IB Physics - 12.1.2 - The failures of classical Physics

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this is a love of physics video

investigating where classical physics

failed and why we need quantum and

modern physics the first failure of

classical physics that we will look at

is blackbody curves so we know from unit

8 that black body is a perfect emitter

of radiation and we know that the

blackbody curve looks something like

that with a peak wavelength

corresponding to the temperature so we

know that black bodies look like this

but if we use classical physics to work

out what the shape of this graph should

be it's kind of similar at these longer

wavelengths but then it goes up and up

and up until it reaches an intensity of

infinity now obviously nothing gives out

an infinite amount of energy so we know

that something must be wrong with the

classical way of looking at things now

it turns out that the blackbody curve

can only be explained using quantum

physics it can only be explained if this

energy is being emitted not as a

continuous wave but as a stream of

photons with energy HC over lambda what

that means is the amount of energy

needed to just omit one photon these

very short wavelengths is so so so so

big that it makes it very very unlikely

so the intensity is very low so quantum

physics and the idea of waves being

particles light waves being particles

called photons is the only way we can

explain blackbody curves now the second

problem that scientists faced at this

time at the beginning of the 20th

century was the model of the atom now we

know that

the nucleus is in the middle of the atom

and there are electrons orbiting around

the nucleus

now classical physics tells us that if a

charged particle accelerates it gives

out energy now if this electron is

accelerating which we know it is because

circular motion says that there's a

constant acceleration towards the center

of a circle then this electron must

constantly be giving our energy if the

electron is constantly giving out energy

then it should slowly spiral into the

nucleus as it loses energy and when it

reaches the nucleus it should annihilate

electrons should not be allowed to orbit

nuclei by the rules of classical physics

now obviously this is wrong we know that

there are electrons we see it if we've

ever done any sort of chemical

experiment we know that there are

electrons are they interact with each

other and that's why we get chemistry at

all so what is happening here well it

turns out this can be explained with

quantum physics as well and the way that

we can explain this very simply is that

for a lone electron to spiral it needs

to gradually give out energy and we know

from quantum physics that it can only

give out energy in the form of photons

and the photons have to be discrete

packets of energy with a certain

wavelength so if we were to slowly

spiral we'd be gradually giving out

energy but that doesn't happen because

of these this discrete energy of these

photons we have to give out energy and

chunks not gradually like this spiral

now it turns out as we will see later on

in this chapter that these electrons

actually form standing waves but that's

for later on in Chapter twelve point one

the third failure of classical physics

was down to the photoelectric effect

now the photoelectric effect involves

shining a light of a certain frequency F

onto a metal plate usually in these

demonstrations zinc is used and what

happens when you shine visible light for

example on on some zinc it's absolutely

nothing nothing happens nothing is

emitted however if you shine an

ultraviolet light that's light with a

slightly higher frequency than visible

light then you start to get electrons

being emitted immediately now the

strange thing about that is that

according to classical physics what

should happen is if light is a wave then

the energy should slowly build up and

yeah if the frequency is lower then the

light has less energy so it should take

longer but the energy should build up in

the zinc and you should start getting

electrons released however it only

releases electrons when light above a

certain frequency is incident on the

zinc now the other strange thing is that

all of these electrons all have exactly

the same kinetic energy if light was

just a regular wave then these electrons

would be ejected with a total variety of

kinetic energies but they're not and

they always have exactly the same

kinetic energy which depends on the

frequency of the light

now we know because of quantum physics

now that light isn't in fact just a

continuous wave but it is made up of

incredibly small particles called

photons and each of those has an energy

H

so now we can say that when one of these

photons hits one of the electrons in the

zinc it either doesn't have enough

energy to completely it ionized the

electron to completely remove it from

the zinc or it does but each time that

energy if the frequency is the same the

energy of the photon is exactly the same

and so if the energy of the photon is

the same and the amount of energy needed

to ionize the electron from the zinc is

the same then there's always the same

amount of energy left over which is why

the electrons always have the same

kinetic energy

so overall quantum physics again had the

answer to this challenge that was a love

at physics video if you liked it and you

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