where is the uncertainty in a particle’s position and is the uncertainty in its
momentum.
According to the uncertainty principle, if you know exactly where a particle is, you
have no idea how fast it is moving, and if you know exactly how fast it is moving, you have
no idea where it is. This principle has profound effects on the way we can think about the
world. It casts a shadow of doubt on many long-held assumptions: that every cause has a
clearly defined effect, that observation has no influence upon experimental results, and so
on. For SAT II Physics, however, you needn’t be aware of the philosophical conundrum
Heisenberg posed—you just need to know the name of the principle, its meaning, and the
formula associated with it.
Nuclear Physics
Until now, we’ve taken it for granted that you know what protons, neutrons, and
electrons are. Within the past century, these objects have gone from being part of vaguely
conjectured theories by advanced physicists to common knowledge. Unfortunately, SAT
II Physics is going to test you on matters that go far beyond common knowledge. That’s
where we come in.
Basic Vocabulary and Notation
As you surely know, atoms are made up of a nucleus of protons and neutrons orbited by
electrons. Protons have a positive electric charge, electrons have a negative electric
charge, and neutrons have a neutral charge. An electrically stable atom will have as many
electrons as protons.
Atomic Mass Unit
Because objects on the atomic level are so tiny, it can be a bit unwieldy to talk about their
mass in terms of kilograms. Rather, we will often use the atomic mass unit (amu, or
sometimes just u), which is defined as one-twelfth of the mass of a carbon-12 atom. That
means that 1 amu = kg. We can express the mass of the elementary
particles either in kilograms or atomic mass units:
Particle Mass (kg) Mass (amu)
Proton 1.0073
Neutron 1.0086
Electron
As you can see, the mass of electrons is pretty much negligible when calculating the mass
of an atom.