Electricity and magnetism chapter 2 and 3 notes

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 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 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.


• 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).


• 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.


• 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.


• 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.


• 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.


• 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.


• 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.


• 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.


• 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.


•Faraday found that a changing magnetic field would induce an electric current in a wire through a process called electromagnetic induction.


• 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.


•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).


•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.


• 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.


•Power companies use these transformers to transmit high voltage electricity to your street and then step it down before being transferred to your house.

Electricity and magnetism chapter 2 and 3 notes

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Transcript of Electricity and magnetism chapter 2 and 3 notes