Magnetism and Electromagnetism SPH4C

Permanent Magnets A permanent magnet has two poles: North and South.

Permanent Magnets A permanent magnet has two poles: North and South. Like poles repel. Unlike poles attract.

Permanent Magnets These repulsive or attractive forces can act at a distance (no contact is required). The region in space over which these forces can act is called a magnetic field.

Magnetic Fields The iron filings in the picture below show the magnetic field lines.

Magnetic Fields Are Different Magnetic lines are drawn out of the North Pole and into the South Pole but they don’t stop and start there: the magnetic field lines are drawn through the poles.

No Monopoles Every magnet is a dipole: it must have two poles. If a dipole magnet is broken in two,

No Monopoles Every magnet is a dipole: it must have two poles. If a dipole magnet is broken in two, it becomes two dipoles:

Many Dipoles Why does this happen?

Many Dipoles Why does this happen? A bar magnet is made up of many smaller dipoles, each with North and South poles, all aligned.

Many Dipoles Why does this happen? A bar magnet is made up of many smaller dipoles, each with North and South poles, all aligned. The dipoles may be knocked out of alignment by heating or otherwise abusing the material.

Repairing Magnets A bar magnet may be re-magnetized by placing it in a magnetic field. This is induced magnetism.

Ferromagnetic Not all materials may be easily magnetized. Those that can are called ferromagnetic. They include iron ore (lodestone), cobalt, zinc, and nickel.

Rare-Earth Magnets The strongest permanent magnets are made from rare earth (lanthanoid) elements, the strongest of these being neodymium-iron-boron (NIB) magnets,

Rare-Earth Magnets The strongest permanent magnets are made from rare earth (lanthanoid) elements, the strongest of these being neodymium-iron-boron (NIB) magnets, now greatly reduced in price and used in children’s toys.

Electromagnets Current (moving charge) will also produce a magnetic field. This is called Oersted’s Principle.

Showing Directions To show that a current, field line, or force is directed out of the page (towards us), we draw: To show that a current or field line is directed into the page, we draw:

Right-Hand Rule #1 When the thumb is pointed in the direction of conventional current flow, the fingers curl in the direction of the magnetic field.

Solenoids To strengthen and straighten the magnetic field, we coil the current-carrying wire into a solenoid.

Right-Hand Rule #2 When the fingers are curled in the direction of conventional current flow, the thumb indicates the direction of magnetic North.

Magnetic Forces Since current will produce a magnetic field, the interaction of this field with an external magnetic field will result in a force acting on the moving charge. This is the Motor Principle.

Magnitude of Force The magnitude of the magnetic force FM on a current-carrying wire is directly proportional to the current I and length L of the wire and to the magnitude of the external magnetic field B.

B Note that B, the magnetic field, is measured in Tesla (T).

B Note that B, the magnetic field, is measured in Tesla (T). 1 T is a very strong magnetic field.

Magnitude of Force The magnitude also depends on the angle q between the magnetic field vector and the current vector. When q = 90o, (the current is moving perpendicular to the field), the force is maximum. When q = 0o or 180o (the current is moving parallel to the field), the force is zero.

Direction of Force: Right-Hand Hule #3 The direction of the force is given by another right-hand rule: if the right thumb follows the direction of conventional current flow and the extended fingers point in the direction of the magnetic field, the force is in the direction the palm would push.

Quick Quiz What is the direction of the force (if any) on the particle in each of the following cases?