C is for concepts
The A to Z of concepts continues to grow. Thanks to those subscribers who have suggested possible additions. In this first pass through the alphabet we have decided to limit each letter to a maximum of four concepts. The good news is we can always add to this list at a later date! The next letter is C and we already have too many to choose from. The initial short list was as follows:
- Conservation laws
- Charge and the Coulomb
- Current
- Circular motion
- Centripetal force
- Carnot cycle
- Control variables
- Collisions
- Centigrade
Convection, conduction…
We've gone with the ones in bold but hope to be able to add some of the others at a future date. As ever feedback from teachers and/or their students would be extremely welcome and will help shape content for future letters. Just add a comment to this blog below. In the meanwhile, here's a taster from the C concepts which will be published soon…
C is for Conservations laws
A lot of our everyday understanding of the world around is based on observational conservation laws. If you notice that something remains constant during a change, then that probably tells you something important.
An example of a simple conservation law is that liquids have a constant volume. When you pour a liquid from a glass into a glass with a different shape, the height of the liquid up the glass changes but the overall volume stays the same.
The volume of the liquid in the two centre glasses (and thus the liquid heights) are the same. The liquids were then transferred to each of the empty glass next to them. The level is higher up the narrower glass on the far right when compared with the wider glass on the far left.
The change in height in the different glasses (as a result of the conservation of volume law) may seem obvious to you, but young children do not see things the same way. A seven-year old would probably tell you that the glass on the far right has more liquid in it compared to the one on the far left. This is despite watching the liquid being poured from the central glasses. The age at which children understand the conservation of volume is an important development of a child's cognitive abilities. This was identified by the Swiss psychologist Jean Piaget*. Typically conservation of volume is first understood at an age of about eleven.
When we bring the rigour of Physics to our observations, the understanding often deepens. If you add solid salt (Sodium Chloride) to significant volume of water, you would probably expect the overall volume to increase as you have added more “stuff”. Careful measurements show that the volume of salty water after the addition is slightly less than the volume of pure water before! This is because water molecules are polar (the oxygen end is slightly negative and the hydrogen ends are slightly positive). The addition of positive sodium ions and negative chloride ions in the solution causes the molecules to, ever so slightly, compact together.
Much of our understanding of the fundamental particles that make up the universe came from careful observations of properties that are conserved during particle interactions.
Two very important conceptual conservation laws in Physics are the conservation of energy and the conservation of momentum. In both cases it is important to understand the contexts for the conservation. There are plenty of obvious everyday example that seem to show situations when neither energy nor momentum are conserved.
For example when thinking about the conservation of energy, consider an object that slides along a surface and comes to rest as a result of friction. The object is clearly loosing energy so the kinetic energy of the object is not conserved when friction is acting on it. In this case, the kinetic energy has been converted into thermal energy. When we consider the system that is the block and the surface, the total energy of system is conserved. This leads to the concept of a closed system (one where neither matter nor energy can enter of leave).
Detailed observations of nuclear reactions means that the mass is now considered as a form of energy. The data booklet contains many example of the formulae that exist for calculating the energy that an object has in different situations. The conservation of energy can be used to “audit” what changes must have taken place and even identify the amount of energy that has been “lost” from a given object.
In another example, this time thinking about the conservation of momentum, consider an object accelerates as it falls towards the Earth. The object's velocity is clearly increasing, so its momentum is not constant. The conservation of momentum only applies to systems where there is no resultant external force. The falling object has an external force acting on it from the Earth so it's momentum is not constant. If you consider the system that includes the Earth and the object then the gravitational forces are all internal to that system. The falling object gains a momentum towards the Earth, but the Earth must also gain a momentum (equal and opposite) towards the object. Thus the total momentum of the Earth/object system is conserved.
*Jean Piaget, (1896-1980), is best know for his work on child psychology. In 1934 he was Director of the International Bureaux of Education and famously said “only education is capable of saving our society from possible collapse, whether violet of gradual”