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EET103 TEKNOLOGI ELEKTRIK KEMAGNETAN DAN KEELEKTROMAGNETAN Magnetism And Electromagnetism Kemagnetan dan keelektromagnetan • Kemagnetan: magnet, medan magnet dan fluks, domain magnet, magnet kekal, magnet sementara. • Keelektromagnetan: medan magnet dan arus elektrik, daya gerak magnet dan ketepuan, keengganan litar magnet, kuantiti dan unit magnetik, histerisis, daya elektromagnet, kilas dalam gegelung. • Aruhan elektromagnet: voltan aruhan dalam gegelung, hukum lenz dan aturan tangan kanan flemming, dan arus pusar. KEMAGNETAN: MAGNET • Magnetism: a force field that acts on some materials but not on the materials • Magnets: physical devices that possess magnetism • Lodestone: natural magnet KEMAGNETAN: MEDAN MAGNET DAN FLUKS • Magnetic field: the force of magnetism • Flux (Φ): invisible lines of force that make up the magnetic field • Magnetic field is strongest at the ends of the magnet. Medan Magnet dan Fluks • Most magnets have a north pole and a south pole. • Flux leaves the north pole and enters the south pole. • Like poles repel; unlike poles attract. Creation of poles • Each time the magnet is broken, a new pair of poles is created Reaction of Like Poles S N N S Like poles produce a repelling force on these held-in-place magnets. The flux loops are distorted. Reaction of Like Poles S N N S Like poles produce a repelling force on these held-in-place magnets. When freed, the magnets move apart and the distortion decreases. Reaction of Like Poles S N N S Like poles produce a repelling force. The magnets move apart and the distortion decreases. Reaction of Like Poles S N N S Like poles produce a repelling force. The magnets move apart and the distortion decreases. Reaction of unlike poles • Unlike poles attract each other. • The force of attraction is greatest when the poles are touching. KEMAGNETAN: DOMAIN MAGNET, MAGNET KEKAL DAN MAGNET SEMENTARA • Magnetic materials: materials that are attracted by magnetic fields • Common magnetic materials: iron, iron compounds and alloys containing iron or steel • Nonmagnetic materials: materials that are not attracted by magnets • Nonmagnetic materials does not stop magnetic flux • Magnetic materials have magnetic domains. • Current-carrying conductors produce magnetic fields Magnetic Domains in a Magnetic Field (case 1) N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S TEMPORAR Y MAGNET Domains are randomly arranged in this unmagnetized temporary magnetic material. When subjected to a magnetic force field, the domains are aligned with the field. When the magnetic force field is removed, the domains return to a random arrangement. Magnetic Domains in a Magnetic Field (case 2) N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S PERMANEN TMAGNET Domains are randomly arranged in this unmagnetized permanent magnetic material. When subjected to a magnetic force field, the domains are aligned with the field. When the magnetic force field is removed, the domains remain aligned. Kemagnetan dan keelektromagnetan • Kemagnetan: magnet, medan magnet dan fluks, domain magnet, magnet kekal, magnet sementara. • Keelektromagnetan: medan magnet dan arus elektrik, daya elektromagnet, daya gerak magnet dan ketepuan, keengganan litar magnet, kuantiti dan unit magnetik, histerisis, daya elektromagnet, kilas dalam gegelung • Aruhan elektromagnet: voltan aruhan dalam gegelung, hukum lenz dan aturan tangan kanan flemming, dan arus pusar, histerisis, kilas dalam gegelung. KEELEKTROMAGNETAN: MEDAN MAGNET DAN ARUS ELEKTRIK • Electromagnetism: production of a magnetic field by current flowing in a conductor. • The magnetic field has no poles. Dot: current flowing out cross: current flowing in Current in a conductor produces flux around the conductor. A larger current produces more flux. Conductor Use the left-hand rule to determine the direction of the flux. Left hand rule • The direction of the flux around a conductor can be determined by using: Force between conductors Flux Around Parallel Conductors The conductors repel each other when the currents are in opposite directions. Flux Around Parallel Conductors The conductors repel each other when the currents are in opposite directions. Flux Around Parallel Conductors The conductors repel each other when the currents are in opposite directions. Flux Around Parallel Conductors The conductors repel each other when the currents are in opposite directions. Flux Around Parallel Conductors The conductors attract each other when the currents are in the same direction. Flux Around Parallel Conductors The conductors attract each other when the currents are in the same direction. Flux Around Parallel Conductors The conductors attract each other when the currents are in the same direction. Coils • The magnetic field around a straight wire is not very strong. • A strong field can be made by coiling the wire around a piece of soft iron. • This electromagnet is sometimes called a solenoid. • The shape of the magnetic field is the same as a bar magnet. Left hand rule for coils Coils • The strength of the magnetic field around the coil can be increased by: 1. Using a soft iron core (core means middle bit). 