1. 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?

2. The Force of Gravity Isaac Newton (among others) – Gravity is an Inverse Square Law: F grav = GM 1 M 2 / R 2 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.

3. The Electrical Force Henry Cavendish: Electricity is an Inverse Square Law: F el = kq 1 q 2 / R 2 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.

4. 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...)

5. So gravity isn’t enough.. Let’s look at other forces that can attract and repel. Like magnetism... Magnetism is produced by electric currents.

6. 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...

7. 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 × 10 8 m/s! Speed of light!

8. 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.

9. 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”??

10. So why “Strong”? Still need one more interaction Radioactivity The Weak Nuclear Force.