2A small current in one circuit is used to switch on a much larger current in another circuit. When the small current is switched on the electromagnet A activates and the iron lever is attracted to it. This causes the arm to move which closes the contact B in the other circuit.Reed relay
3A The bell push is closed which allows a current to flow. B The electromagnet becomes magnetized and the armature is pulled to the magnet this makes the hammer hits the gong.As this happens the contact is broken so it not a complete circuit so the electromagnet is de-magnetized.C The armature is no longer attracted so it jumps back.This closes the contact, so electricity flows again and the armature is once more attracted to the electromagnet.The whole cycle is repeated until you stop pressing the bell push.
45-18. Electromagnetic Induction The effect of producing an induced current is known as electromagnetic induction. The direction of the induced current can be reversed by reversing the motion of the wire or reversing the field direction. The strength of the current depends on the strength of the magnetic field and the speed of the wire's motion.
5Electric MotorsAn electric motor uses the sideways push of a magnetic field to turn a current-carrying wire loop. Electric motors use a commutator to change the direction of the current in the loop. Alternating current electric motors do not use commutators.
8Michael Faraday (1791-1867) built the first electric motor and discovered magnetic induction.
9Loudspeaker – Moving Coil A moving coil is moved by a changing input current.The paper cone compresses and rarefies the air.Input
10MicrophoneA moving coil activated by voice vibrations is used as a microphone.The coil induces a current in the magnet that can be amplified or recorded.
11Alternating and Direct Current Alternating current (ac) is current that flows in a back-and-forth manner; household current changes direction 120 times each second (60 Hz). Direct current (dc) flows in one direction.The ac generator (or alternator) produces an ac current and can be modified to produce dc current by…..Use of a rectifier which permits current to pass through it in only one direction.
12AC GeneratorAs the coil turns the direction and size of the induced current is varied
14TransformersA transformer is a device composed of two unconnected coils, usually wrapped around a soft iron core, that can increase or decrease the voltage of ac current.
15Transformers = This how A.C. changes voltage up or down V In V Out Turns 2Turns 1=
16Better Transformers Thick wire to reduce heat loss Laminate core to reduce EDDY CURRENTSSoft iron reduce the Hysteresis losses as the core is magnetised and demagnetisedFigure 8 shape to cut flux leakage
17GalvanometerDevice used in the construction of ammeters and voltmeters.Based on principle that a current in a magnetic field experiences a forceScaleCurrent loop or coilRadialMagneticFieldSpring
18Galvanometer Magnet forms a radial field The springs balance the moment of the force or torque from the current in the magnetic fieldScaleCurrent loop or coilRadialMagneticFieldSpring
19Galvanometer used as Ammeter Typical galvanometer have an internal resistance of the order of 60 Ω - that could significantly disturb (reduce) a current measurement.Built to have full scale for small current ~ 1 mA or less.Must therefore be mounted in parallel with a small resistor or shunt resistor.Galvanometer60 WRp
20The equivalent resistance of the circuit is also small! Galvanometer60 WRpLet’s convert a 60 Ω, 1 mA full scale galvanometer to an ammeter that can measure up to 2 A current.Rp must be selected such that when 2 A passes through the ammeter, only A goes through the galvanometer.Rp is rather small!The equivalent resistance of the circuit is also small!
21Galvanometer used as Voltmeter Must mount a large resistor in series to limit the current going though the voltmeter to 1 mA.Must also have a large resistance to avoid disturbing circuit when measured in parallel.RsGalvanometer60 W
22Maximum voltage across galvanometer: 60 WRsMaximum voltage across galvanometer:Suppose one wish to have a voltmeter that can measure voltage difference up to 100 V:Large resistance
23Photodiode A photodiode is a PN junction in reverse bias. When a photon of sufficient energystrikes the diode, it excites an electron,thereby creating a mobile electronand a positively charged electron hole.
24LEDWhile all diodes release light, most don't do it very effectively. In an ordinary diode, the semiconductor material itself ends up absorbing a lot of the light energy. LEDs are specially constructed to release a large number of photons outwards.
25Bipolar TransistorThe small current between the base and emitter controls the large current from collector to emitter
26Transistor as amplifier Input signal controls the larger voltage outputLooks exactly like the input but bigger amplitude
27Transistor as switch Small change in base voltage turns on larger lamp Warnings, alarms and computer circuits
42Lenz’s Lawgives the direction of the induced electromotive force (emf) and current resulting from electromagnetic induction. The law provides a physical interpretation of the choice of sign in Faraday's law of induction, indicating that the induced emf and the change in flux have opposite signs.
43Induction Motor No brushes to wear out Induction motors use shorted wire loops on a rotating armature and obtain their torque from currents induced in these loops by the changing magnetic field produced in the stator (stationary) coilsBarsarereallyloops
44Practical Induction Motor This implies that there is no current supplied to the rotating coils. These coils are closed loops which have large currents induced in them because of their low resistance.Barsarereallyloops
45Induction CoilFather Nicholas Joseph Callan (1799 – 1864) was a Irish priest and scientist from Darver, Co. Louth, Ireland. He was Professor of Natural Philosophy in Maynooth College from 1834, and is best known for his work on the induction coil.
47Induction CoilIt is a type of electrical transformer used to produce high-voltage pulses from a low-voltage DC supply.To create the flux changes necessary to induce voltage in the secondary, the DC current in the primary is repeatedly interrupted by a vibrating mechanical contact called an interrupter.
48As in the relay the coil becomes an electro-magnet This time it disconnects the supply.This turning off and on creates a large current in the greater windings of the secondary