2. Using more turns of wire on the coil. 3. Using a bigger current. • Reversing the direction of the current will reverse the magnetic field direction. KEELEKTROMAGNETAN: DAYA GERAK MAGNET DAN KETEPUAN • Daya gerak magnet (magnetomotive force, mmf): effort exerted in creating a magnetic field • Ketepuan (saturation): A magnetic material is saturated when an increase in mmf no longer increase the flux in the material. KEELEKTROMAGNETAN: KEENGGANAN LITAR MAGNET • Reluctance (keengganan litar magnet): opposition to magnetic flux • Magnetic materials are attracted to a magnet because of their low reluctance • Permeability: refers to a material's ability to attract and conduct magnetic lines of flux Low Reluctance Flux Path Iron Flux Flux bends to follow a low reluctance path. Adding a Low-Reluctance Path in a Magnetic Field N S Opposite magnetic poles produce a flux in the air between the poles. When a low-reluctance path is provided, the flux increases and is concentrated in the low-reluctance path. KEELEKTROMAGNETAN: KUANTITI DAN UNIT MAGNETIK QUANTITIES UNITS magnetomotive force (mmf) ampere-turn (At) magnetic field strength (H) ampere-turn/meter (At/m) flux (f) weber (Wb) flux density (B) tesla (T) permeability (m) Wb/(Atm) relative permeability (µr) unitless Magnetomotive force (mmf) • Base unit: ampere-turn (At) • One ampere-turn is the mmf created by 1A flowing through one turn of a coil. Magnetic Field Strength (H) • Magnetic field strength = field intensity = magnetizing force • Amount of mmf available to create a magnetic field for each unit length of a magnetic circuit. • Base unit: ampere-turn per meter (At/m) mmf(A t) H length(m) Magnetic Field Strength (H) 4t mmf = 3 A x 8 t = 24 At 8t 3A Core length = 0.4 m mmf = 3 A x 4 t = 12 At 24 At H= = 60 At / m 0.4 m 3A Core length = 0.2 m H= 12 At 0.2 m = 60 At / m The magnetic field strength is the same for the two circuits. Flux (Φ) • Base unit: Weber (Wb) • One weber is the amount of flux change required in 1s to induce 1V in a single conductor. Flux Density (B) • Flux density: the amount of flux per unit cross-sectional area • Base unit: Tesla (T) • One tesla is equal to one weber per square meter. flux(Wb) Flux density(T) 2 area(m ) Flux Density (B) A B Material B carries twice as much flux as material A. However, the flux density (B) is the same in both materials because the cross-sectional area of B is two times larger than that of A. Permeability (µ) • Permeability (µ): refers to ability of a material to conduct flux • Base unit: weber per ampere-turn-meter (Wb/Atm) Flux density(B) Permeabili ty( m ) magnetic field strength(H ) Relative permeability (µr) • Relative permeability (µr): compares the permeability of the material with the air • Example: µr of iron = 600 (iron carries 600 times as much flux as an equal amount of air) KEELEKTROMAGNETAN: HISTERISIS • Hysteresis is the tendency of a magnetic material to retain its magnetization. • Hysteresis causes the graph of magnetic flux density versus magnetizing force to form a loop rather than a line. • The area of the loop represents the difference between energy stored and energy released per unit volume of material per cycle. This difference is called hysteresis loss. Hysteresis loop Hysteresis • Retentivity - material's ability to retain a certain amount of residual magnetic field when the magnetizing force is removed after achieving saturation. (The value of B at point B on the hysteresis curve.) • Coercive Force - The amount of reverse magnetic field which must be applied to a magnetic material to make the magnetic flux return to zero. (The value of H at point C on the hysteresis curve.) KEELEKTROMAGNETAN: DAYA ELEKTROMAGNET • Motor effect: the force on a wire in a magnetic field when current flows through the wire • One side of the wire, the fields have the same direction and repel the wire • On the other side, the field have opposite directions and attract the wire. • We can predict which way the wire will move by using Fleming’s Left Hand Rule Motor effect (catapult effect) Gambarajah ini menggunakan right hand rule! Flemming’s Left Hand Rule (Motor Rule) • Use: To determine the direction of a force on a current carrying conductor in a magnetic field • The carbon rod is NOT magnetic. • When no current flows, the rod is stationary • When we turn on the current, the rod experiences a force that makes it move. • The direction of the force is determined by Fleming' Left Hand Rule KEELEKTROMAGNETAN: KILAS DALAM GEGELUNG • B – magnetic field • F – Force • Use Fleming’s left hand rule to determine F • No current flow • Because of momentum, the coil will spin • There is current flow, therefore there is F. Torque in coil • The torque in coils can be determine: T Fd T