Materials Week 6 Flashcards
Why is the Tungsten filament coiled into a shaped spring?
So that it can accommodate the expansion and contraction of the W metal as it is heated and cooled.
To be able to fit the long length of the W wire into the small space provided for the filament. The W filament must have a certain resistance so that the joule heating effect will allow it to get to the optimum operating temp to give off light using incandescence. The resistance depends on the material and the diameter and length.
Why is incandescence considered to be more efficient at generating visible light the higher the temperature?
At higher temperatures, a greater fraction of the spectrum of light given off by incandescence is centred over the visible spectrum. At low temps, the spectrum of light emitted by incandescence is centred at long wavelengths, particularly the infrared. As the temp increases the peak in the intensity shifts to shorter wavelengths and at 550K it is centred in the visible spectrum. It’s more efficient at higher temps than lower.
Why do metals heat up when you pass a current through them?
The conducting electrons bump into the atoms and transfer some of their kinetic energy to other atoms. This results in increased atomic vibration of the atoms, which is another measure of the temp of a material. More vibrations means a higher temp.
How do you expect the resistivity of metals to change with an increase in temp?
We expect the resistivity to increase at higher temps. The increased vibrational amplitudes of atoms at higher temps means there is an increased chance of conducting electrons bumping into the atoms. This decreases the average mobility of the conducting electrons and hence increases the resistivity.
How do you expect the resistivity of the W-AL alloy to change as you decrease the temp to absolute 0?
We expect the resistivity to decrease as the temp approaches 0K because the atomic vibration of atoms will decrease. At 0K resistivity won’t be 0 because the Al atoms are still not the same size as the W atoms and therefore this solute misfit will mean the conducting electrons can still bump into atoms.
Why don’t Tungsten filament bulbs last forever? What determines their lifetime?
The evaporation of Tungsten atoms from the filament. In a local area of the filament if some atoms leave the filament by evaporation, then the local cross section will be reduced. If the local cross section is reduced then the resistance increase which leads to greater heating and more evaporation. The process is self supporting and once started is difficult to stop and eventually breaks the filament
Why are incandescence light bulbs banned?
Because they are terribly inefficient. Max 6-7% conversion efficiency from electrical energy to visible light.
What is a band gap?
Band gap is a range of energies that is forbidden for electrons to occupy. A typical band gap for insulators if more than 2eV and for semi conductors typically 1-2eV
Explain why Argon is found in Tungsten filament light bulbs?
The 3rd shell of Ar is full with 8 electrons (3s2-3p6). Ar doesn’t want to accept or give off any other electrons to or from other electrons. It is therefore extremely unreactive. It is used in incandescent light bulbs to protect the W filament from oxidation at high temp
What limits the operating temperature of Tungsten filament bulb?
We don’t have conducting materials with melting temperatures higher than Tungsten.
Provide a physical interpretation of the correlation that is observed between the thermal
expansion coefficient and the melting temperature of materials (Fig. 6.3 e-pub).
We tend to observe that the higher the melting temperature of a material, the lower is
the thermal expansion coefficient. We may think of melting a material as a process of
providing enough thermal energy to vibrate the atoms apart from their equilibrium
separation. If this process requires lots of energy (ie. high melting point) then the
energy minimum in the interatomic potential curve will be deep. The thermal
expansion coefficient is a measure of how much the atoms vibrate apart when we
increase the temperature of a material by 1K. If we need to vibrate the atoms apart to
some critical distance for a material to melt, and the melting temperature is really high,
then the thermal expansion coefficient must be low. This is reflected in the interatomic
potential curve which shows when the energy minimum is deep, the curve must also be
sharper (leading to smaller thermal expansion coefficients).
