Quantum physics is often thrown around as a buzzword. It just oozes scientific charisma, am I right? Like, if you ran into someone and they said, “Oh, I’m a quantum physicist” …. Don’t pretend you wouldn’t instantly want to be their best friend.
Am I delusional? Probably. Moving on…
Scientific prestige aside, quantum physics is a pretty hefty part of the HSC Physics course. This is cool, given that it is the newest and current realm of scientific thought. All of modern theoretical and experimental physics is in the quantum realm. Getting to learn about it is awesome, but it can be tricky. It’s definitely not easy. It took some of the greatest scientific minds ever to develop it into something worth teaching (Einstein and Hawking are the most famous).
So, how do you understand something at this level of complexity?
That’s what this guide is for. I’m going to get you acquainted with the basic ideas of Quantum Physics in about 10 minutes. We’ll cover the Ultraviolet Catastrophe, Wave/Particle Duality, and Planck’s Law. I’m not touching the optional ‘Quanta to Quarks’ part of the course, but some of this may still prove relevant.
Reading this article may provoke questions, which you can get answered fast in our Physics Forums!
Classical vs Quantum Physics: What is the Difference?
The big question to start is pretty simple, what is the difference between classical and quantum physics?
‘Classical Physics’ as a term normally refers to the Newtonian Physics that we know so dear. It encompasses all ideas of Physics that you have learnt up until this point: Forces, momentum, relativity, energy, etc.
‘Quantum Physics’ is the current realm of scientific thought. Basically, it arose from the fact that classical physics did an excellent job of predicting and explaining behaviour on a large scale. When we get to the atomic level, it breaks down. Quantum Physics as a whole is our answer to that, as it provides a new way of examining things that still works at the atomic level.
Quantum physics is characterised by some pretty hefty mathematical formulations. Don’t worry, you’ll not touch the bad ones unless you go on to study physics at university.
Ultraviolet Catastrophe: The Failure of Classical Physics
Okay, so why the hell did we need Quantum Mechanics anyway? Didn’t the old stuff work fine?
Well, sort of. Classical Laws that you know (such as, say, Newton’s Theory of Universal Gravitation) do work very well for most circumstances. Predicting planetary orbits, or determining how attractive (ba-dum tish) you are to your classmate beside you? Beautiful, works a charm. But trying to predict the behaviour of atoms is not so nice. It breaks, fairly massively.
Of course, the big failure you learn about in this course is referred to as the Ultraviolet Catastrophe, which concerned the radiation curves for things called black bodies.
Let’s take stock. A black body is an idealised physical body (that is, a perfect one doesn’t actually exist), that absorbs all incident electromagnetic radiation. A black body at a constant temperature is said to be at thermal equilibrium, and emits black body radiation. This radiation can be visualised with a black body radiation curve. These are the curves you study in HSC Physics.
The issue was this. The measured black body curves contradicted the classical theories of physicists at the time. The big theory that failed was Rayleigh Jean’s Law. This law predicted that the radiance/intensity of a black body would become extremely large with decreasing wavelength/increasing frequency. In actuality, the curves have a small peak and then cut off. The law worked for high wavelengths, but not low ones.
It was Max Planck who first empirically derived a solution to the issue, and we now know this solution to be Planck’s Law. This new law was able to correctly model the black body radiation curves. The two laws, Rayleigh-Jean’s and Planck’s, can be proved through some fairly simple math to agree for long wavelengths. It only disagrees for shorter wavelengths.
Quantified Energy
Of course, we know that Planck’s Law is based on the one equation you do need to know for this part of the course:
E = hf
Planck determined that, in order for his laws to make sense, energy must be released from a black body in discrete quantities. That is, it is released in small packets, and these packets are called quanta.
We now know that his idea had a much larger consequence. We had realised that energy is quantised. That is insane! And it leads to all sorts of crazy physical phenomena that I won’t explain here. It also leads you into the realm of some mentally scarring mathematics.
So how does this apply to black bodies? Well, it is quite simple. It was formulated that a black body released these quanta in response to energy changes within its atomic structure. Electrons lose energy, reactions take place, etc. The excess energy is released as a quantum packet of radiation energy. This conversion of thermal energy into electromagnetic energy is normal for any object.
Depending on the object, most of these quanta will have a similar amount of energy. By the equation above, this must also mean that most of these quanta have a similar frequency. This corresponds to the characteristic/peak wavelength on a black body radiation curve.
Why does the curve flatten out for other frequencies? This is a little harder to explain, but it again comes down to the atomic structure of the black body. Sure, some energy packets released might be hugely energetic, and thus have a very high frequency. However, most black bodies don’t release many quanta with high frequencies, so although each packet has more energy, less packets are released. This corresponds to the drop in intensity we see on a black body radiation curve.
Wave/Particle Duality and Photons
It was Einstein who first took Planck’s hypothesis of quanta and applied it to light. Funny fact, he didn’t actually coin the term ‘photon.’ It had been used previously by several people. Einstein did postulate that all forms of electromagnetic radiation could only exist in discrete energy packets.
We can model electromagnetic waves such as light, using the photon particle model. How do we do this? Quite easy. The frequency of electromagnetic radiation corresponds to the energy in each photon. More energy per photon means a higher frequency. The intensity of light, as suggested earlier, is just the number of photons striking some surface per unit time.
It can be pretty tricky to wrap your head around this idea, but note that we now believe that ALL elementary particles exhibit the behaviour of both waves and particles. We call this the wave-particle duality. Funnily enough, the first hint of this way of thinking was the discovery of the electron. Those who do Quanta to Quarks will learn more about De Broglie’s Hypothesis, which links the momentum and wavelength of every wave/particle.
This is complex stuff, but the basic idea is this. Light exhibits behaviour which is consistent with the wave model. It exhibits other behaviours consistent with the particle model. Both are valid models for light!
And this is pretty much the basics of quantum physics, at least the parts you need to start applying to other principles. The photoelectric effect, the operation of photo cells and solar cells; all of these rely on a solid understanding of quantum physics.
If you need some clarification of anything in this article, or anything for that matter, try our FREE HSC TUTORING! We answer questions, mark essays, and give all the help we can to help you get the results you deserve.
