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Name: Mylisha
Status: Other
Grade: 6-8
Location: MA
Country: United States
Date: May 2007


Question:
Does a light bulb's life depend on its wattage?



Replies:
Hi Mylisha,

Light bulbs fail primarily because, over time, the tungsten filament actually evaporates slowly at the very high temperatures it operates at. When there is an area that becomes a little thinner than the rest of the filament, that area has a little more resistance, and as a result gets hotter. The increased heat at that point causes a more rapid rate of evaporation of the filament there, causing more heat and more evaporation, and so on. Eventually that area gets so thin and hot, it burns out.

A 40 Watt light's filament (for example) operates at much the same temperature as the filament in a 100 Watt light. The difference between them is that the 100 Watt light has a thicker filament and uses more current. But their filament temperatures are much the same, so their rate of evaporation of their filaments is similar.

You might think that the slightly thicker filament of a higher wattage light would allow a longer time before it evaporated enough to burn out. However, because the 100 Watt light uses more current, the minimum thickness of the filament before it will overheat and fail, is thicker than for a 40 Watt light. So the result is the lifetime is much the same. Of course this comparison only applies for lamps that are similar to each other except for wattage. You cannot compare, for example, a Christmas tree light with a 100 Watt lamp because there are many other things that are different about them.

Regards,

Bob Wilson.


Wattage is but one variable in determining the "lifetime" of a light bulb -- there are numerous others. Some examples: geometry of the filament (assuming an incandescent light and not a fluorescent bulb, an LED, or a laser), the operating temperature, the filament material, the "on - off" cycling of the lamp, and other variables too. As a rule, higher wattage lights "tend" to have a shorter lifetime, but it is difficult to make a hard and fast rule.

Vince Calder


No, the lifetime does not depend directly on the wattage (power). Rather, the lifetime depends most directly on the temperature of the filament, all other things being equal.

The hotter the filament, the shorter the life of an average incandescent bulb.

A bulb that is powered so that the filament is glowing a brilliant blue-white will soon burn out, regardless of the power. And a bulb that is powered so that filament is glowing a dim yellow, will last for years, regardless of the power.

Commercial bulbs are rated for a certain voltage. That is, the manufacturer specifies the voltage necessary to make the bulb operate properly - to give a reasonable bright light without burning out too fast. It might be 120V I a house, or maybe 12V in a car.

The light output and lifetime of lamp filaments is known pretty accurately as a function of voltage. Briefly: the amount of light that a bulb produces goes as the 3.5 power of the voltage ratio. The lifetime goes as the inverse twelfth power of the voltage ratio. This is for a normal light bulb. Also, the current goes as the 0.55 power of the voltage ratio. Voltage ratio is the applied voltage divided by the rated voltage.

So, for example, if a 220V bulb is operated at 110V, the voltage ratio is 0.5. The light output will be 0.5 raised to the 3.5 power, which is 0.088. It will be very dim! It will make less than one-tenth as much light as if it were run on 220V. It will also be orange in color.

The bulb will last a LONG time at this low output. The lifetime will be increased by 0.5 raised to the inverse twelfth power, or 2 raised to the twelfth power, which is 4096 times as long! It will last for thousands of years.

Light bulbs for photographic work are sometimes overdriven to make a lot of light, but they do not last as long.

For example, suppose a 120V bulb is driven at 140V. The voltage ratio is 140/120 = 1.167. The light output is 1.7 times as much. The actual power consumed is 1.27 times as much, so the bulb is more efficient. However, the lifetime of the bulb is cut down by a factor of 6.35.

For fun, I ran a 12V light bulb on 22V. It was very bright. It lasted 2 hours.

Bob Erck



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