IN 1897 , JJ Thomson discovered the electron. After the development of the planetary model of the atom in 1911, Rutherford proposed another subatomic particle and named it the proton, in 1919.
However, there were still limitations in that planetary model , in terms of mass. Why were there heavier atoms? They could have more protons, bu they did not have a significant increase in charge.Rutherford proposed a particle that was possible a proton and electron joined together.The stage was set for work that eventually culminated in the discovery of the neutron by James Chadwick.
In the second half we discuss the principles of radioactivity: the types of radioactivity as well as the physics principles that govern it.
However, there were still limitations in that planetary model , in terms of mass. Why were there heavier atoms? They could have more protons, bu they did not have a significant increase in charge.Rutherford proposed a particle that was possible a proton and electron joined together.The stage was set for work that eventually culminated in the discovery of the neutron by James Chadwick.
In the second half we discuss the principles of radioactivity: the types of radioactivity as well as the physics principles that govern it.
1. 1897 - JJ Thomson and his electron
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And the principle at play here is the manipulation of the charge with electric and magnetic fields
Prior to 1897, the atom was seen as indivisible. In fact, the term Thomas means indivisible. Indeed in the seminal experimental work of JJ Thompson in 1897, using electromagnetic principles, JJ Thompson discovered the first subatomic particle which we now know as the electron. This experiment radically changed our understanding of matter and eventually lead to the development of quantum mechanics. |
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A brief summary of the work of JJ Thomson, useful for review
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- A beam of electrons travels through a set of crossed electric and magnetic fields. What is the speed of the electrons if the magnetic field is 82 mT and the electric field is 5.8 x 104 N/C? (415 N?C)
- Electrons, moving at 7.5 x 10^7 m/s, pass through crossed magnetic and electric fields undeflected. What is the size of the magnetic field if the electric field is 4.4 x 10^4 N/C? (5.87 x 10^-4 T)
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This demonstration of an aspect of the JJ Thomson experiment is done in conjunction with University of Sydney Kickstart program and Crooked Science
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1a. 1909 - Millikan's Oil Drop Experiment
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Robert Millikan's famous oil drop experiment showed that charge is discreet, and that its value could be determined. As a result of his work he was able to also determine mass of the electron.
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Here is a simple yet instructive interactive for Millikan's experiment.
Best process is
1. Record both voltage and radius of oil droplet in a spreadsheet
2. For each, determine the mass by calculating the volume and using the supplied density
3. Work out the charge for each by equating the two forces (electric field and gravitational field) (You will have to determine the electric field from the voltage)
4. Carefully examine the charge values - they are multiples of a set value
Best process is
1. Record both voltage and radius of oil droplet in a spreadsheet
2. For each, determine the mass by calculating the volume and using the supplied density
3. Work out the charge for each by equating the two forces (electric field and gravitational field) (You will have to determine the electric field from the voltage)
4. Carefully examine the charge values - they are multiples of a set value
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This activity allows students to simulate a simplified version of Robert Millikan's Oil-Drop experiment. Instructions are given under the simulation. By Tom Walsh
2. 1911 - Rutherford and the atomic planetary model
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modeling the scattering experiment
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JJ Thompsons discovery of the electron lead him to develop a basic model of the atom called the plum pudding model. Ernest Rutherford and with the work of geiger and Marsden, develop the experiment to test this model, the results of which radically changed our understanding of the structure of the atom.
A great model which you can use in the classroom is provided in the TAB above |
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This pHET interactive allows you to see a model of the Rutherford scattering and compare it to the Plum Pudding model
modeling the scattering experiment
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In this video, using a 3D printed model I demonstrate how the scattering experiment works giving you the viewer to participate. I also then discuss how science relies on the development of models that explain observations and are able to therefore predict outcomes
If you wish tp print out your own version of this model you can find it here. It was designed by ScoolLab , based at CERN |
3. 1913 - The Bohr model of the atom
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Although Rutherford planetary model was a much better model than JJ Thompsons plum pudding model, it had some major flaws. This led Niels Bohr to hypothesise a model using planck's concept of energy being discreet
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4. 1932 - Chadwick and the discover of the neutron
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By 1919, Rutherford had developed a model of the atom where the electron was in orbit around a central nucleus which contains protons. However his model could not explain the discrepancy in mass. This was eventually resolved by James Chadwick's work and led to the understanding that the nucleus contains not only protons but also neutrons
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We now turn our attention to radioactivity.
