# Can work function be changed by charge-bias?

1. Sep 3, 2010

### AshsZ

Hypothetical device:

Vacuum Tube
Two tungsten electrode plates at each end

The work function of tungsten is ~4.5V.

Lets attach a power supply to the two electrodes with a potential difference of 4.0V.

Question is, by biasing the two plates in such manner, is the thermionic discharge potential at the cathode reduced to 0.5V?

rephrase: If 0.5V of thermal-equivalent heat is applied to this already voltage-biased cathode, will the conduction electrons possess ample kinetic energy to escape?

2. Sep 4, 2010

### WarPhalange

The work function of a given material is in terms of energy, not potential difference. The thing you also have to remember is that electrons "feel" electric field, not voltage potential. I can have a voltage difference of 4V between two plates, but have their separation be several miles. Clearly that is almost 0 electric field, though, so the electrons won't feel much of a force.

3. Sep 4, 2010

### AshsZ

I see. How about in the case of using a much higher voltage power supply with this tube - would make it analogous to a CRT and electrons will flow through the tube, which I know would work. From your description the reason for the flow of electrons is because there is a sufficient electrostatic field between these two plates.

With this modified hypothetical vacuum tube, having a power supply that we can adjust the voltage output, we adjust the voltage just to the point where we can measure a current. Then we back the voltage down just to the point where flow ceases. In this condition, we then add heat to the cathode.

As the cathode increases in temperature, its valence electrons will increase in kinetic energy. Would the combination of the increased electron energy (through heating) and the large electric field that is just below threshold allow the electrons to finally flow through the tube?