Magnets & Electromagnets Pg

Auroras  Have you ever seen the Northern lights? (aurora borealis)  Did you know that you are really watching the interplay of electricity and magnetism?  Why do they move and swirl?

Permanent Magnets  1. Where is the magnetic field the strongest?  At the poles!  **the magnetic field lines of a bar magnet are similar to the electric field lines of an electric dipole  **while iron filings will align with the magnetic field, the electric dipoles align with the electric field

Permanent Magnets  Recall that the magnetic field lines of a bar magnet extend from the north pole to the south pole outside the magnet and from south to north inside the magnet, forming a closed loop. (gr. 11)  Magnetic field lines always form closed loops – you can never have only a south pole or only a north pole

Permanent Magnets  Just as opposite electric charges attract, opposite magnetic poles attract  The effect is different – when like poles approach each other, they repel……but….magnetic poles are electrically neutral

Permanent Magnets - summary  All magnets have magnetic poles  Opposite poles attract and like poles repel  A magnetic field surrounds all magnets and flows from north to south outside the magnet and from south to north inside

Earth’s Magnetic Field  The largest magnet on Earth is Earth itself  If we were to place compasses at different spots in Earth’s magnetic field, each compass needle would be aligned parallel to the field  **Earth’s north geographic pole is actually a south magnetic pole  **It is convention to refer to this south magnetic pole as Earth’s north magnetic pole

Earth’s Magnetic Field  Earth’s magnetic field affects the motion of cosmic rays  Charged particles entering Earth’s magnetic field are deflected and spiral along the field lines toward the magnetic poles  This motion results in a concentration of charged particles at Earth’s north and south magnetic poles (i.e. the aurora borealis and aurora australis)

Earth’s Magnetic Field  The circled x on the right indicates that the force is directed into the page for that particle and vice versa for the particle on the left (….right-hand rule) To find the direction of the force of a magnetic field: Point fingers in the direction of the velocity Curl fingers to the direction of the magnetic field Thumb points in the direction of the force

Earth’s Magnetic Field  Our knowledge of what causes Earth’s magnetic field is incomplete, but several clues point to an explanation  Earth’s magnetic poles move slowly from day to day and year to year  Second, geological studies show that Earth’s magnetic field has completely reversed direction many times during the planet’s history

Earth’s Magnetic Field  Electric currents in Earth’s core probably cause this behaviour of the magnetic field  Earth’s core is made of liquid metal which conducts electricity  The spin of Earth on its axis causes the liquid to circulate much like the current in a conducting loop……which causes a magnetic field  Scientists believe that circulation within Earth’s core has a complicated flow pattern that varies with time….these variations cause changes in the magnetic field

Earth’s Magnetic Field (summary)  Resembles that of a bar magnet  Changes orientation over time  Is able to redirect the motion of charged particles from space (auroras)

Did you know?  Scientists have discovered many living creatures that use Earth’s magnetic field in different ways:  Magnetotactic bacteria (photo)  Honey bees  Homing pigeons  Dolphins *the dark round dots inside the cell are magnetite crystals

Electromagnetism  In 1820, Danish physicist Hans Christian Oersted was demonstrating how a wire becomes warmer when electric charge flows through it  In the course of his demonstrating, he noticed that the needle in a nearby compass moved each time he switched on the electricity  This strange event let Oersted to conclude that a magnetic field surrounds moving electric charges  …..this is now known as the principle of electromagnetism

Electromagnetism  Moving charges, like those in an electric current, produce a magnetic field  Current in a straight wire or other long, straight conductor creates a magnetic field whose lines look like circles centred on the wire  You can determine the direction of the magnetic field lines around a straight wire using the right-hand rule for a straight conductor

Electromagnetism  If you make a circular loop from a straight wire and run a current through it, the magnetic field will circle around each segment of the loop  The field is strongest inside the coil because the field lines are closer together  To determine the direction of the magnetic field in coiled water, you must use the right-hand rule for a solenoid

Electromagnetism (summary)  Moving electric charges produce a magnetic field  Straight conductor  Thumb points in direction of current  Curled fingers indicate direction of magnetic field lines  Coiled conductor  Fingers curl in direction of current  Thumb indicates direction of magnetic field lines through the centre of the cell (i.e.. Thumb points to the north pole) Pg. 385, #5