Temperature:

Why does a filament give off light?

A light bulb is “turned on” by passing an electric current through its filament, heating the filament until it is “white hot.” The high temperature of the filament--up to 3,000^oC, causes it to give off visible light by a process called incandescence. Incandescence is defined as the emission of visible light by a hot body. Any hot object gives off incandescent light. The higher the temperature of the object, the brighter the light given off. Examples of incandescent objects include hot coals in a campfire or barbecue grill, the sun, light bulb filaments and the burners on electric stoves, which glow dull red when their temperature is on high. Electric stove burners are just above the minimum incandescence temperature visible to the human eye, about (390^oC).

In describing incandescence, most temperatures are given in degrees Kelvin (K). The Kelvin temperature scale is directly related to the Celsius scale given in degrees centigrade (^oC) by:        

      T (K)  = T (^oC) + 273.15

The Fahrenheit temperature scale is also directly related to the Kelvin scale:

 
         T (K)  =  (5/9)T(^oF) + 255.37

Use the Interactive Thermometers to explore the relationship between Kelvin (K), Centigrade (^oC) and Farenheit (^oF) scales as water changes from ice to water to gas. Also investigate the broader Interactive Temperature Scales which range up to and beyond typical light bulb filament operating temperatures.

After you have explored the applets, take the quiz on the right. You can always go back and experiment with the applets, to answer the questions.

Incandescence:

How Does Temperature Affect Incandescence?

The incandescence of objects at any temperature can be predicted using Planck's law, which assumes an object is a black body radiator. A black body radiator emits only its own light and does not reflect outside light. Planck’s law describes the energy intensity of all the different colors, or wavelengths of light, emitted.

Planck's Law:

gives the total energy radiated from the object ( ), in watts per square centimeter, as a function of wavelength ( ), in micrometers ( µm ) and the surface temperature ( T ), in Kelvin.

The color of the light emitted is related to the wavelength at which the light intensity is greatest, which is given by Wien’s law (obtained from Planck’s law by differentiation):

Wien’s Law: max T = 2898

where, max is the emission wavelength with the greatest intensity and T is the surface temperature.

Together, these laws predict the light given off by a light bulb or any incandescent source. Use the interactive incandesent spectrometer on the right to explore the color/wavelength and intensity of light given off from any incandescent light source as a function of it's temperature. The sliding bar controls the temperature. The intensity of visible light is proportional to the area under the curve in the visible (colored) region of the spectrum. The color of the light given off results from the combined intensity of the various wavelengths of visible light emitted. The light to the right of the visible region is in the infrared region of the spectrum. The light to the left is in the ultraviolet region.

Use the Interactive Incandescent Spectrometer on the right to explore the relationship between light emission and temperature.

After you have explored the relationship between temperature and the wavelength, color and intensity of light, take the quiz on the right. You can always go back to the applet, to arrive at your answers.

Invent:

What features should a light bulb filament material have?

Using Plank’s law and Wien’s law you can predict the wavelength and intensity of light emitted by a filament at different operating temperatures. Since, only light visible to the eye is useful, the greater the fraction of visible light emitted by the filament, the more efficient it will be. The largest fraction of visible light is emitted from a filament operating at ~5,000 K. More light is emitted at higher temperatures, but more of it is out of the range of visible light. Inventors have used these principles to invent and improve incandescent filaments.

Now take a short quiz on the essential features a filament material should have.

Invent your own Incandescent Light Bulb Filament

Many filament materials were pursued to optimize visible light emission and filament longevity in the race to build a better incandescent light bulb. Some of the more prominent materials explored for use in incandescent filaments were carbon (C), osmium (Os), tantalum (Ta), platinum (Pt) and tungsten (W).

To optimize the visible light emitted and longevity of a filament, the material from which it is made must withstand high temperatures. Two of the most significant failure mechanisms to be avoided are melting and evaporation, or sublimation, of the filament.

Use the Filament Tester to explore different filament materials to see which performs the best. Remember, the brightness and longevity of the filament are the two crucial test criteria. Click on any element to see its melting point and sublimation temperature at 10-3 torr (note that some elements have already melted before their vapor pressure reaches 10-3 torr, the sublimation point). After clicking on the element, you can test its melting resistance by increasing the current in the light bulb filament using the sliding control. Using the Sublimation Test button you can test the filament's behavior at its sublimation point (the temperature at which the filament has a vapor pressure of 10-3 torr). All materials erode/sublime at the same rate at their sublimation point. At progressively lower temperatures sublimation resistance improves dramatically.

After you have explored the applet, take the quiz on the right. You can always go back and experiment with the applet, to answer the questions.

Why Tungsten?

Why use Tungsten filaments?

The filaments used in early light bulbs were made of carbon. However, the carbon filament can not survive for long at temperatures higher than 2,100^oC. Carbon vaporizes from the filament at these temperatures, shortening the life of the filament. The bulb gives off only dim light at lower filament temperatures.

Tungsten filaments offer the best combination of high melting point and low vapor pressure for all known elemental filament materials. This allows the filament to be heated to higher temperatures and provide brighter light with good longevity. However, even tungsten filaments fail.

Now, let's explore why a light bulb filament burns out. By understanding how tungsten filaments fail, it should be possible to invent even better filaments and light bulbs.