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What holds the atoms together?
Need to start considering Forces
Electricity tends to split atom apart (like charges repel;
unlike charges attract)
So what holds it together? Gravity?
The Force of Gravity
• Isaac Newton (among others) – Gravity is an
Inverse Square Law:
• Fgrav = GM1M2 / R2
• G is a constant that gets the units correct –
the physics is in the product of the masses and
the R-squared in the denominator.
The Electrical Force
• Henry Cavendish: Electricity is an
Inverse Square Law:
• Fel = kq1q2 / R2
• k is a constant that gets the units correct – the
physics is in the product of the charges and
the R-squared in the denominator.
Okay, so the atom is solved!
Electricity holds the electrons in orbit around the
positively charged nucleus – yeah!
Electricity also wants to push the positive charge in the
nucleus apart, but . . .
Gravity holds the nucleus together! Yeah!
Umm . . . Hold on a minute (see the board . . .)
So gravity isn’t enough . .
Let’s look at other forces that can attract and
repel.
Like magnetism . . .
Magnetism is produced by electric currents.
Electricity and magnetism
Okay, electricity produces magnetism . . . But
Faraday showed that magnetism can also
produce electricity! And . . .
It was later shown that electricity can produce
magnetism in more than one way, leading to .
..
Unification => Electromagnetism
Electricity and magnetism are two aspects of the
same fundamental interaction –
Electromagnetism.
James Clerk Maxwell (1865) showed that the
two “fields” interact to form EM waves.
Speed of those waves? 3 × 108 m/s! Speed of
light!
Optics and EM Unification
So Light is an EM wave.
Many other EM waves (like microwaves)
Electricity, Magnetism, and Optics are unified.
But none of them is strong enough to keep the
nucleus together.
So what DOES keep the nucleus
together?
“The Hand of God?” Well . . .
The force is in the nucleus, so we’ll call it . . .
“The strong nuclear force”.
Better than the “Hand of God”??
So why “Strong”?
Still need one more interaction
Radioactivity
The Weak Nuclear Force.