Basic Electronics DC - Magnetism Copyright © Texas Education Agency, All rights reserved.

Presentation Overview An overview of magnetism basics Types of magnets and how to create them Terms and units The rules and relationships between current, forces, and fields Ways of producing artificial magnets Permeability of magnetic materials Magnetic properties Induction Copyright © Texas Education Agency, All rights reserved.

Magnet A material or object that produces a magnetic field An object that will attract iron, nickel, or cobalt and that will produce an external magnetic field There are two types of magnets: permanent magnets and electromagnets There are two types of permanent magnets: hard magnets that do not demagnetize, and soft magnets that lose their magnetism Copyright © Texas Education Agency, All rights reserved.

Magnetism A invisible force that attracts or repels magnetic materials A property of certain materials (e.g. iron, nickel, and cobalt) that exerts a mechanical force on other magnetic materials, and that can cause induced voltages in conductors when relative movement is present Some materials feel no force at all from magnetism (these are called non-magnetic substances) Copyright © Texas Education Agency, All rights reserved.

Picture of Magnetic Field Lines Using Iron Filings Copyright © Texas Education Agency, All rights reserved.

The pattern revealed by the iron filing vividly illustrates that something extremely well-organized exists beyond the surface of the magnet. A common way to refer to magnetic fields is by using the term “magnetic lines of force.” Picture of Magnetic Field Lines Using Iron Filings Copyright © Texas Education Agency, All rights reserved.

The Basics of Magnetism Magnets attract specific metals Magnets have a north and a south pole Like poles repel, unlike poles attract Magnetic and electric fields are related The same quantity that can turn a metal into a magnet is the same property that allows it to be attracted to other magnets Copyright © Texas Education Agency, All rights reserved.

Magnetic Poles One of the two ends of a magnet where magnetic field lines converge or diverge There are two types of poles: north and south Every magnet always has both poles, meaning there are only magnetic dipoles (there are no magnetic monopoles) Magnetic field lines are said to leave the north pole and enter the south pole Copyright © Texas Education Agency, All rights reserved.

What Creates Magnetism? Electrons have several fundamental properties Electrical charge Magnetic dipole moment This dipole moment comes from a more fundamental property of the electron called quantum mechanical spin This means each electron behaves like a tiny magnet Copyright © Texas Education Agency, All rights reserved.

What Creates Magnetism? Magnetic materials have several unpaired electrons with the same spin When these tiny magnetic dipoles are aligned in the same direction, their individual magnetic fields add together to create a measurable magnetic field Electrons usually come in pairs with opposite spins where the magnetism cancels Copyright © Texas Education Agency, All rights reserved.

Permanent Magnet A type of magnet found in nature Usually made out of iron Other types of magnets include Alnico, Neodymium, Samarium, and a type of ceramic, which includes iron oxide as a composite Neodymium magnets are some of the strongest on Earth Copyright © Texas Education Agency, All rights reserved.

Electromagnet Where magnetism is formed by current flowing through an electric wire A magnet that can be turned on and off Copyright © Texas Education Agency, All rights reserved.

Example of an Electromagnet Copyright © Texas Education Agency, All rights reserved.

Force of Attraction The paper clips are not originally magnetized The external magnetic field induces or creates a temporary magnetism in the paper clips They are then attracted to the electromagnet because opposite poles attract The attraction only lasts as long as the paper clips are near the magnetic source The paper clips will turn so the opposite pole end of the paper clip faces the electromagnet This same effect happens with a refrigerator door magnet creating temporary magnetism in the metal door which then attracts the magnet to the door

Important Terms: Retentivity Also called permanence; how long a magnet retains its magnetism Materials that are hard to magnetize generally retain their magnetism longer Relates to the amount of force needed to align magnetic domains Copyright © Texas Education Agency, All rights reserved.

Important Terms: Permeability Copyright © Texas Education Agency, All rights reserved.

