Category: 12 Turning Points

Disc Electron, Duality, Relativity

30 Relativity

This page covers the relativity section of the course. 

The key to understanding the Michelson–Morley experiment (Topic 3.1) is realising that turning the apparatus through 90º alters the relationship between the beam paths and the Earth’s motion – no matter which way the Earth is moving. The null result disproves the ether theory.

Inertial frames of reference can be explained as places where the observer is considered to be at rest compared to the measuring equipment they are using. Since the value of the speed of light is always the same, then the observer’s measuring equipment (their ruler or their clock) must adapt to conform with any relative motion. Thus at high relative velocities their ruler would shrink and their clock would slow so as to indicate the correct readings.

Resources

30 Relativity

30 Relativity Book

A2 Relativity Practice Questions

A2 Relativity Timed Assessment

Relative Motion and Inertial Reference Frames

Did you know that everything is moving? Even you, as you're sitting perfectly still, because the earth is moving, and the sun, and the galaxy, and so forth. For this ...reason, it only makes sense to talk about the motion of some object relative to some other object, just as Galileo told us. Watch this and see what I mean!

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29 Wave Particle Duality

This page covers the wave particle duality section of the course. 

One way of modelling Newton’s corpuscular theory of light (Topic 2.1) is a rough demonstration of refraction using a ball rolling down a slope. Fold and support a large piece of card so that there are two flat surfaces separated by a short broad ramp. If a small ball is made to roll obliquely down the ramp from the higher surface to the lower, its direction changes (towards the normal) as it speeds up. The students can investigate Young’s fringes with either light or 3 cm microwaves, depending on the equipment you have available. An appropriate risk assessment should be carried out, especially if you are using a laser for the light source. You may be surprised to learn that Newton maintained his support for the corpuscular theory despite having studied Newton’s rings – which provide strong evidence in favour of the wave nature of light.

Electromagnetic waves (Topic 2.2) can be demonstrated using 3 cm microwaves. Most microwave kits include the extra equipment needed to demonstrate and investigate the properties of waves. For instance, students should be able to measure the wavelength by setting up a standing wave with an aluminium reflector and using the probe to locate the maxima and minima. Other wave properties that can be studied using 3 cm microwaves include diffraction and polarisation.

We can introduce wave–particle duality via the photoelectric effect (Topic 2.3), which shows why a particle theory of light won favour again. Students should understand the difference between photons and photoelectrons. Simply put, it is the photons that go in and photoelectrons that come out. The idea of a minimum energy may be hard to grasp, but a simple analogy for the photoelectric effect is for students to think of a sculptor chipping away at a stone block. Insufficient energy produces no result; a hard blow chips a little away; a harder blow can send a small chip flying. If you calculate some photon energies for different types of electromagnetic waves (using E = hf or E = hc/λ) they can relate this energy to the position in the electromagnetic spectrum, which they should recall from their GCSE work.

A simple way for understand how to measure the Planck constant is by using LEDs of different colours. Students should measure the forward bias voltage (V) that just switches each LED on. Manufacturers supply values for the wavelength (λ) of the light produced by their LEDs. Once students are clear that in this case the energy flow runs the other way from that in the photoelectric effect, they can substitute their values in eV = hc/λ.

The discovery of electron diffraction (Topic 2.4) substantiated de Broglie’s hypothesis. It is a useful exercise to calculate the de Broglie wavelength for various particles – for instance electrons moving in a TV tube or protons in an accelerator beam.

This chapter can be difficult. The concept of wave–particle duality involves new concepts. You should try to develop confidence with the formulae and calculations used to test understanding of this material in examinations.

Resources

29 Wave Particle Duality

29 Wave Particle Duality Book

A2 Wave Duality Prac Questions

A2 Wave Duality Timed Asssessment

 

Links

SEM Simulation

Liquids Conduct Heat Poorly

This simple idea shows how energy is not conducted well by a fluid. Heat ice and water to see if boil at the top and and still be frozen at ...the bottom.[+] Show More

Permanent link to this article: https://www.animatedscience.co.uk/a-level-physics-topics/12-ks5-turning-points/29-wave-particle-duality

28 Discovery of the Electron

This page covers discovery of the electron section of the course. 

Resources

28 Discovery of Electron

28 Discovery of Electron Book

A2 Discovery of Electron Timed Assessment

Thermionic Emission Explained - The Cathode Ray Tube - Physics

When a metal wire is heated in a vacuum, electrons are 'boiled' off the surface. This effect is called thermionic emission.

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