to Atomic and Nuclear Physics

to Atomic and Nuclear Physics Welcome! Purpose of the course: To provide basic knowledge of the atom, its constituents and nuclear processes Phil Lightfoot, E47, (24533) p.k.lightfoot@shef.ac.uk

to Atomic and Nuclear Physics Welcome! In 1897 few would have imagined that the probing of materials at the atomic level would reveal so much. These early discoveries of atomic constituents and their structure would pave the way for semi-conductor electronics, develop key concepts in physical laws, and offer a replacement energy source for fossil fuels in the form of nuclear power. This course summarises key discoveries in early particle physics and combines historical background with the detailed physics understanding needed to fully appreciate the subject. Phil Lightfoot, E47, (24533) p.k.lightfoot@shef.ac.uk

Most Important Thing !!!!!! I’m always available to help with any aspect of the course My contact details are on the top of your lecture notes

Assessment for A&N Physics Contribution from this course towards final course grade in PHY008 will be: Final Examination 2 assessed homeworks Books and Course PackEverything you need to know is given to you in the notes. However …..Any A level textbook would be useful in addition to the recommended textbooks

What you should now have 1. Full copy of the lecture notes (34 pages including formula sheets). 2. All problem class questions including worked examples. 3. Homework 1 and 2 attached to the problem class questions.

Problem Class Questions The only way to really understand a topic and ensure very high performance in the final exam is to do loads of questions. In every lecture we’ll go through questions together in order to apply what we’ve learned to real examples. I’ve also supplied you with questions to try during the problem class sessions. In addition I’ve included loads of questions with full model answers already supplied to help with the course. If you understand the lectures, and can do these questions, you will do very well in the final examination.

Outline of the Course Chapter 1: The electron and the photon Chapter 2: The Atom Chapter 3: The nucleus Chapter 4: Radioactivity

How much do you know already ??? Questions: Can you calculate the electric field if you know voltage and separation ? What is the charge in coulombs on an electron ? What is the equation for the force on an electron in an electric field E ? What is the equation for the force on an electron in a magnetic field B ? What is the force due to gravity on a body of mass M ? What is the equation relating acceleration with initial and final velocity ? What is Fleming’s left hand rule ?

The electron (cathode rays) The story of atomic and nuclear physics begins in the 1850s. Science lecturers who travelled from town to town delighted audiences by showing them the ancestor of the neon sign. Fill a glass tube with low pressure gas….put wires in opposite ends.. put a high voltage across.... and the interior of the tube glows!!! Around 1897 J.J. Thomson was investigating a long-standing puzzle known as "cathode rays." If the low pressure gas in the glass tube was replaced by a total vacuum then the glow disappeared. However, where the positive high voltage electrode passed through the glass a fluorescent glow was seen.

The electron (cathode rays) Thomson proposed that these mysterious rays are streams of particles called electrons: very small, negatively charged particles that are repelled by the cathode and attracted by the anode The electron gun fires electrons towards the metallic Maltese cross which has a high positive potential. The electrons that hit the cross are stopped by the metal, but those that get past it hit a fluorescent screen on the tube which glows white. Gun Thomson also noted that they could be deflected by electric and magnetic fields. Anode Cathode Here we have deflection by an electric field between D and E

The electron (cathode rays) Thomson wanted to know more about electrons. Two of the most important features of any particle are its mass and charge and Thomson was the first to obtain a ratio for these constants In order to understand what he did we need to know a little physics first…. Electric field : If we have two parallel plates a distance d apart and the voltage difference between them is V, we can say that the electric field is E where V is in volts, d in metres and E in Vm-1. Charge on an electron : The charge on an electron is 1.6 × 10-19 C where C is coulombs (a unit of charge). The mass of the electron is 9.11 × 10-31 kg. Force on a charged particle in an E field : If the particle has a charge of q, then the force on it is qE in newtons when q is given in coulombs and E is given in Vm-1. Newton’s 2nd law : Force in newtons is equal to the mass of an object in kg multiplied by its acceleration in ms-2.

The electron (cathode rays) Example: Thomson didn’t know the charge and mass of an electron but to help show how he designed an experiment to find them, let’s do this example in which we can use their actual values. What is the electric field? What is the force on the electron? Neglecting gravity, in which direction does the electron move and what is its acceleration? UP If it starts at the ground plate, what is its velocity when it hits the top plate? But Thomson didn’t know either charge or mass!!!

The electron (cathode rays) In the last slide we saw the effect of an electric field on a charged particle. Here we look at the effect of a magnetic field on an electron. Magnetic field : When the field lines B andthe particles velocity v are at right angles, the force on a charged particle q is also at right angles to both and is defined as : B is in teslas, v in ms-1, and q in coulombs. The particle travels in a circle as force is always at right angles to motion. Current is defined as the flow of positive charge The direction of the field is either an × to indicate it is passing into the paper or a • to show it is coming out of the paper. The direction of the force on the particle is determined by Fleming’s Left Hand Rule.

The electron (cathode rays) Example of magnetic fields An electron travelling at a velocity of 3×107 ms-1 enters a magnetic field B of strength 1.1 teslas as shown in the diagram. In which initial direction will it experience a force due to the magnetic field? Force is directed up the page by Fleming B What is the force on the electron? How is the effect of this force different to that produced by an electric field? In electric fields the electron is attracted to the positive potential and so the force always points towards this. In magnetic fields the force is always directed at right angles to the motion of the electron and the magnetic field lines and so the electron spins in a circle.

The electron (cathode rays) Let’s see how Thomson did it…. An electron, moving from left to right, passes between two parallel plates, and feels a force towards the positive plate. The force due to the E field is : So the vertical acceleration is : This force acts only while the electron is between the plates, a time of : So the vertical velocity of the electron as it exits the parallel plate is : After this point, electron carries on to screen with constant vx vy .

The electron (cathode rays) If the distance to the screen D>>L, then the time T taken for the electron to travel to the screen is given by : In this time the electron will have also had a vertical velocity vy deduced on the previous page as : So we can say that when the electron hits the screen it has a vertical displacement y given by :- This is very interesting but although we can measure y, D, and L, we cannot even state a ratio for the charge q to mass m for the electron because we don’t know vxthe horizontal velocity of the electron.

The electron (cathode rays) But Thomson had an idea. He left the electric field switched on so that the electrons were still being deflected. Then he applied a magnetic field positioning it in such a way that the force due to the magnetic field was in opposition to the force due to the electric field. He then increased B until the deflection y = 0 and the forces therefore balanced. At this point he was able to say that : and so After substituting back into the expression for y we find : and therefore that

The electron (cathode rays) Thomson had for the first time measured two fundamental properties of the electron. Of course the best he could do was to measure the charge and mass as a ratio of one another but it was better than nothing!!!! He knew that the overall charge of elements is zero, and it was obvious that the charge on an electron was negative. Since he couldn’t find a corresponding positive particle, in 1904 he created the plum pudding model of the atom. In this model the atom is composed of electrons, surrounded by a soup of positive charge to balance the electron's negative charge, like negatively-charged "plums" surrounded by positively-charged "pudding". This concept existed until 1911, when Rutherford built his own model of the atom - the "planetary" one. http://www-outreach.phy.cam.ac.uk/camphy/electron/electron4_1.htm

to Atomic and Nuclear Physics