modern physics, Study notes of Physics

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modern physics

the quantum revolution

➜ all the physics I’ve shown you so far is “deterministic” ➜ if you precisely measure the condition of a system at some point in time ➜ and you know the “equations of motion” ➜ you can predict what will happen for evermore ➜ the “clockwork” universe for example

• projectile motion
• planets orbiting the sun
• electric and magnetic fields from charges and currents ➜ physics was viewed this way until the turn of the 20 th century ➜ when some simple experiments forced us to rethink our views

the quantum revolution

➜ we now think of fundamental physics as “probabilistic” ➜ we can only calculate the relative odds of any particular event occurring ( at least at microscopic scales ) ➜ e.g. in classical electromagnetism :

if we measure the position and velocity of the electron, we can use equations of motion to predict the exact path of the electron heavy positive charge electron electron hits here

the quantum revolution

➜ we now think of fundamental physics as “probabilistic” ➜ we can only calculate the relative odds of any particular event occurring ( at least at microscopic scales ) ➜ e.g. in the quantum theory :

can only determine the relative probability that the electron will hit at each place on the wall heavy positive charge electron high probability lower probability lower probability

the quantum revolution

➜ how did we come to this? ➜ TRYING TO EXPLAIN EXPERIMENTAL RESULTS! the scientific method ➜ one of the first ‘troubling’ results was the ‘photoelectric effect’

the photoelectric effect

➜ a simple experimental observation ➜ shine UV light onto a charged electroscope and it discharges ➜ OK, interesting, let’s do a controlled experiment, varying properties of the light and the metal and see what happens

the photoelectric effect

3. current only flows for light with frequency above some threshold, f > f 0

the photoelectric effect

5. reversing and increasing the potential, the current flow can be stopped, and the potential required, - V stop , is independent of the light intensity

the photoelectric effect

explain by conservation of energy then Δ V = -V stop corresponds to the energy needed to stop K max the light provides energy to the electrons it ‘costs’ a certain amount of energy to pull the electron out of the metal whatever is left over goes into kinetic energy of the freed electron maybe the ‘cost’ can vary depending how ‘deep’ the electron is in the metal but there is a minimum cost and hence a maximum K.E.

the photoelectric effect

so what’s the problem?

2. the current appears without time delay when the light is switched on 3. current only flows for light with frequency above some threshold, f > f 0 our wave theory of electromagnetism says that energy arrives continuously, with more energy arriving for more intense light the frequency should be irrelevant!

the photoelectric effect

the solution : if the frequency of the light is too low, a single photon doesn’t have enough energy to overcome the work function

light is particles now? really?

sorry, but yes ... consider a double slit experiment performed with light of very low intensity some aspects of light are wave-like and some are particle-like e.g. interference pattern e.g. the individual arrival don’t like this? it’s going to get worse!

‘lumpy’ energy

➜ there are other experiments that suggest energy can come in discrete amounts ➜ atomic spectroscopy ➜ diffraction grating spectrometer

atomic spectroscopy

➜ diffraction grating spectrometer results continuous spectrum - all colors discrete spectra

• only certain special wavelengths only certain photon energies are emitted by ‘excited’ atoms