Chapter 17 Electromagnetic Induction The prime link between electricity and magnetism is… MOTION Magnetism Creates Electricity Through Motion Electricity Creates Magnetism Through Motion Anytime a charged particle moves, a magnetic field is generated Magnetism Creates Electricity Through Motion Electricity Creates Magnetism Through Motion Anytime a charged particle moves, a magnetic field is generated If the direction of the current changes, the resulting magnetic field reverses as well If a conductor (wire) is passed through a magnetic field a voltage is induced across the conductor. (Current flows through the wire) N S This phenomenon is called electromagnetic induction Left Hand Rule The magnetic field with circular lines of forces is in a plane perpendicular to the current in the wire. Determining Magnetic Polarity (N vs S) You can also determine magnetic polarity using the Left Hand Rule. Using your left hand, curl your fingers in the direction that current is flowing through a coil of wire. Explaining Left hand rule vs Right hand rule In our class we use the left hand rule because we use the electron flow model. If we were using the traditional current flow model, the right hand rule would be used It is useful for illustration purposes to denote when current is going toward you or away from you. X means current is going into the board. A dot means is coming out. Notice the direction of magnetic field follow the left hand rule. Notice current going in the same direction aids each other’s magnetic field. This is why current going through a coil makes a large electromagnet! (See next slide) Remember! You can go from electricity to magnetism, or magnetism to electricity. You can either move a magnet through a coil of wire inducing current, or… You can move current through a coil of wire inducing a magnetic field. If the magnet inside the coil of wire below was stationary, would an electromagnetic field be induced? Faraday’s Law “A voltage is induced by a changing or moving magnetic flux cutting the turns of a coil.” When current is produced an induced voltage is also produced. This occurs when there is relative motion between a magnetic field and/or a current carrying conductor 3 factors effect the size of induced voltage 1. The number of turns in the coil 2. The strength of the magnetic field (Number of flux lines) 3. The relative speed between the coil and the magnetic field. Example A magnetic flux of 1000 uWb cuts across a coil of 2000 turns in 0.1 s. Calculate the induced voltage. Lenz’s Law “The induced voltage in a circuit causes a current to flow. This current’s magnetic field is in the opposite direction of motion of the original magnetic field, thus opposing it.” If this law weren’t true, what would happen? Lenz’s law in action http://www.youtube.com/watch?v=k eMpUaoA3Tg Magnetic Field of and Electromagnet Depends on 3 things. The 3 C’s ◦ Current ◦ Coils ◦ Core AC Generators Converts mechanical energy into electrical energy (Faraday’s Law) What does the opposite? (Converts electrical energy into mechanical) ◦ A Motor How much voltage can a generator make? The amount of voltage depends on 4 things: 1. The strength of the magnetic field 2. The angle at which the conductor cuts the magnetic field 3. The relative change in speed between conductor and magnetic field 4. The length of the conductor within the magnetic field. How an AC generator works… http://www.listenonrepeat.com/watc h?v=uYfTzCa71SE http://www.listenonrepeat.com/watc h?v=gqA3WoOunEA&feature=relate d See magnetic field handout N S How do DC Generators work? DC generators produce DC voltage. However this voltage is not a nice flat voltage, it is pulsating. See video on next slide. Same as AC generator except slip rings are replaced with two piece commutator. How do DC generators work? http://www.listenonrepeat.com/watc h?v=Xi7o8cMPI0E&feature=related Generators are not perfectly efficient Not all 100% of the mechanical energy is converted into electrical energy. The efficiency depends on 3 things. 1. I2*R or copper losses in the winding ◦ (Energy is lost as heat, heat is reduced by using Small Awg wire) 2. Eddy current loss in the core material ◦ Currents that go in the opposite direction as the current being made. This can cause a circulating flow of electrons, or a current, within the body of the conductor. These circulating eddies of current create induced magnetic fields that oppose the change of the original magnetic field due to Lenz's law. 3. Hysteresis loss (magnetic friction) ◦ (Magnetic dipoles constantly change directions, this molecular friction produces heat. Heat goes into armature causing its resistance to increase.) What about motors? Motors are opposite of generators. Motors convert electrical energy into mechanical energy. Note: The DVD movie after lunch will do a good visual job of explaining how motors work Types of DC motors Series Motors: Windings are connected in series with Armature Starting torque is greater than other DC motors Speed varies widely with load. Types of DC motors Shunt Motors: Windings are connected in parallel with Armature. Starting torque is smaller than other DC motors. Speed does not vary widely with load. Types of DC motors Compound Motors: Stepper Motors: A mixture of series and shunt. Motors that turn a specific amount of time. Used in printers, scanner, disk drives… (Micro-controllers drive stepper motors) Brushless DC Motors In a BLDC motor, the electromagnets do not move; instead, the permanent magnets rotate and the armature remains static.