The Restless universe
|Introduction to The restless Universe|
1 The lawful Universe2 The clockwork Universe
Other titles in the Physical World series
|2 The clockwork Universe2.2 Energy and conservationPart 1 of 2 | Part 2For a printable version of '2 The clockwork Universe' click here|
Newtonian mechanics is concerned with explaining motion, yet it contains within it the much simpler idea that some things never change. Take the concept of mass, for example, which appears throughout Newtonian mechanics, including the law of gravitation. In Newtonian mechanics, mass is conserved. This means that the mass of the Universe is constant and the mass of any specified collection of particles is constant, no matter how much rearrangement occurs within the system. A chemist might take one kilogram of hydrogen and let it react with eight kilograms of oxygen to produce water. According to the law of conservation of mass , nine kilograms of water will be produced, the same as the total mass of the ingredients (1 kg + 8 kg = 9 kg). You may think this is trivial, but it is not. Conservation laws are rare and wonderful things. There is no general law of conservation of volume for example. The initial volume of the hydrogen and oxygen is far greater than the final volume of the water. The fact that mass is conserved really is a deep discovery about the checks and balances that exist in our Universe.
Newtonian mechanics introduced several other important conservation laws, including the celebrated law of conservation of energy. Not too surprisingly, this law states that the total energy of the Universe is constant and the total energy of an isolated system of particles is constant. But the full meaning of these words will only become apparent once the concept of energy has been properly defined.
For the moment, it is sufficient to note that we all have some familiarity with the concept of energy. We pay money for gas, electricity and petrol precisely because they are sources of energy, and we use that energy to heat and light our homes and to drive cars. From this, it is apparent that energy has many different forms - chemical energy in gas or electrical energy can be converted into light energy, thermal energy, or the energy of a whirring vacuum cleaner. It is possible to change energy from one form into another but, crucially, when all these forms are properly quantified, the total amount of energy remains constant. Energy is neither created or destroyed because it is a conserved quantity.
Relevant LinksA note on powers of ten and significant figures
|S207 The Physical World|