A Strangely Attractive Topic Magnetism A Strangely Attractive Topic

History #1 Term comes from the ancient Greek city of Magnesia, at which many natural magnets were found. We now refer to these natural magnets as lodestones (also spelled loadstone; lode means to lead or to attract) which contain magnetite, a natural magnetic material Fe3O4.

History #2 Chinese as early as 121 AD knew that an iron rod which had been brought near one of these natural magnets would acquire and retain the magnetic property…and that such a rod when suspended from a string would align itself in a north-south direction. Use of magnets to aid in navigation can be traced back to at least the eleventh century.

Finally, the Science Not until 1819 was a connection between electrical and magnetic phenomena shown. Danish scientist Hans Christian Oersted observed that a compass needle in the vicinity of a wire carrying electrical current was deflected! In 1831, Michael Faraday discovered that motion of a magnet toward or away from a circuit could produce the same effect.

Let This Be a Lesson! Joseph Henry (first Director of the Smithsonian Institution) failed to publish what he had discovered 6-12 months before Faraday

The Connection is Made So.... SUMMARY: Oersted showed that magnetic effects could be produced by moving electrical charges; Faraday and Henry showed that electric currents could be produced by moving magnets So....

A Sheep in a Cow Suit? All magnetic phenomena result from forces between electric charges in motion.

Looking in More Detail Ampere first suggested in 1820 that magnetic properties of matter were due to tiny atomic currents All atoms exhibit magnetic effects Medium in which charges are moving has profound effects on observed magnetic forces

Magnets What do we already know?

What are we going to learn today? What the characteristics of magnets are That magnets can repel and attract each other How magnets behave What happens when you put two magnets together

What are the characteristics of magnets? They can attract some materials They can also repel other magnets They are usually made of iron They have two ends called magnetic poles

What do all these new words actually mean? Magnet A stone or a piece of metal that attracts some other metal. Attract To pull towards each other. Repel To push away from each other. Poles The ends of a magnet.

What do the poles do? When a magnet is held from a string: The north pole points north The south pole points… can you guess? How do we know this? We can measure it with a compass! On school magnets, the RED pole is the north pole and the BLUE pole is the south.

But what happens if you put two magnets together? Talk to your partner- what do you think? So let’s try it. First of all, north to south… south to north… north to north… south to south…

What did we find out? So now we know that “like” poles repel each other… and that “opposite” poles attract each other. They do this because there is a FORCE between them.

Top Ten List What We Will Learn About Magnetism 1. There are North Poles and South Poles. 2. Like poles repel, unlike poles attract. 3. Magnetic forces attract only magnetic materials. 4. Magnetic forces act at a distance. 5. While magnetized, temporary magnets act like permanent magnets.

Top Ten continued 6. A coil of wire with an electric current flowing through it becomes a magnet. 7. Putting iron inside a current-carrying coil increases the strength of the electromagnet. 8. A changing magnetic field induces an electric current in a conductor.

Top Ten Continued 9. A charged particle experiences no magnetic force when moving parallel to a magnetic field, but when it is moving perpendicular to the field it experiences a force perpendicular to both the field and the direction of motion. 10. A current-carrying wire in a perpendicular magnetic field experiences a force in a direction perpendicular to both the wire and the field.

Figure: Origin of magnetic dipoles: (a) The spin of the electron produces a magnetic field with a direction dependent on the quantum number (b) Electrons orbiting around the nucleus create a magnetic field around the atom.

For Every North, There is a South Every magnet has at least one north pole and one south pole.  By convention, we say that the magnetic field lines leave the North end of a magnet and enter the South end of a magnet.  If you take a bar magnet and break it into two pieces, each piece will again have a North pole and a South pole.  If you take one of those pieces and break it into two, each of the smaller pieces will have a North pole and a South pole.  No matter how small the pieces of the magnet become, each piece will have a North pole and a South pole.  S N S N S N

No Monopoles Allowed S N It has not been shown to be possible to end up with a single North pole or a single South pole, which is a monopole ("mono" means one or single, thus one pole).  Note: Some theorists believe that magnetic monopoles may have been made in the early Universe. So far, none have been detected. S N

Magnets Have Magnetic Fields We will say that a moving charge sets up in the space around it a magnetic field, and it is the magnetic field which exerts a force on any other charge moving through it. Magnetic fields are vector quantities….that is, they have a magnitude and a direction!

Defining Magnetic Field Direction Magnetic Field vectors as written as B Direction of magnetic field at any point is defined as the direction of motion of a charged particle on which the magnetic field would not exert a force. Magnitude of the B-vector is proportional to the force acting on the moving charge, magnitude of the moving charge, the magnitude of its velocity, and the angle between v and the B-field. Unit is the Tesla or the Gauss (1 T = 10,000 G).

Scientists Can Be Famous, Too! Tesla

Famous, continued Gauss

The Concept of “Fields” Michael Faraday realized that ... A magnet has a ‘magnetic field’ distributed throughout the surrounding space

Magnetic Field Lines Magnetic field lines describe the structure of magnetic fields in three dimensions.They are defined as follows. If at any point on such a line we place an ideal compass needle, free to turn in any direction (unlike the usual compass needle, which stays horizontal) then the needle will always point along the field line. Field lines converge where the magnetic force is strong, and spread out where it is weak. For instance, in a compact bar magnet or "dipole," field lines spread out from one pole and converge towards the other, and of course, the magnetic force is strongest near the poles where they come together.

Field Lines Around a Magnet

Field Lines Around a Doughnut Magnet

Field Lines Around a Bar Magnet

Field Lines Around a Magnetic Sphere

Field Lines of Repelling Bars

Field Lines of Attracting Bars