Torque (Nm) F Force (N) d distance of F (m) Kemagnetan dan keelektromagnetan • Kemagnetan: magnet, medan magnet dan fluks, domain magnet, magnet kekal, magnet sementara. • Keelektromagnetan: medan magnet dan arus elektrik, daya elektromagnet, daya gerak magnet dan ketumpatan, keengganan litar magnet, kuantiti dan unit magnetik, histerisis, daya elektromagnet, kilas dalam gegelung • Aruhan elektromagnet: voltan aruhan dalam gegelung, hukum lenz dan aturan tangan kanan flemming ARUHAN ELEKTROMAGNET: VOLTAN ARUHAN DALAM GEGELUNG Induced Voltage X Voltage is induced when conductors cut lines of flux. Green arrow shows direction of conductor movement. Dot and X show direction of current caused by voltage. No voltage is induced when conductors move parallel to the to the lines of flux. FARADAY’S LAW If current produces a magnetic field, why can't a magnetic field produce a current ? Michael Faraday In 1831 two people, Michael Faraday in the UK and Joseph Henry in the US performed experiments that clearly demonstrated that a changing magnetic field produces an induced EMF (voltage) that would produce a current if the circuit was complete. • When the switch was closed, a momentary deflection was noticed in the galvanometer after which the current returned to zero. • When the switch was opened, the galvanometer deflected again momentarily, in the other direction. Current was not detected in the secondary circuit when the switch was left closed. • When the switch is closed, the current begins to flow and an induced magnetic field is set up around the primary coil. The current increases from zero to some value over a short period of time. The changing electrical current produced a changing magnetic field which is the cause of the induced current. • When the switch is opened, the current decreases which results in a decreasing magnetic field. The result is an induced current in the secondary circuit, in the opposite direction. When a constant current flows in the primary circuit, the induced magnetic field is constant. A constant magnetic field does not induce a current in the secondary circuit. An e.m.f. is made to happen (or induced) in a conductor (like a piece of metal) whenever it 'cuts' magnetic field lines by moving across them. This does not work when it is stationary. If the conductor is part of a complete circuit a current is also produced. • Faraday found that the induced e.m.f. increases if (i) the speed of motion of the magnet or coil increases. (ii) the number of turns on the coil is made larger. (iii) the strength of the magnet is increased. Faraday’s Law f EN Δt • E = Electromotive force (emf) • Φ = Flux • t = Number of turn • Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be "induced" in the coil. No matter how the change is produced, the voltage will be generated. • The change could be produced by changing the magnetic field strength, moving a magnet toward or away from the coil, moving the coil into or out of the magnetic field, rotating the coil relative to the magnet, etc. • Inserting a magnet into a coil also produces an induced voltage or current. • The faster speed of insertion/ retraction, the higher the induced voltage. ARUHAN ELEKTROMAGNET: HUKUM LENZ Lenz's Law states that the induced current will always set up a magnetic field that will oppose the movement of the external magnetic field Heinrich Friedrich Emil Lenz • Therefore, if we move a bar magnet through a coil of wire, according to Lenz's Law, the generated magnetic field ("effort") will oppose the magnetic field of the bar magnet (The "cause" that produced the induced current with its associated magnetic field). Lenz Law experiment: • A small force acting on the magnet causes a changing magnetic field in the coil which will induce a current. • If the current flowed in the indicated direction, an induced magnetic field would put the south pole opposite the north pole in the external magnet. • This would attract the magnet, causing it to accelerate. The increased speed of the magnet would induce a larger current which would pull even harder on the magnet and so on. • If we think about this situation, we see that a small amount of work input generates a larger output of energy. This arrangement would violate the law of conservation of energy • If the induced current flows in the opposite direction, the induced current in the coil would set up a magnetic field with the north pole opposite to the external magnet. • In order to generate a current, we would have to exert a force that would be opposed by the induced magnetic field. • The harder we push on the magnet, the more repulsion we'd feel from the induced magnetic field. • To increase the energy output, we would need to increase the work input. This would be consistent with what we know about the law of conservation of energy. Flemming’s Right hand rule (Generator Rule) • Use: To determine the direction of the induced emf/current of a conductor moving in a magnetic field.