UCSB Science Line
Sponge Spicules Nerve Cells Galaxy Abalone Shell Nickel Succinate X-ray Lens Lupine
UCSB Science Line
Home
How it Works
Ask a Question
Search Topics
Webcasts
Our Scientists
Science Links
Contact Information
Why is tungsten used for filaments in light bulbs when nichrome's resistivity is so much higher?
Answer 1:

Thanks for sending us two great questions this week! Your observation about Nichrome having a much higher resistivity than Tungsten is a very good one.

However, when choosing a material for a particular application, many different properties of materials must all be considered together (such as: melting point (MP), boiling point (BP), thermal conductivity, electrical resistivity, cost, etc.). For example, while Nichrome does have a much higher resistivity than Tungsten, Tungsten has a much higher melting point than Nichrome (Nichrome MP = 1400 C, Tung MP = 3422 C). For a typical incandescent light bulb, the Tungsten filament operates at approximately 2500 Celsius; notice that this is actually above the melting point of Nichrome!

Therefore, when you compare the melting points of the materials to the operating temperatures of a light bulb, you can quickly see why Tungsten is the preferred choice for filament wires. I hope that this helps answer your question, and we hope to hear from you again soon!


Answer 2:

Great Questions!

Pure tungsten has some amazing properties including the highest melting point (3695 K), lowest vapor pressure, and greatest tensile strength out of all the metals.Because of these properties it is the most commonly used material for light bulb filaments. It can reach high temperature before melting and therefore emit a brighter light than nichrome can. Nichrome on the other hand, although it has a pretty high melting point (1673 K) it is not as high as tungsten, but it is often used for heating devices, such as a blowdryer. An electric current can heat the filament around 2000-3300 K, which stays below the melting point of tungsten but above that of nichrome. Check out this website on light bulbs.


light-bulbs

Actually in order to be in the visible spectrum certain temperatures must be reached.


Answer 3:

The incandescent bulb emits light through black body radiation. This is the process by which every object with heat is constant emitting electromagnetic radiation. The wavelength of the radiation that is emitted depends on the temperature of the object. All electromagnetic radiation has a wavelength, long wavelengths from 100 meters to 1 millimeter comprise radio waves, going to smaller wavelengths from roughly 100micrometer to 750 nanometers is what we call infrared radiation, and 750nm (red) to 400nm (blue/violet) is visible light. At even smaller wavelengths there is ultraviolet, x-rays, and gamma rays. You probably know that an infrared camera can see a person in complete darkness because people give off infrared radiation, because we are at a temperature of about 37 degrees celsius. At higher temperatures the wavelengths emitted are shorter. In order to emit light that we can see, i.e. 750nm to 400nm,the filament of the bulb needs to be at a temperature of about 3000degrees celsius, which is incredibly hot. Nichrome would melt at this temperature, but Tungsten is just fine!


Answer 4:

Incandescent light bulbs use heat generated by the resistance of the filament to create light by something called black body radiation. To get white light you need the filament to reach 2700-3300K to give off white yellow light. The reason tungsten is used is because it has the highest melting temperature of any element at 3600K, where as nichrome melts at 1600 K which is almost to cold to give off light (maybe dark red).

To generate a lot of heat you need a big resistance. Resistance(R) is dependent on resistivity (p) as you said, but also on the length (l) of the wire and the width (w).



R = (p*l)/w

Having a big resistivity helps to create a big resistance but since I can change the thickness and length of the filament in my light bulb (the filament is looped to increase the length) I can control the resistance. Since the wall sockets put out 120V, this is how you can create bulbs of different power ratings (Power = current * voltage = current2 * resistance).



Click Here to return to the search form.

University of California, Santa Barbara Materials Research Laboratory National Science Foundation
This program is co-sponsored by the National Science Foundation and UCSB School-University Partnerships