PHYS 124 lt 2

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Published on November 13, 2007

Author: Techy_Guy

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PHYS 124:  PHYS 124 Origins and Evolution Lecture 2 Dr Chris Bowdery Origins and Evolution:  Origins and Evolution This time we will be looking at Magnetism Earth’s Magnetism:  Earth’s Magnetism Fridge Magnets:  Fridge Magnets Basic Facts of Magnetism:  Basic Facts of Magnetism A phenomenon with an associated force Magnetic force is weaker than electric force but stronger than gravity (so can hold paper on fridges) Iron-containing rocks known to have magnetic properties centuries ago 19th century: links established between magnetism and electricity Moving or spinning electric charges create magnetic fields Magnetic field - the force of magnetism everywhere over a wide region Origin of Magnetism:  Origin of Magnetism What is the connection with electricity? Imagine a row of +ve ions and a row of –ve electrons Moving electrons is an electric current Origin of Magnetism:  Origin of Magnetism Electron speed is very slow. Just millimetres per second in copper wire. Even so, Einstein’s relativity applies. Block of electrons shrink! So the electric charge of the electrons is not cancelled by that of us ions! The imbalance is magnetism Magnetism & Relativity:  Magnetism & Relativity Magnetism = electrons in motion + Einstein’s Relativity Theory Relativity is not only about fast moving spaceships! Slow speeds plus very strong electric charge  magnetism Earth’s Magnetism:  Earth’s Magnetism S N Can be treated as if a giant bar magnet sits inside the Earth ‘South’ magnetic pole near the north geographic pole Because north-seeking compasses are attracted to a magnetic south pole (opposite poles attract) For simplicity, it is always called the north magnetic pole Earth’s Magnetic Field:  Earth’s Magnetic Field Origin: complex motion of electric currents in the liquid iron of the Earth’s outer core crust mantle outer core inner core Earth’s Magnetic Field:  Earth’s Magnetic Field Evolution: magnetic pole flips occur every few hundred thousand years S N Evidence from magnetism in volcanic rocks that solidified During flip, magnetic field has complex form – many zones of N & S poles Earth’s Magnetic Field:  Earth’s Magnetic Field During transitions lasting tens to thousands of years ‘N’ ‘N’ ‘N’ ‘S’ ‘S’ ‘S’ Little magnetic shield to protect Earth from solar and cosmic rays health hazard spectacular auroras Navigation problems – for birds at least Earth’s Magnetic Field:  Earth’s Magnetic Field The Earth is overdue for the next flip! Some indications from falling magnetic field strength that it might happen ‘soon’ ‘N’ ‘N’ ‘N’ ‘S’ ‘S’ ‘S’ Earth’s Magnetic Field:  Earth’s Magnetic Field Long term Evolution: magnetic field may cease if iron core currents stop Perhaps in several billion years time? Mars has no magnetic shield anymore. Only remnant field in its rocks. Could lose most of our atmosphere because of solar wind! Fridge Magnets:  Fridge Magnets Key Substance: iron From where: Earth’s crust Iron nuclei are the heaviest nuclei cooked in the most massive red giant stars. They can only be released in supernova explosions. Magnetic Iron:  Magnetic Iron Key Substance: lone electron From where: near the outside of iron atoms How does that work?:  How does that work? Spinning electron creates a magnetic field around itself A tiny magnet in its own right A piece of magnetic iron is just a vast collection of magnetic iron atoms each containing a lone spinning electron Vast collection of spinning e-:  Vast collection of spinning e- What is the nature of electric charge?:  What is the nature of electric charge? A profound question! Simple answer: a charge which causes electric (and magnetic) effects But what is a charge #!? Charges:  Charges A property of particles that causes attraction/repulsion. List of fundamental properties Charges Spin (angular momentum) Energy/momentum Is that all of them? Yes! Charges:  Charges So what charges are there then? You really want to know? Charges:  Charges Well there are charges for all the forces that exist Strong nuclear Weak nuclear Electric Gravity Gravity Charge:  Gravity Charge Gravity is often said to be caused by MASS So is mass a type of charge then? Let’s say YES for now Gravity Charge:  Gravity Charge The origin of mass is coming in a later lecture. Let’s “weight” and see Strong Nuclear Charge:  Strong Nuclear Charge ‘Strong nuclear’ is the force that binds quarks into protons and neutrons So what is the charge called? Colour charge Colour Charge:  Colour Charge More about this is in the Origin of Mass lecture! Weak Nuclear Charge:  Weak Nuclear Charge ‘Weak nuclear’ is the force that is too weak to hold anything together So what does it do? Controls star burning Weak Nuclear Charge:  Weak Nuclear Charge Star burning? Yes. It affects the rate at which protons turn into neutrons proton + proton  proton + neutron Proton and neutron then fuse to form deuteron Weak Nuclear Charge:  Weak Nuclear Charge Can you show me that? Weak Nuclear Charge:  Weak Nuclear Charge Where is the weak charge? The quarks and leptons carry it. It determines what type they are up up down proton Weak Nuclear Charge:  Weak Nuclear Charge up up down proton up down proton ↓ neutron Weak nuclear force changes one type of quark to another Cause of radioactive beta decay! Turns electron into its neutrino too Weak Nuclear Charge:  Weak Nuclear Charge Actually 2 types #1 #2 up +3/6 +1/6 down -3/6 +1/6 neutrino +3/6 -3/6 electron -3/6 -3/6 Sum of charges is the electric charge! +2/3 -1/3 0 -1 Electric Charge:  Electric Charge Shock! Horror! Electric charge is not elementary but a mix of so-called weak charges! Not a lot of people know that! Origin of Magnetism:  Origin of Magnetism is moving or spinning electrons which carry electric charge Origin of Electric Charge is the mix of mysterious weak charges that first happened at 10-12 s from start of Big Bang

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