Fundamental Forces �of the Universe

There are four fundamental forces, or interactions in nature.

• Strong nuclear
• Electromagnetic
• Weak nuclear
• Gravitational

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Strong nuclear force

• Holds the nuclei of atoms together
• Very strong, but only over very, very, very short distances (within the nucleus of the atom)

Electromagnetic force

• Causes electric and magnetic effects
• Like charges repel each other
• Opposite charges attract each other
• Interactions between magnets
• Weaker than the strong nuclear force
• Acts over a much longer distance range than the strong nuclear force

Weak nuclear force

• Responsible for nuclear decay
• Weak and has a very short distance range

Gravitational force

• Weakest of all fundamental forces, but acts over very long distances
• Always attractive
• Acts between any two pieces of matter in the universe
• Very important in explaining the structure of the universe

Remember…

• The weak nuclear force is NOT the weakest of the fundamental forces.
• GRAVITY is the weakest force, but most important in understanding how objects in the universe interact.

Nuclear Reactions

• There are two kinds of nuclear reactions:
• Fusion
• Fission

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• Protons and neutrons are the two most important subatomic particles in the nucleus and participate in these reactions.

Fusion

• Fusion is the process of combining nuclei of atoms to make different atoms.

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• This reaction is going from SMALL to LARGE particles.

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• Think of fusing two things together.

Fusion

• Nuclear fusion happens at the sun.

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• One atom of hydrogen-3 and one atom of hydrogen combine to form a helium atom, a neutron and lots of energy!!!

hydrogen-3

hydrogen

atom

helium atom

neutron

Fusion

• Where does the energy come from?
• Energy is gained when the two hydrogen atoms break apart.
• Some of this energy is used up to create the helium atom, but the rest is given off as light.
• Mass is converted to energy!
• E = mc² (c = 3.0 X 10⁸ m/s)
• Since the speed of light is so large even a small mass will be converted to a very large energy.

Fission

• Fission is the process of breaking up the nucleus of an atom.

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• This reaction is going from LARGE to SMALL particles.

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• Think of breaking two things apart.

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Fission

• Nuclear fission happens on earth.

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• Nuclear fission begins when a neutron hits the nucleus of large atom.

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• Adding this neutron makes the nucleus unstable and it splits into two smaller nuclei and two neutrons.

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Fission

Chain Reactions

• If there are other ₂₃₅U atoms nearby, the neutrons that came from splitting the first ₂₃₅U nucleus can hit other atoms.

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• The nuclei of these other atoms will release more neutrons and split more ₂₃₅U atoms.

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• This is called a chain reaction.

Chain Reactions

Radioactivity

• Radioactivity is the process where the nucleus emits particles or energy.

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• There are three types of radioactive decay:
• Alpha decay
• Beta decay
• Gamma decay

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Alpha decay

• A particle with 2 protons and 2 neutrons is released from an unstable nucleus.

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• Alpha decay can be stopped by clothing, skin, a few centimeters of air, or cardboard.

Beta decay

• This occurs when a neutron in the nucleus of a radioactive isotope splits into a proton and an electron.

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• The electron is emitted.

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• Beta decay can be stopped by dense clothing or wood.

Gamma decay

• This involves the release of high-energy, electromagnetic radiation from the nucleus of the atom.

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• Gamma rays have even more energy than X-rays.

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• It can only be stopped with thick walls of concrete or lead.

Radioactive Decay

Half-Life

Half-Life

• The half-life of a radioactive element is the TIME it takes for HALF of the radioactive atoms to decay to stable ones.

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• If there are 80 grams of a radioactive element that has a half-life of 1000 years, then after 1000 years half of the element, or 40 grams of the element, will remain.

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• Now that there are only 40 grams left, how many grams will be left after another 1000 years has passed?
• There will be only 20 grams remaining.

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Half-Life

• The number of half-lives that occur for an element is found by dividing the total time by the half-life of a radioactive element.

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• Half-life ÷ total time = # of half-lives

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• An element has a half-life of 1000 years. How many half-lives have occurred after 2000 years has passed?
• Two half-lives because 2000 years ÷ 1000 years = 2

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Half-Life

• To find the fraction of the original amount think of the original amount as 1 and then divide by 2.
• 1 ÷ 2 = 1/2, one-half is how much remains after one half-life occurs.
• If two half-lives occur then divide the original amount by 2 twice.
• 1 ÷ (2×2) = 1 ÷ 4 = 1/4, one-fourth is how much remains after two half-lives occur.
• What do you do if three half-lives occur?
• 1 ÷ (2×2×2) = 1 ÷ 8 = 1/8, one-eighth is how much remains after three half-lives occur.

Half-Life Practice Problem #1

• The radioactive isotope Fluorine-11 has a half-life of 11.0 s. How many half-lives occur in 11.0 s for Fluorine-11?
• Only one half-life occurs because the half-life of Fluorine-11 is 11.0 s.

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• If you started with 30 g, how many grams are left after 11.0 s?
• Since one half life occurs, 30 g is divided by 2 and there are 15 g left.

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• What fraction of the original amount is left?
• One-half of the original amount is left.

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Half-Life Practice Problem #2

• The radioactive isotope Carbon-15 decays very fast and has a half-life of 2.5 s. How many half-lives occur in 5.0 s for Carbon-15?
• Two half-lives occur because 5.0 s ÷ 2.5 s = 2.

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• If you started with 100 g, how many grams are left after 5.0 s?
• Since 2 half-lives occur, the 100 g must be divided by 2 twice: 100 g ÷ 4 = 25 g.

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• What fraction of the original amount is left?
• 1 ÷ (2×2) = 1 ÷ 4 = 1/4, one-fourth remains.

Half-Life Practice Problem #3

• Neon-15 has a half-life of 30 s. How many half-lives occur in 1.5 min?
• Three half-lives occur because 1.5 min = 90 s and 90 s ÷ 30 s = 3.

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• If you started with 56 g, how many grams are left after 1.5 min?
• Since 3 half-lives occurred, the 39 g must be divided by 2 three times: 56 g ÷ 8 = 7 g.

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• What fraction of the original amount is left?
• 1 ÷ (2×2×2) = 1 ÷ 8 = 1/8, one-eighth is left