Compare and Contrast

A really useful revision tactic (and also quite a common type of exam question) involves looking at the differences that exist between two related things in the same area of physics. These comparisons can be very simple, such as the definitions of scalars and vectors, or they can be more complicated, such as competing theories for the birth and evolution of the Universe.

You should know that scalars and vectors are different types of quantities (measurements). Scalars and vectors both have a magnitude (size) but vectors also have a direction.

To give an example, mass is a scalar quantity; it is a way of counting how many particles (atoms) are contained in an object and the result is given in kilograms. Weight, on the other hand, is a force, measured in newtons, that is created when a mass is inside a gravitational field. Like all forces, weight has a specific direction, which we often call “downwards”. Weight is therefore a vector quantity because it has both a magnitude and a direction. There is more about vectors and scalars here.

Another comparison is between nuclear fission and nuclear fusion. These are both processes that change the nuclei of atoms but in one case we take a large nucleus and break it up (fission) whereas in the other case we take two smaller nuclei and join them together (fusion).

Both processes release energy and both also produce free neutrons, which in the case of fission (only) help to keep the process running. Fission is currently used for electricity generation in nuclear power stations but it is hoped that fusion might take over in the future as it is a safer process – but also one that is harder to keep running.

There is a brilliant short summary of nuclear fission and fusion on the Science in School website, at https://www.scienceinschool.org/content/fusion-vs-fission. And there is a great video, created by the UK Atomic Energy Authority, about the fusion process that powers the Sun and how it could become a possible power source for Earth. It really is very easy to understand so please do view it (on YouTube) here. For a slightly more detailed explanation about fusion as a energy resource for electricity generation, watch this video.

Fusion’s link to the stars brings us to astronomy and two opportunities to compare and contrast different things; models of the structure of the Universe (geocentric versus heliocentric) and its evolution (Big Bang versus Steady State).

In the geocentric model, the Earth was at the centre of the Universe with the Sun, planets and stars all going around the Earth. Seeing the Sun rising in the East and setting in the West was obvious “proof” that this version of events was correct.

The geocentric model was proved wrong when Galileo used a telescope to see the moons of Jupiter. These objects were in orbit around a different planet and that immediately proved that not everything is going around the Earth. This led to the idea of everything going around the Sun and we call this the heliocentric model (helios is the Greek word for the Sun). Of course, we now know that more distant objects, such as other stars in our galaxy and other galaxies beyond, don’t orbit around our Sun so the heliocentric model isn’t completely correct either.

Moving from what we know to what we believe, there are two ideas about the evolution of the Universe and both are “theories”, which means that we can’t prove which one of them (or perhaps even neither) is the true version of things.

We know that the Universe is expanding because of galactic red shift, by which we mean that light from distant galaxies is moved to the red end of the spectrum by amounts that depend on how far away the galaxies are from us (and therefore how fast they are moving). This fact fits both the Big Bang Theory and the Steady State Theory – because a static Universe would surely collapse in on itself due to gravitational attraction between all the masses.

So why do we believe that the Big Bang Theory is more likely to be correct? The answer is to be found in the Cosmic Microwave Background radiation. This can be explained as the energy that was released in the Big Bang: as time has gone by and the Universe has expanded, the wavelength of the radiation was stretched out until it now sits in the microwave region of the electromagnetic spectrum. This “left over” energy of the Big Bang is a microwave (radio) signal that we can detect coming from every direction in space.

For more detail about the two theories of the Universe’s birth and evolution, read this article, which was posted about a year ago when we were studying the Astronomy topic.

Mention of the electromagnetic spectrum (remember Red Monkeys In Vans Use X-ray Glasses?) brings us to the two types of waves. All electromagnetic waves are transverse, which means the movement of the “particles” is at right angles (perpendicular) to the direction of the wave’s travel. This is different from longitudinal waves, such as sound, where the movement of the particles is parallel to the direction of the wave’s travel. Mexican waves are a good model for longitudinal waves, as shown in this video.

For a more scientific visualisation of the two wave types, try this recorded animation or the interactive simulations on this page.

If you want more resources to help with your revision for the next Paper 1 trial examination (in February) then re-read the post that was published to help you prepare for last November’s trial examination: you can access that article by clicking here.