Ferromagnetism Ferromagnetic materials are made from iron and form permanent magnets Magnetic materials with high values of permeability that range from 50 to 5000 Responsible for the type of magnetism found in nature (NOTE: Iron, cobalt, nickel, alloys that include these materials [like alnico], some compounds of rare earth metals, and certain minerals like lodestone are ferromagnetic materials.) Copyright © Texas Education Agency, All rights reserved.

Units of Magnetism Copyright © Texas Education Agency, All rights reserved.

Magnetic Field Direction One form of right hand rule shows the direction of a magnetic field around a conductor with current I Thumb in the direction of current flow Fingers wrap around the conductor in the direction of magnetic field lines Copyright © Texas Education Agency, All rights reserved.

The Direction of Magnetic Force A magnetic field creates a force on a conductor with current flow The direction of force is perpendicular to the magnetic field Copyright © Texas Education Agency, All rights reserved.

Another Right-hand Rule Pointing the thumb of the right hand in the direction of the conventional current and the fingers in the direction of the magnetic field lines B, the force on the conductor points up from the palm. Copyright © Texas Education Agency, All rights reserved.

Left-hand Rule When current flows in a wire, and an external magnetic field is applied across that flow, the wire experiences a force perpendicular both to that field and to the direction of the current flow. A left hand can be held, as shown in the illustration, so as to represent three mutually orthogonal axes on the thumb, first finger, and middle finger. Each finger is then assigned to a quantity (mechanical force, magnetic field, and electric current). The right and left hand are used for generators and motors respectively. Copyright © Texas Education Agency, All rights reserved.

Use of the LHR: Hall Effect The charge carriers of a current carrying conductor placed in a transverse magnetic field experience a sideways Lorentz force This results in a charge separation in a direction perpendicular to the current and to the magnetic field The resultant voltage in that direction is proportional to the applied magnetic field. The Hall effect is often used to measure the strength of a magnetic field. Copyright © Texas Education Agency, All rights reserved.

Electricity and Magnetism Current flowing through a wire creates a magnetic field around that wire This is what creates an electromagnet This principle is used in a motor A conductor moving through a magnetic field creates (induces) a voltage in that conductor This principle is used in a generator Copyright © Texas Education Agency, All rights reserved.

Motors and Generators A changing magnetic field creates an electric field This is used in a generator The electric field produces a force on an electron If the electron is in a conductor, current will flow This is known as Faraday's law A changing electric field generates a magnetic field This is known as Maxwell's correction to Ampère's law This is used in a motor Copyright © Texas Education Agency, All rights reserved.

Transformers and Generators A changing magnetic field can be created several ways A magnetic field expanding or collapsing This is how a transformer works Requires AC voltage Through relative motion This is how a generator works Relative motion can be created by moving a conductor through a magnetic filed or by moving a magnetic field past a conductor Both of these require a prime mover Copyright © Texas Education Agency, All rights reserved.

Motor Action The interaction of two magnetic fields produce mechanical forces These forces can be used to make a rotor spin in a motor The torque created by the rotor can be used to perform work Copyright © Texas Education Agency, All rights reserved.

AC Motors Rotating Magnetic fields The rotating magnetic field is a key principle in the operation of alternating-current motors A permanent magnet in such a field rotates so as to maintain its alignment with the external field The rotating magnetic filed is created by AC voltages that are different in phase being applied to magnetic poles (created by coils) that are at different angles to each other Copyright © Texas Education Agency, All rights reserved.

Ways of Producing Artificial Magnets A. Electrical Coil Method B. Stroking Method Copyright © Texas Education Agency, All rights reserved.

Permeability of Magnetic Materials High permeability Iron, steel, nickel, cobalt Commercially made alloys of iron, nickel, cobalt, and other elements Silicon steel (used in transformers) Alnico (used in audio speakers) Medium permeability Aluminum, platinum, manganese, and chromium Low permeability Bismuth, antimony, copper, and zinc Rare metals (mercury, gold, and silver) Nonmagnetic materials (diamagnetic) Glass, paper, rubber, wood, and air Copyright © Texas Education Agency, All rights reserved.

