a-brief-history-of-physics

But what are atoms made of?Electron • From the Greek word for amber [ ήλεκτρον] • Electrons can be easily knocked out of atoms e.g., by rubbing with fur • Mass is slightly more than 1/2

A Brief History of Physics

(in 23 PowerPoints)

Two pillars of physics

Matter = Stuff

Properties of matter

say “mass”)

Density = mass / volume

A digression on units

Scientists use SI units (Système International)

[derived from the metric system]

Length: metre (m)

[1/10,000,000th of distance from pole to equator]

Volume: derived from length units

[e.g., cubic decimetre (dm3), or litre (l)]

(1 dm = 0.1 m = 10 cm)

Mass: kilogram (kg)

[originally the mass of 1 litre of water]

Therefore density has units like kg/m3 or kg/dm3 (= kg/l)

Time: second (s)

Matter is composed of ?

Democritus

atoms

Kinds of atoms [Elements]

Properties of atoms

• Almost indestructible

(but in the late 19th century, some were found to naturally self-destruct [radioactivity], and in the 20th century, we humans learned how to destroy them)

• Radius: ~50-250 pm (picometer = 10-12 m)

• Mass: ~10-27-10-25 kg

(about 1-250 times the mass of a hydrogen atom, or

~1-250 u [u = unified atomic mass unit])

But what are atoms made of?

Electron

• From the Greek word for amber [ ήλεκτρον]

• Electrons can be easily knocked out of

atoms (e.g., by rubbing with fur)

• Mass is slightly more than 1/2000th of the mass of the lightest atom (hydrogen)

• Carries one basic unit of negative electric

charge

• Given the symbol e–

But what are atoms made of?

• Atoms are electrically neutral, so they

must contain a positive charge equal to

their number of electrons

• At first, this was thought to be uniformly

distributed throughout the atom

• Then in 1912, Ernest Rutherford

discovered that the positive charge was located in a tiny region in the centre of the

atom, its nucleus

But what is the nucleus made of?

Proton

• Mass about the same as a hydrogen atom

(~6×1023 [Avogadro’s number] protons make one gram)

• Carries one basic unit of positive electric

charge

• Given the symbol p+

But what is the nucleus made of?

• A hydrogen atom contains one proton

(99.9+% of its mass) and one electron

• Opposite charges attract, keeping the two together to form the hydrogen atom

• Heavier atoms were assumed to have

more protons and electrons

• But like charges repel, so what keeps the

protons together in the tiny nucleus?

But what is the nucleus made of?

Neutron

• Finally in 1932, James Chadwick

discovered the nuclear glue—the neutron

• Mass is ~1% greater than the proton’s

• Zero electric charge (neutral)

• Given the symbol n0

• Outside a nucleus, the neutron breaks

down in ~15 minutes to a p+ and an e–

We’re ready to build an atom

• Decide what kind of atom you want, and find its atomic

number (Z) in the periodic table

• Count out Z protons

• You will also need at least Z neutrons for glue—more for heavier atoms (about 50% more for lead)

• Too few (or too many) neutrons will make your atom

unstable (radioactive)

• Combine the protons and neutrons to make your nucleus

• Add Z electrons for electrical neutrality—they will

automatically take up positions around the nucleus

• Any atom with Z>83 will be radioactive (as will Z=43 or 61), but some will survive for a reasonable amount of

time (say a few billion years)

• Enjoy your new atom!

Example: Building a helium atom

Nucleus much too large!

-• Neutrinos (ν) [Greek nu]

emitted in nuclear reactions; mass <10-9 u

• From the Greek for “activity”

• Anytime something happens, energy is involved

• The SI unit of energy is the joule (J)

– 1 J = 1 kg m 2 /s 2 = 1 kg (m/s) 2 = 10 7 erg

– This is about the energy of a small apple dropping 1 m onto a physicist’s head

– We won’t really use this unit

• Typical forms of energy include:

– Motion

– Heat

– Light

– Electricity

Force and Energy

• Physics initially developed through the concept of “forces”(pushes and pulls) acting on matter Forces included, apart from direct contact, those acting through space, like gravity, electricity and magnetism

• Gravity was always an attracting force, that all matter

exerted on other matter, proportional to mass

• Electricity could be either repulsive or attractive (in the

physicist’s sense!), depending on whether like or unlike

charges were involved

• Magnetism was similar, but more complex and mysterious

• At the same time, the concept of energy was being

developed and applied, and found to be in many ways more fundamental than that of force, though the force concept

remains useful in practice to the present day

Electricity and magnetism

• In the 19th century, it was discovered that electricity and

magnetism were related—that electric currents (moving

charges) created magnetic fields, and that moving magnets could create electric currents

• This is the basis for electric generators and motors

• In the mid-to-late 1800’s, physicists showed that electricity and magnetism were fundamentally linked They derived equations predicting a “new” phenomenon, electromagnetic waves, that could carry energy through space

• Since the equations said these waves should travel at the speed of light, it was reasonable to suppose that light was

an electromagnetic wave, but not the only one

• Since then, radio waves, x-rays, gamma rays, and other

parts of the electromagnetic “spectrum” have been

discovered and applied, differing only in their wavelength

The electromagnetic spectrum

• A wave is described by its

speed (velocity) v, in m/s frequency f, in cycles/s [Hertz or Hz], and wavelength λ [Greek “lambda], in m/cycle

• These are related by the equation

velocity = wavelength × frequency

v = λf

• Speed for electromagnetic waves is always the speed of light, ~3×108 m/s, so

λ = 3×108/f m and f = 3×108/λ Hz

Matter and Energy

• Until 1905, it was thought that both matter and energy were conserved; that they

could be transformed, but not created or destroyed

• Then along came Einstein

E = mc2

c = 3×108 m/s

So 1 kg × c2 = 9×1016 kg m2/s2 = 9×1016 J

Matter and Energy

• Relativity removed the distinction between matter and energy

• Quantum mechanics (with help from Einstein!)

completed the process

• The main carrier of energy (in every process we’re interested in) is the photon, a “particle” with no

mass, that travels at the speed of light—in fact, it’s just our electromagnetic wave in disguise!

• The unique feature that quantum mechanics adds

is that the energy carried by a single photon

depends on its frequency

E = hf (or E = hν, Greek “nu”) where h is Planck’s constant, ~6.6×10-34 J·s

Quantum mechanics

• Planck’s constant, h, and the idea that energy comes in discrete packets (“quanta”) lies at the heart of quantum mechanics

• Quantum mechanics forms the basis of modern science and technology, including this computer

• In fact, the world itself depends on quantum

mechanics, though it took millions of years of human development for us to realize that.