Magnetism

Single moving charge and the magnetic field it creates

Magnetic field created by a loop of wire with a current flowing through it

What causes magnetism?

• The movement of any chargedparticle creates a magnetic field.

• Even the spinning of a single electron as itorbits a nucleus produces a magnetic field.

• Magnetic fields of different movingcharged particles then interact to produceforces of attraction or repulsion.

Magnetic Domains

Lodestones

• Natural Magnets

–Magnetite, Fe3O4 (an oxide of iron)• Ancient civilizations (Greek 590 BCE,

Chinese 2600 BCE) realized that these stones would cling to iron tools.• A suspended, pivoting lodestone always

pointed along the North South magnetic axis on a map.

Naturally magnetic things

• Magnetite crystals have been found in living organisms

• · Magnetotactic bacteria!• · Migratory Bird brains!!• · Other migratory animals: bees, fish• · Human brains!!!• · YOU HAVE ROCKS IN YOUR HEAD!!!!!

Magnetic Poles• The force exerted by a magnet on another

object is strongest at the two poles of the object– Like magnetic poles repel.– Opposite magnetic poles attract

• Most magnets have two poles (dipole), but can have three or more!

Monopole (not Monopoly)• Monopole: piece of a magnet

that is simply a north pole or a south pole

• Many have tried to isolate a monopole by breaking magnets in half.

• No matter how we break a magnet, the pieces are always dipoles!

• A monopole cannot be isolated.

• Do not pass GO.• Do not collect $200.

Magnetic Fields

• Travel from North to South OUTSIDE the magnet

Magnetic Fields

• From South to North INSIDE the magnet

Compasses and magnets

• A compass can be used to identify the North and South poles of a magnet.

Magnetic Field lines are Continuous

Magnetic Field Lines Never Cross

• And always form closed loops (they loop through the inside of the magnet. In the South and out the North

Magnetic Fields Between Bar Magnets

Earth’s Magnetism

• Magnetic Field has REVERSED direction about 300 x in the last 170 million years

• The poles “Wander”

• Magnetic and geographic poles are not the same. Declination is about 11.5°

• What is strange about the picture?

Earth’s Magnetism

• Magnetic North & South poles are opposite geographic North & South poles and are not exactly in the same place (about 11.5 degrees “off”.

The Earth’s Magnetic Field

• A bit more complex than a magnet

The earth’s magnetic field

Shaped by the solar wind. Compressed near the sun and elongated on the other side.

Electricity and Magnetism

• When more than one charged particle moves in a common direction, what is formed?

• A CURRENT!

• The motion of the charges creates a magnetic field. In which direction does the magnetic field travel? Use the 1st right hand rule to figure this out.

First Right Hand Rule• Thumb points in the direction of the current

(RIGHT HAND ONLY). Fingers curl in the direction of the magnetic field

• Note the compass readings. This is used for CURRENT carrying wire!

All current carrying wires generate magnetic fields

• Long, straight wire– magnetic field is cylindrical around the wire– Draw magnetic field lines as concentric circles

• Loop of wire:– magnetic field forms rings around the wire & through the loop

– Note the direction of current in and out and the magnetic field direction

What about a coil of wire (many loops)

• Coil of wire (Solenoid Coil):– Magnetic field of each loop adds together– Overall field looks like that of a bar magnet – Out one end of the coil, back in the other end

Iron Core Solenoids

• An iron core has the effect of multiplying greatly the magnetic field of a solenoid compared to an air core solenoid.

Electromagnets

• Electromagnets are solenoids which have an iron core.

• The ferromagnetic property of the iron core causes the internal magnetic domains of the iron to line up with the magnetic field produced by the current in the solenoid. The effect is the multiplication of the magnetic field by factors of tens to even thousands.

Electromagnet (Solenoid with solid core)

Why do I care about a solenoid?

• Solenoids are commonly used as starters in cars. Often when a vehicle's starter is malfunctioning the solenoid is the problem.

• Solenoids are mainly used in cars as actuators. A solenoid will move a component a certain distance after electricity has been applied.

• Another common usage in vehicles is as a relay. A solenoid closes a circuit when electricity is applied.