In the late 1890s and Henri Becquerel, and then later Pierre and Marie Curie discovered that certain materials emitted radiation.
This began understanding of radioactivity spontaneous emission of particles and or energy from matter. The following videos starts by examining what radioactivity is, and then examined for physics underpinning it
In the late 1890s and Henri Becquerel, and then later Pierre and Marie Curie discovered that certain materials emitted radiation.
This began understanding of radioactivity spontaneous emission of particles and or energy from matter. The following videos starts by examining what radioactivity is, and then examined for physics underpinning it
5. What is radioactivity?
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In this lesson we are going to examine the nature of radioactivity, first discovered by Henri Becquerel in 1897. In the following video we introduce what radioactivity is and the three types of radioactive decay that occurs when a nucleus transmutes into another form. As you watch the video, become familiar with the properties of the three possible decay products, as well as the nuclear reactions that represent the decay types. |
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- Geiger activity - In this activity you can model the penetration ability for the different types of reactive decay
6. Half Life Explained
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What is the decay rate of a head of beer? This video looks at that and uses the results to help you understand the concept of half-life in radioactivity.
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If you wish to the expert yourself , here is the experiment on half life
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Given that the half lives as shown (As-81 = 33s; Au-198 - 2.69 days; C-14 = 5730 years)
a. How long does it take a 100.00g sample of As-81 to decay to 6.25g? (4 x 33 = 132s)
b. What percent of a sample of As-81 remains un-decayed after 43.2 seconds? (40.3%)
c. How old is a bone if it presently contains 0.3125g of C-14, but it was estimated to have originally contained 80.000g of C-14? (45,840 y)
a. How long does it take a 100.00g sample of As-81 to decay to 6.25g? (4 x 33 = 132s)
b. What percent of a sample of As-81 remains un-decayed after 43.2 seconds? (40.3%)
c. How old is a bone if it presently contains 0.3125g of C-14, but it was estimated to have originally contained 80.000g of C-14? (45,840 y)
7. Understanding Mass Defect and Binding Energy
Mass Defect
When nucleus transmutes, such as in the case of alpha or beta decay, energy is released. But where does that energy come from? (It also occurs in nuclear fission and fusion, which we will discuss shortly)
The answer simply is from the matter itself.
Like a chemical reaction where you have reactants and products, so too, in nuclear reactions you have a reactant or reactants which results in the production of the products.
During the process of the nuclear reaction there is less mass in the products than the reactants.
This seems to violate one of the conservation laws: one of the conservation of matter. But the fact is the mass lost is converted into energy - the mass defect.
Some more correctly the conservation laws is about the conservation of mass-energy, mass is just the concentration of energy by way of E=mc^2
This is referred to as the mass defect.
When nucleus transmutes, such as in the case of alpha or beta decay, energy is released. But where does that energy come from? (It also occurs in nuclear fission and fusion, which we will discuss shortly)
The answer simply is from the matter itself.
Like a chemical reaction where you have reactants and products, so too, in nuclear reactions you have a reactant or reactants which results in the production of the products.
During the process of the nuclear reaction there is less mass in the products than the reactants.
This seems to violate one of the conservation laws: one of the conservation of matter. But the fact is the mass lost is converted into energy - the mass defect.
Some more correctly the conservation laws is about the conservation of mass-energy, mass is just the concentration of energy by way of E=mc^2
This is referred to as the mass defect.
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And it is that mass difference that converts to energy by way of Einstein's famous equation
But in nuclear physics, is is more helpful to to use a non SI unit for mass, as well as for energy So instead of using the joule (J) for energy we can use the electron volt (eV) And for mass , instead if using the kilogram, we use the atomic mass unit (u). The video covers this Before you continue however, make sure you are familiar with the electron volt (eV) as a unit of energy - If not, please review here. |
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This video now examines the concepts of Binding energy.
