing oxygen to enter the bulb. It is during the contracting phase, which happens as the bulb cools after being turned off, that eventually the seal is lost. When this happens, oxygen is allowed to enter and the next time the bulb is turned on, the filament, now exposed to the oxygen, burns out.
Why does a light bulb filament act as resistor?
The filament of a light bulb isn't like a resistor ... it is a resistor. The only difference from the ones on circuit boards is the it's designed to operate at a much higher temperature. So hot that it glows. The glass envelope is there to prevent oxygen from getting in and promptly burning it. When the filament becomes too hot it breaks breaking the current that was lighting it in the first place. That is why the light bulb "burns" out.
