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Name: Sunita
Status: student
Grade: 9-12
country: India
Date: Fall 2013


Question:
Tungsten is a good electron emitter but it is bad conductor. Why? If it can emit electrons easily why is it unable to conduct? Does it mean that if a substance is a bad conductor of electricity it is a good electron emitter (has low work function) ?



Replies:
Hi Sunita,

When comparing different metals at the same temperature, tungsten is no better an emitter of electrons, than any other metal. The reason tungsten is used as an electron emitter (such as when it is used as a cathode in a vacuum tube), is simply that tungsten is able to withstand operation at very high temperatures without damage.

To get significant electron emission, requires operation of a cathode at very high temperature; it must be red hot. Most metals will either melt or become very weak at this temperature. The only reason tungsten is used as an electron emitter (such as in the cathode of a vacuum tube) is that tungsten is one of the few metals that can withstand operation at the extreme temperature needed to get significant electron emission.

To summarize, in order for a cathode to achieve significant amount of electron emission, it must operate at red heat. Tungsten is usually used because it is the cheapest metal that can withstand extended operation at such high temperature. Other than its ability to withstand extreme temperature, there is no other advantage to using tungsten as a cathode. Regards, Bob Wilson


Sunita,

One way to view this is to think that electron emission goes by a different mechanism than conduction. In order for an electron to be emitted, the most energetic electron(s) of that atom must be of a high enough energy so that it requires very little energy to ionize that atom. On the other hand, for a substance to be a good electrical conductor, the electrons must require very little energy to be promoted to a conduction band. So it is the difference of taking an electron out of an atom, versus having a readily accessible orbital within the atom that an electron can get promoted to.

More importantly, in my opinion, ionization (the ejection of an electron) is an atom property. We can view it as an atom acting separately from other atoms. Conduction on the other hand is a bulk property. It is the result of a lot of atoms acting together. In metals, electrical conduction can be viewed as a result of the fact that in the crystal lattice of most metals, electrons are freely shared between atoms, electrons do not necessarily stay confined to one atom. This is referred to as the "electron sea model". It so happens that in tungsten, the electrons are more bound to their atoms. Its conduction band is not as easily accessible. So the tungsten atom should not be compared to the tungsten bulk.

Greg (Roberto Gregorius) Canisius College


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