What if we wanted to MAKE a chlorine atom? We would have 17 protons, 17 electrons and 18 neutrons. But if we added their masses we would get a total mass that is greater than the mass of a chlorine atom. If still have a mass defect! Watch this video as we tie the mass defect to Binding energy. (this video does covers the atomic mass unit which you can watch for review (and helpful if you are still a little unsure) , but if you wish to go straight to Binding Energy , scroll to 3:25) |
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A quick summary on Binding energy, useful for review
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Tritium is an isotope of hydrogen. The mass of the tritium isotope, H-3, is 3.0160490 u.
- What is the mass defect of this isotope? (0.009106 u)
- What is the binding energy of this isotope? (8.48 MeV)
- Find the binding energy per nucleon. (2.83 MeV)
- What is the mass defect of this nucleus? (1. 0.98940 u)
- What is the binding energy of this nucleus? (92.1 MeV)
- Find the binding energy per nucleon. (7.68 MeV)
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8. Strong Nuclear Force Explained
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We have talked about mass and energy, and now tie in the concept of force
Protons are positively charges and thus should repel. On a simple level, we know it requires energy to hold a nucleus together, and this is the binding energy So we can all understand it, we can also examine it in terms of force. There must be a force that holds the protons together that is greater than the repulsive forces due to charge. This is the strong nuclear force |
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There are a number of ways we can explore this concepts further.
One aspect of most high school physics course on kinematics, is that they only concern themselves with constant acceleration. In reality however, acceleration, like displacement and velocity, can chance with respect to time.
Velocity is the rate of change of displacement. Unit: m/s2
Acceleration is the rate of change of velocity.
So what is the rate of change of acceleration?
The answer to that is the jerk. So slope of the acceleration vs time graph is the jerk. Unit: m/s3
We can go further.
What is the rate of change of the jerk?
Well it's the snap. Unit: m/s4
Can we go further? Yep. The rate of change of snap is the crackle. Unit: m/s5
I think you can guess the next one.
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- pHET graphing animation - this interactive from the University of Colorado pHet team is a great way to demonstrate the relationship between motion and its graphical analysis. That why I used it in my video. At this time its Java based so will only work on PC/Mac
When we think of nuclear processes so far we have discussed radioactive decay, that is alpha, beta and gamma decay.
As well producing the nuclear products, energy is also released. However, there are other nuclear processes that release energy but do not involve radioactive decay.
In brief, fission is the process by which a large atom can be broken into two smaller atoms with the result of the release of energy. In fusion, two smaller atoms are combined to produce one larger atom but also release excess energy.In both cases these processes can be under uncontrolled, releasing the energy very quickly, or controlled, allowing the energy to be released progressively.
As well producing the nuclear products, energy is also released. However, there are other nuclear processes that release energy but do not involve radioactive decay.
In brief, fission is the process by which a large atom can be broken into two smaller atoms with the result of the release of energy. In fusion, two smaller atoms are combined to produce one larger atom but also release excess energy.In both cases these processes can be under uncontrolled, releasing the energy very quickly, or controlled, allowing the energy to be released progressively.
9. Fission Explained
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We will start with fission, which is the production of energy when a larger atom is split into two smaller atoms. There is a mass defect as a result, which is converted into energy.
The following video examines fission, first in a historical context and then the discussing physics principles. Ensure you have a reasonable understanding of mass defect and binding energy before you watch the video. |
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10. How a nuclear reactor works
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A nuclear reactor is an essence a fission reaction and that is controlled. Since the nuclear fission rate is determined by the number of neutrons available and their speed, a nuclear reactor can control the rate of fission by controlling the amount of neutrons and the speed of the neutrons.
Although nuclear reactors are very complex structures they all work on similar principles, and this video discusses the key components of a nuclear reactor. |
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There are some great nuclear reactor interactives available for you to try
- https://playgen.com/nuclear-simulator/
- https://www.nuclearinst.com/Nuclear-Reactor-Simulator - it does take little time to load
11. Fusion Explained
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What is nuclear fusion and what doe sit generate energy? Where does the energy from the sun come from? This video answers both questions as it reviews how hydrogen fuses to form helium and generate lots of energy.
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