MAGNETISMSPH3U

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

• Like poles repel. Unlike poles attract.

• 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.

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?

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.

Magnetic Fields• A magnet that is moved in space near a second magnet

experiences a magnetic field.• A magnetic field can be represented by field lines.

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

• The strength of the magnetic field is greaterwhere the lines are close together and weaker where they arefarther apart.

Magnetic Fields• 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.

• By convention, the lines proceed from S to N inside a magnet and from N to S outside a magnet, forming closed loops

• The lines do not cross one another.

Magnetic Poles• At the poles, the magnetic fields lines are closer together.

Direction of Magnetic Field Lines• These lines are a map of the magnetic field around a bar

magnet. The needle of a magnetic compass will follow the lines, with the north end showing the direction of the field.

The Source of Magnetic Fields• Permanent Magnets:

• Moving electrons produce magnetic fields.• In most materials these magnetic fields cancel one another and

neutralize the overall magnetic effect.• In other materials such as iron, cobalt, and nickel, the atoms

behave as tiny magnets because of certain orientations of the electrons inside the atom.

• Atoms themselves have magnetic properties due to the spin of the atom’s electrons.

• Groups of atoms join so that their magnetic fields are all going in the same direction

• These areas of atoms are called “domains”.

What are Magnetic Domains?• Magnetic substances like iron, cobalt, and nickel are composed

of small areas where the groups of atoms are aligned like the poles of a magnet. These regions are called domains. All of the domains of a magnetic substance tend to align themselves in the same direction when placed in a magnetic field. These domains are typically composed of billions of atoms.

Induced Magnetism• Each “domain” in this first image has randomly distributed

magnetic fields that cancel each other out.

• When an unmagnetized substance is placed in a magnetic field, the substance can become magnetized.

• This happens when the spinning electrons line up in the same direction.

The Earth is a Big Magnet• The Earth’s magnetic field is thought to originate with

moving charges.• The core is probably composed of iron and nickel, which

flows as the Earth rotates, creating electrical currents that result in the Earth’s magnetic field.

• Note that the magnetic north pole and the geographic North Pole are not in the same place.

• The magnetic north pole acts as if the south pole of a huge bar magnet were inside the earth.

• You know that it must be a magnetic south pole since the north end of a magnetic compass is attracted to it and opposite poles attract.

When a charged particle enters a magnetic field, an electric force is exerted on it. If a charged particle moves at an angle to a magnetic field, the magnetic force acting on it will cause it to move in a spiral around the magnetic field lines.

The solar wind is constantly bombarding the Earth’s magnetic field. Sometimes these charged particles penetrate that field. These particles are found in two large regions known as the Van Allen Belts.

The Earth’s magnetic field extends far into space. It is called the “magnetosphere.”

When the magnetic particles from the sun, called “solar wind”, strike this magnetosphere, we see a phenomenon called…

MAGNETIC FIELDS PRODUCED BY CONDUCTORSSPH3U/SPH4C

Oersted’s Discovery• In 1819, the Danish

physicist Hans Christian Oersted (1777-1851) discovered the connection between electricity and magnetism by accident while lecturing at the University of Copenhagen. He noticed that a compass needle placed closely to a current carrying wire would take up a position nearly perpendicular to the direction of the current.

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:

Principle of Electromagnetism• Whenever an electric current moves through a conductor,

a magnetic field is created in the region around the conductor.

• The magnetic field lines for a straight conductor are concentric circles around the conductor.

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.

Parallel Wires

Electromagnets• A device that exerts a magnetic force using electricity.• The magnetic field around a straight conductor can be

intensified by bending the wire into a loop.

Coil or Solenoid• The magnetic field can be further intensified by combining

the effects of a large number of loops would close together to form a coil, or solenoid.

How is the scrap metal held up by the crane?

Right-Hand Rule #2• If a coil is grasped in the right hand with the curled fingers

representing the direction of electric current, the thumb points in the direction of the magnetic field inside the coil.

Parallel Coils

Factors Affecting the Magnetic Field of a Coil

• Current in the Coil• The more current, the greater the concentration of magnetic field

lines in the core.

• Number of Coils• The more loops, the stronger the magnetic field since the magnetic

field is the sum of the field of each loop.

• Type of Core Material• The core of a coil can greatly affect the coil’s magnetic field

strength.• A core of iron will increase the strength compared to that of air.

Type of Core Material• The core material becomes an induced magnet, as its

atomic dipoles align with the magnetic field of the coil. The core itself becomes an induced magnet.

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

• 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.

Type of Core Material• Ferromagnetism

• Materials that become strong induced magnets when placed in a coil.

• Iron, nickel, cobalt, and their alloys.

• Paramagnetism• Materials that magnetize slightly when placed in a coil and increase

the field strength by a barely measurable amount.• Oxygen and aluminum.

• Diamagnetism• Materials that cause a very slight decrease in the magnetic field of a

coil.• Copper, silver, and water.

Applications of Electromagnetism

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.