MAGNETS AND ELECTROMAGNETISM

Lesson#14

MAGNETSMagnetism Is A Force That Acts On Certain Materials. without goinginto too much detail it is useful to remember that it is a fundamental forcethat attracts or repels certain types of materialsWHAT MATERIALS? ALLOYS OF COPPER,NICKEL, ALUMINIUM, IRON, COBALT. The unique class of materials that are strongly affected by magnetismare called ferromagnetic and they will attract anything that contains iron.A permanent magnet is a material that, when placed into a strong magnetic field, will exhibit a magnetic field of its own, and continue to exhibit amagnetic field once it has been removed from the original field. Thismagnetic field is continuous without losing strength, as long as it is notsubject to changes in environment such as temperature, a de-magnetising field or being hit. Magnetism is a di cult concept as all you can see are theeffects of magnetism. The invisible lines, called flux lines, can only be seen bysprinkling iron lings on to paper that is close to a magnet.Ferromagnetic metals that are attracted by magnets include iron, steel andother iron alloys, but also nickel and cobalt.

Rules of magnetismThere are several features that magnets display:• Magnets have two poles, North and South.• The magnetic lines of force ( flux) never cross.• Flux lines always form a closed loop.• If the flux lines distort when brought close to another magnetic fieldthey will always return to their original shape when moved away again.• Outside of the magnet, flux lines run North to South.• A magnet placed in a magnetic field experiences a force on it.• The higher the concentration of flux lines, the stronger the magnet.• Like poles repel.• Unlike poles attract.

The concentration of flux lines determines howstrong the magnet will be. Magnetic flux, withthe symbol Φ, is measured in webers, Wb. Theconcentration of flux lines or magnetic fluxdensity, B, is how many flux lines are in a specialarea of the magnet. Hence, it is measured inwebers/m² and found by the formula:B = Φ /Awhere A is the cross-sectional area measured inm² and Φ, the magnetic flux, is measured in Wb.It is worth noting that magnetic flux density isalso measured in tesla, T. It has two units thatare both perfectly acceptable.

Strength of a magnet

Worked example 1Calculate the magnetic flux density of a magnet with a flux 2 Wb andcross-sectional area of 0.13 m².B = Φ /AB = 20.13 = 15.38

Magnetic field around a straight wire

ElectromagnetismWhen current passes through a conductor, a magnetic field is induced around that conductor. The strength of the magnetic field is proportional to the amount of current passing through the conductor. It can only exist while the current is owing. Control of an electromagnet can be achieved,by simply putting a switch into the circuit. It is important for lots of applications to know the direction in which the magnetic field is going. Motor movement is caused by the interaction of magnetic fields, so it is a good idea to know which way the motor will start spinning. The concentric circles of magnetic flux lines stretch along the whole length of a current-carrying conductor and the flux direction is relative to the direction of current, as can be seen in Figure

Maxwell’s screw ruleAn easy way to work out which way the flux lines are running is to imagineputting a screw into a piece of wood. As the screw is turning clockwise itis going into the wood. The direction of the screw is the direction of thecurrent flow (away from you and into the wood) and the clockwise rotationrepresents the rotation of flux lines, as per Figure below. This is knownas Maxwell’s screw rule and is named after the scientist who discovered it.

Direction of flux is found by the RIGHT hand rule.

The solenoidA solenoid is a number of turns of insulated conductor wire closely wound in the same direction to form a coil. The coil is held in place by a core called a former. If the coils are connected to a circuit with a current supplyand a switch, a solenoid is created. The direction of the current dictates the polarity of the electromagnet. Imagine taking your right hand and wrapping your fingers around the solenoid in the direction of the current flow; your thumb will point to the North Pole.

The solenoid is a temporary magnet that acts like a permanent bar magnet when switched on. Applications include relays, contactor controls, fire doors, bells and buzzers, residual current devices (RCDs) and miniaturecircuit breakers (MCBs).Consider replacing the battery source with an ammeter and then pushing amagnet into the coil at speed – what will happen? The ammeter will show current is flowing.

Magnetic MaterialsMagnetic material are attracted to magnetic fields.Examples of ferromagnetic materials; iron iron compounds alloys of iron or steel

nonmagnetic materials:( no attraction) metals such as copper, brass, al,silver zinc, tin.flux travels thru nonmagnetic materials.

Permanent magnets Domains remain aligned. Examples:Fe (ferrous) with 0.8% carbonalloys like alnico (compound of fe cobalt ,nickel, al, and cu)also certain ceramic materials

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