Modern Physics
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This text is meant to accompany class discussions. It is not everything there is to know about uniform circular motion. It is meant as a  prep for class. More detailed notes and examples are given in the class notes, presentations, and demonstrations. See the links below.


Click here for reading questions.


The atom

In general the atom is made from two groups of particles, nucleons and electrons. Nucleons are the protons and neutrons that are found in the nucleus of an atoms. (Hence the name "nucleon.") Electrons are found outside of the nucleus.

Electrons can be removed from the atom. Protons can be removed from the nucleus. Neutrons cannot be removed from the nucleus. When they are removed, neutrons are unstable and decay into something else. Neutrons cannot be removed from the nucleus and remain stable. The number of protons in an atom determine the element. The number of neutrons and electrons can vary. But if the number of protons changes, then the element has also changed. For example if carbon with 6 protons, increases the number of protons to 7 then the element is no longer carbon. It is now changed to nitrogen.


If an atom is "neutral," then number of charges must equal each other. Neutrons carry no charge. Protons carry a charge of +1.6x10-19C. Electrons carry a charge of -1.6x10-19C. If an atom is neutrally charged, then the number of protons and neutrons must equal each other. (In the diagram above, the atom is not neutral. It would be considered positively charged because there are more protons than neutrons.

An isotope is when the number of nucleons has changed but the number of protons has stayed the same. For example, Carbon has 3 isotopes, carbon-12, carbon-13 and carbon-14. Each of these elements has 6 protons. But the number of nucleons is different between them, (12, 13, and 14 nucleons.)

Carbon-12 has 12 nucleons: 6 protons and 6 neutrons
Carbon-13 has 13 nucleons: 6 protons and 7 neutrons
Carbon-14 has 14 nucleons: 6 protons and 8 neutrons

Carbon-14 is the only unstable isotope of carbon. After 5730 years, half of the carbon-14 that was on the planet will change to nitrogen-14. 5730 years is consider to be half-life of carbon-14. Different isotopes of different elements will have varying half lives. A half-life determines the time it takes for half of a sample to under go radioactive decay into a a new material. Sometimes the new material is a new element. Other times the new material is a different isotope.



A 200 gram sample of the isotope silver-112 has a half-life of 3.13 hours. After 9.39 hours, how much silver-112 will remain?

A 200 gram sample of the isotope silver-112 has a half-life of 3.13 hours. After 9.39 hours, how much silver-112 will remain?

Half-lives Time Amount of 112Ag present
0 t = 0 200 grams
1 t = 3.13 hrs 200/2 grams = 100 grams
2 t = 2(3.13) = 6.26 hrs 100/2 grams = 50 grams
3 t = 3(3.13) = 9.39 hrs 50/2 = 25 grams


ANSWER: After 9.13 hours, (3 half-lives,) 25 grams will remain.

Note: Silver-112 transmogrifies into cadnium-112. (This was looked up on a table found in the Internet.) The mass of material that I started with after 9.39 hours is still 200 grams. But from the original 200 grams of Silver-112, 175 grams is now cadnium-112 and 25 grams is silver-112 after 3 half-lives.

Fundamental Particles


Fundamental particles are the smallest possible particle that make up the parts of an atom. An electron is a fundamental particle. Scientists have not been able to crack it open to see if anything smaller exists inside of it.

Protons and neutrons are NOT fundamental particles. They can be broken up into smaller particles. They are made of particles called "quarks." The study of quarks is in itself another course or series of courses. Suffice it to say there are 6 basic quarks. there are several characteristics that describe the quarks. Below are quarks and their respective charges.

Quark Charge
up +2/3e
down -1/3e
strange -1/3e
charm +2/3e
top +2/3e
bottom -1/3e
"e" is the magnitude of the charge on a proton or electron.

A proton is made from 2 up quarks and 1 down quark. A neutron is made from 1 up quark and 2 down quarks.

Now the model of an atom is looks more detailed that before.


The nucleus has a lot of like charged particles very close to each other. Coulomb's law says these are pushing each other away from the center of the nucleus with a force equal the inverse square of the distance apart. A new force is needed to hold the nucleus together. This new force is called the "strong force." It is stronger than the repulsion dues to coulomb's laws. But it only extends a little farther than the diameter of an atoms nucleus. The strong froce only exists in the nucleus of atoms. According to Coulomb's law the farther apart you get the weaker the Coulomb force gets. The strong force gets stronger at particles inthe nucleus are pushed apart. But there is a limit just beyond the diameter of the radius where the strong force will stop working.

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by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)