1. PHY115 – Sault College – Bazlurslide 1 Electromagnetic Induction

2. PHY115 – Sault College – Bazlurslide 2 Electromagnetic Induction In 1820 Oersted found that magnetism was produced by current-carrying wires. In 1831 Michael Faraday in England and Joseph Henry in the United States – discovered that electricity can be produced from magnetism.

3. PHY115 – Sault College – Bazlurslide 3 Induced Voltage Faraday and Henry both discovered that electric current can be produced in a wire simply by moving a magnet in or out of a coiled part of the wire. They discovered that voltage is caused, or induced by the relative motion between a wire and a magnetic field. Voltage is induced whether the magnetic field of a magnet moves near a stationary conductor or the conductor moves in a stationary magnetic field.

4. PHY115 – Sault College – Bazlurslide 4 More Induced Voltage When the magnet is plunged into the coil, voltage is induced in the coil and charges in the coil are set in motion. This phenomenon of inducing voltage by changing the magnetic field in a coil of wire is called electromagnetic induction.

5. PHY115 – Sault College – Bazlurslide 5 More Induced Voltage The greater the number of loops of wire that move in a magnetic field, the greater the induced voltage. Pushing a magnet into twice as many loops will induce twice as much voltage; pushing into ten times as many loops will induce ten times as much voltage; and so on.

6. PHY115 – Sault College – Bazlurslide 6 Faraday's Law Electromagnetic induction is summarized by Faraday's law, which states, The induced voltage in a coil is proportional to the product of the number of loops and the rate at which the magnetic field changes within those loops.

7. PHY115 – Sault College – Bazlurslide 7 Use of Electromagnetic Induction Electromagnetic induction is all around us. –trigger traffic lights –security systems of airports –ATM card –tape recorder –Car alternators –hydroelectric

8. PHY115 – Sault College – Bazlurslide 8 Generators and Alternating Current When one end of a magnet is repeatedly plunged into and back out of a coil of wire, the direction of the induced voltage alternates. The frequency of the alternating voltage induced equals the frequency of the changing magnetic field within the loop.

9. PHY115 – Sault College – Bazlurslide 9 Generators and Alternating Current It is more practical to induce voltage by moving a coil rather than by moving a magnet. This can be done by rotating the coil in a stationary magnetic field. This arrangement is called a generator. The construction of a generator is in principle identical to that of a motor. In a motor, electric energy is the input and mechanical energy is the output; in a generator, mechanical energy is the input and electric energy is the output. Both devices simply transform energy from one form to another.

10. PHY115 – Sault College – Bazlurslide 10 Power Production Fifty years after Michael Faraday and Joseph Henry discovered electromagnetic induction, Nikola Tesla and George Westinghouse put those findings to practical use and showed the world that electricity could be generated reliably and in sufficient quantities to light entire cities.

11. PHY115 – Sault College – Bazlurslide 11 Turbogenerator Tesla built generators much like those still in use today - but quite a bit more complicated than the simple model we have discussed. Tesla's generators had armatures - iron cores wrapped with bundles of copper wires - that were made to spin within strong magnetic fields by means of a turbine, which in turn was spun by the energy of falling water or steam. The rotating loops of wire in the armature cut through the magnetic field of the surrounding electromagnets, thereby inducing alternating voltage and current.

12. PHY115 – Sault College – Bazlurslide 12 Generator Steam drives the turbine, which is connected to the armature of the generator.

13. PHY115 – Sault College – Bazlurslide 13 MHD (magnetohydrodynamic) Generator Instead of making charges move in a magnetic field via a rotating armature, a plasma of electrons and positive ions expands through a nozzle and moves at supersonic speed through a magnetic field. Oppositely directed forces act on the positive and negative particles in the high- speed plasma moving through the magnetic field. The result is a voltage difference between the two electrodes. Current then flows from one electrode to the other through an external circuit. There are no moving parts; only the plasma moves.

14. PHY115 – Sault College – Bazlurslide 14 Lenz's law German physicist Heinrich Lenz in 1833 gives the direction of the induced electromotive force (emf) resulting from electromagnetic induction, thus: The emf induced in an electric circuit always acts in such a direction that the current it drives around a closed circuit produces a magnetic field which opposes the change in magnetic flux. Motion upward causing a current in this direction Current in this direction produces a thrust downwards

15. PHY115 – Sault College – Bazlurslide 15 Fleming's left hand rule for Motors http://en.wikipedia.org/wiki/Fleming%27s_left_hand_rule Fleming's left hand rule is a rule for finding the direction of the thrust on a conductor carrying a current in a magnetic field.

