Consider a beam of electrons with energy 1 eV, flying exactly
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Transcript of Consider a beam of electrons with energy 1 eV, flying exactly
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Consider a beam of electrons with energy 1 eV, flying exactly in x-direction (i.e. no momentum in y-direction, py=0). You now measurethe y-position of the electrons by placing an aperture with 5 nm widthinto the beam. What is the consequence? y
x(A) I now know the y-position of the electrons within 5 nm. The electrons continue straight on along the x-direction, just like before the measurement.
(B) I now know the y-position of the electrons within 5 nm. Some of the electrons are deflected and acquire some momentum in y-direction, now flying faster than before.
(C) I now know the y-position of the electrons within 5 nm. Some of the electrons are deflected and acquire some momentum in y-direction, slowing down accordingly w.r. to their momentum along x-direction.
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Consider a beam of electrons with energy exactly 1 eV, flying exactly in x-direction (i.e. no momentum in y-direction, py=0). You now measurethe y-position of the electrons by placing an aperture with 5 nm widthinto the beam. What is the consequence? y
x(A) I now know the y-position of the electrons within 5 nm. The electrons continue straight on along the x-direction, just like before the measurement.
(B) I now know the y-position of the electrons within 5 nm. Some of the electrons are deflected and acquire some momentum in y-direction, now flying faster than before.
(C) I now know the y-position of the electrons within 5 nm. Some of the electrons are deflected and acquire some momentum in y-direction, slowing down accordingly w.r. to their momentum along x-direction.
Before slit: py = 0 we do not know the y-position of the electrons at all!... y =
After slit: y = 5 nm py = ħ/10 nm .... we cannot circumvent HUP!
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Note: The slit results in diffraction of the matter wave, whichintroduces the uncertainty in py as shown below. You already knowthis behavior from diffraction of light at a slit!
y
x
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The conservation of total mechanical energy of a frictionless particle means that …
(A) … the sum of heat and kinetic energy is a constant.
(B) … the potential energy of the system is a constant.
(C) … the sum of kinetic and potential energy is a constant.
(D) … the particle has always the same velocity.
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The conservation of total mechanical energy of a frictionless particle means that …
(A) … the sum of heat and kinetic energy is a constant.
(B) … the potential energy of the system is a constant.
(C) … the sum of kinetic and potential energy is a constant.
(D) … the particle has always the same velocity.