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TFM
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Homework Statement
A thin square tungsten foil, 10mm x 10mm, is supported in the centre of a vacuum chamber by thin wires which have negligible thermal conductance. A cooled shield surrounds the foil such that its initial temperature is -37 C. Two parallel slits, 1[tex]\mu[/tex]m wide and 1mm long and whose centres are separated by 6.6[tex]\mu[/tex]m, have been cut in the foil. The foil is now heated by electron bombardment with a 10mA current of 5keV electrons, whose energy is completely absorbed by the foil. Calculate the change in separation of the slits.
How would you go about measuring this change in separation from outside the vacuum chamber?
For tungsten, the emissivity should be taken as (1/5.7) and the coefficient of linear expansion as 5 x [tex]10^{-6} K^{-1}[/tex]. (The linear expansion coefficient [tex]\alpha[/tex], is defined as the fractional change in linear dimension per degree Kelvin, i.e. [tex]\alpha = \DeltaL/L0[/tex] where [tex]\Delta[/tex]L is the change in length for a 1K change in temperature and L0 is the original length.)
Homework Equations
[tex] \frac{\Delta L}{L_0} = \alpha \Delta T[/tex]
The Attempt at a Solution
Okay, so to get the change in length, Delta L, I need the formula
[tex] \Delta L = L_0 \alpha\delta T [/tex]
However, I am unsure how to get T.
When I saw that there was a current, I thought of using the power law
P = VI
and
V=IR
but we are not given any resistance. we are given the energy of each electron, But I am unsure how to get through to the current.
TFM