Magnetic Properties Magnetic lines of force Continuous and form complete loops Never cross each other Cause like poles (north-north, south- south) to repel each other Cause unlike poles (north-south, south-north) to attract each other Parallel lines going in the same direction repel each other Attract other lines going in the opposite direction Exert tension along their lengths, tending to shorten themselves Pass through all materials, both magnetic and nonmagnetic Always enter or leave magnetic material at right angles to the surface Tend to flow in paths of least opposition Copyright © Texas Education Agency, All rights reserved.

Magnetic Properties (cont’d.) Magnetic field Area around magnet through which force lines flow Direction of flow is always from north pole to south pole Magnetic flux Sum total of magnetic field force lines flowing from north pole to south pole Symbol for magnetic flux- Greek letter phi (φ) Unit of flux- Maxwell; one maxwell (Mx) equals one line of force (example: if a magnetic field contains 6 lines of force, the flux of the magnet is 6 maxwells, or φ= 6Mx Flux density- number of force lines per given are Copyright © Texas Education Agency, All rights reserved.

Magnetic Induction Method Place iron bar in vicinity of permanent magnet Do not allow iron bar to touch magnet Effect Magnetic field lines of force flow through the iron bar The iron bar becomes magnetized Pole polarity is reversed End of bar near north pole of magnet becomes south pole of bar End of bar near south pole of magnet becomes north pole of bar The permanent magnet attracts the iron bar (NOTE: this constitutes more action). Copyright © Texas Education Agency, All rights reserved.

Practical Applications of Induction in the Electronics Field Radio and television transmission and reception Transformers Relays and solenoids Coils, chokes, and inductors Audio speakers Motors and generators Magnetic memory Copyright © Texas Education Agency, All rights reserved.

Math That Involves Magnetism Magnetic field strength around a wire B is magnetic field strength in Tesla I is current in amps D is distance in meters Copyright © Texas Education Agency, All rights reserved.

Terms and Definitions Magnetism- a property of certain materials (e.g. iron, nickel, and cobalt) that exerts a mechanical force on other magnetic materials, and that can cause induced voltages in conductors when relative movement is present Magnet- an object that will attract iron, nickel, or cobalt and that will produce an external magnetic field Natural magnet- any material found in the earth that exhibits the properties of magnetism. Example: The lodestone, which contains magnetite, a form of iron, and that has been magnetized by the earth’s magnetic field Artificial magnet- a device that has been made magnetic by induction Magnetic induction- where a magnetic field causes an un-magnetized ferromagnetic substance to become magnetized Magnetic lines of force- a set of imaginary, curved lines, around a magnet that indicates the strength and direction of the magnetic field Copyright © Texas Education Agency, All rights reserved.

Terms and Definitions (cont’d.) Magnetic field- the area around a magnet through which the lines of force flow Permanent magnet- a magnetic device that remains its magnetism after it is removed from a magnetic field Electromagnet- a core of soft iron that is temporarily magnetized by sending current through a coil of wire wound around the core Permeability- the ability to pass or conduct magnetic field lines (NOTE: some materials such as iron have high permeability, others such as aluminum have medium permeability, and others such as silver and gold have low permeability.) Magnetic poles- one of the two ends of a magnet where magnetic field lines converge or diverge (NOTE: by convention, the north-seeking pole is marked with N, or plus, or is colored red.) Ferromagnetic- magnetic materials with high values of permeability that range from 50 to 5000 (NOTE: steel, cobalt, nickel, and alnico are ferromagnetic materials. ) Diamagnetic- non-magnetic materials; these have a permeability of less than one (NOTE: diamagnetic materials include bismuth, antimony, copper, and zinc.) Copyright © Texas Education Agency, All rights reserved.

Presentation Summary Terms and definitions Types of magnets Ways of producing artificial magnets Permeability of magnetic materials Magnetic properties The use of the Left-hand Rule for conductors and coils Induction Practical applications of induction in the electronics field Copyright © Texas Education Agency, All rights reserved.