Magnetism and Electromagnetism • The Nature of Magnets • Earth As a Magnet • Magnetism in Action • Magnetism From Electricity • Electricity From Magnetism
The Nature of Magnets • The Greeks first discovered magnetism. • Magnetism is the force of attraction or repulsion caused by magnetic materials. • Every magnet has two poles: North and South. • Opposite poles attract and like poles repel.
The Nature of Magnets • The area over which magnetic force is applied is called the magnetic field. • Magnets can be temporary or permanent. • Many permanent magnets are created from mixtures of aluminum, nickel, cobalt, and iron (Alnico).
The Nature of Magnets • The atomic structure of groups of atoms determines their magnetic properties. • When atoms line up similarly they create a magnetic domain. • When groups of domains line up, the object becomes magnetic.
The Nature of Magnets • Striking or heating magnets can mix up their domains, causing them to lose their magnetic properties. • Similarly, exposing substances to magnets can causes their domains to line up and give them magnetic properties.
The Earth As a Magnet • The earth exerts magnetic forces which are strongest near its poles. • The magnetism is believed to be caused by the motion of iron and nickel in the earth’s core. • Scientists have also been able to tell by examining rocks that the earth’s magnetic field has reversed many times in it’s history (every 500,000 years).
The Earth As a Magnet • Compasses are small magnets which align themselves with the earth’s magnetic field and are used to help find direction. • The north pole of a magnet points in the northward direction, meaning that the geographic North Pole is actually a magnetic south pole.
The Earth As a Magnet • In addition, the magnetic and geographic poles do not line up. • They are actually 1500 kilometers from each other. • The angular difference of the poles is called magnetic variation or declination. • This must be taken into account when using a compass.
The Earth As a Magnet • Generally, the closer a compass is to the equator, the more accurate it is. • Other planets and even our sun have magnetic fields. • Our sun’s magnetic field is thought to play a role in the sunspot cycle.
Magnetism in Action • The earth’s magnetic field deflects charged particles radiated from the sun. • The region of the earth’s atmosphere where the magnetic field lines run is called the magnetosphere. • The interactions can cause the northern and southern lights (aurora borealis and australis).
Magnetism in Action • These same types of interactions allow astronomers to use radio waves when studying stars. • Additionally, magnetism is used to confine hot plasmas used in nuclear fusion (an energy source scientists are trying to perfect).
Magnetism From Electricity • In the 1800’s Hans Oersted discovered that a current carrying wire would deflect the needle of a compass. • He inferred that an electric current would induce a magnetic field with direction dependent upon that of the current.
Magnetism From Electricity • In playing with his new discovery, Oersted found that twisting the wire into loops (a solenoid) would create a strong magnetic force. • By placing an iron core inside the solenoid, an even stronger magnetic force can be generated. • This combination is called an electromagnet.
Magnetism From Electricity • Electromagnets are strong temporary magnets that can be turned on or off. • Since forces always occur in pairs, the wire not only exerts a force on the magnet, but magnets exert a force on the current carrying wire.
Magnetism From Electricity • There are many practical applications of Oersted’s discovery. • Electric motors use electromagnets that are free to rotate called armatures. • The armature sits in between the pole of a permanent magnet and spins by being attracted and repelled by constantly changing its poles due to an AC circuit.
Magnetism From Electricity • DC circuits can also be used in electric motors when a device called a commutator is incorporated in the electromagnet to continually switch the direction of the current.
Magnetism From Electricity • A galvanometer is another device that relies on electromagenetism. • Galvanometers measure current strength and direction. • Other uses of electromagnetism include doorbells, car starters, telephones, and telegraphs.
Electricity From Magnetism • After scientists learned of Oersted’s discovery, many wondered if electricity could be produced from magnetism. • In 1831 two scientists, working independently, found it to be possible: Michael Faraday and Joseph Henry.
Electricity From Magnetism • Faraday found that a changing magnetic field would induce an electric current in a wire through a process called electromagnetic induction.
Electricity From Magnetism • Electric generators, the type used at power plants to create our electricity, use electromagnetic induction to produce electricity. • When a power source spins a turbine, electromagnets are spun between coils of wire creating an electric current in the wire.
Electricity From Magnetism • The resulting current produced is alternating current. • It is carefully setup so that the current changes direction 120 times/sec or at 60Hz (cycles/sec).
Electricity From Magnetism • Transformers are devices that increase or decrease voltage in a wire. • Two insulated wires are wound around the same iron core the alternating current in one induces a current in the other.
Electricity From Magnetism • If the induced wire has fewer coils (step down transformer), the voltage is decreased. • If the induced wire has greater coils (step up transformer), the voltage is increased.
Electricity From Magnetism • Power companies use these transformers to transmit high voltage electricity to your street and then step it down before being transferred to your house.