16. PHY115 – Sault College – Bazlurslide 16 Fleming's left hand rule for Motors The left hand is held with the thumb, first finger and second finger mutually at right angles.thumbfirst finger second fingerright angles The First finger represents the direction of the Field. The Second finger represents the direction of the Current (in the classical direction, from positive to negative).positive negative The Thumb represents the direction of the Thrust or resultant Motion.

17. PHY115 – Sault College – Bazlurslide 17 Fleming's Right-hand rule for Generator http://en.wikipedia.org/wiki/Right-hand_rule The induced current from motion in a magnetic field known as Fleming's right hand rule. –The appropriately-handed rule can be recalled by remembering that the letter "g" is in "right" and "generator". Induced

18. PHY115 – Sault College – Bazlurslide 18 Right hand grip rule http://en.wikipedia.org/wiki/Right_hand_grip_rule The right hand rule is a physics principle applied to electricity passing through a solenoid, resulting in a magnetic field. When you wrap your right hand around the solenoid with your fingers in the direction of the current, your thumb points in the direction of the magnetic north pole. It can also be applied to electricity passing through a straight wire, the thumb points in the direction of the current, and the fingers in the direction of the magnetic field.

19. PHY115 – Sault College – Bazlurslide 19 Generator – converts energy Generators of whatever kind, of course, don't produce energy - they simply convert energy from some other form to electric energy. Some fraction of the energy from the source, whether fossil or nuclear fuel or wind or water, is converted to mechanical energy either to drive the turbine or to produce the plasma, and the generator converts most of this to electrical energy. The electricity that is produced simply carries this energy to distant places.

20. PHY115 – Sault College – Bazlurslide 20 Transformers Energy can be transferred from one device to another across empty space with the simple arrangement as shown.

21. PHY115 – Sault College – Bazlurslide 21 Transformers Instead of opening and closing a switch to produce the change of magnetic field, suppose that alternating current is used to power the primary. Then the frequency of periodic changes in the magnetic field is equal to the frequency of the alternating current. Now we have a transformer. If you place an iron core inside the primary and secondary coils, the magnetic field within the primary is intensified by the alignment of magnetic domains. The field is also concentrated in the core and extends into the secondary, which intercepts more of the field change.

22. PHY115 – Sault College – Bazlurslide 22 Stepped up Transformer If the secondary coil has more turns than the primary, the alternating voltage produced in the secondary coil will be greater than that produced in the primary. In this case, the voltage is said to be stepped up.

23. PHY115 – Sault College – Bazlurslide 23 Transformers A practical and more efficient transformer. Both primary and secondary coils are wrapped on the inner part of the iron core (yellow), which guides alternating magnetic lines (green) produced by ac in the primary.

24. PHY115 – Sault College – Bazlurslide 24 Stepped down Transformers If the secondary has fewer turns than the primary, the alternating voltage produced in the secondary will be lower than that produced in the primary. The voltage is said to be stepped down. So electric energy can be fed into the primary at a given alternating voltage and taken from the secondary at a greater or lower alternating voltage, depending on the relative number of turns in the primary and secondary coil windings.

25. PHY115 – Sault College – Bazlurslide 25 Transformers Stepped-up voltage may –light a neon sign or –operate the picture tube in a television receiver or –send power over long distances. Stepped-down voltage may safely operate –a toy electric train –Calculator –Computer –Radio

26. PHY115 – Sault College – Bazlurslide 26 Transformers The relationship between primary and secondary voltages with respect to the relative number of turns is given by

27. PHY115 – Sault College – Bazlurslide 27 Transformers The primary gives no more than the secondary uses, in accord with the law of conservation of energy. If the slight power losses due to heating of the core are neglected, then Electric power is equal to the product of voltage and current, so we can say We see that if the secondary has more voltage than the primary, it will have less current than is in the primary. The ease with which voltages can be stepped up or down with a transformer is the principal reason that most electric power is ac rather than dc.

28. PHY115 – Sault College – Bazlurslide 28 Self-Induction Current-carrying loops in a coil interact not only with loops of other coils but also with loops of the same coil. Each loop in a coil interacts with the magnetic field around the current in other loops of the same coil. This is self-induction. A self-induced voltage is produced. This voltage is always in a direction opposing the changing voltage that produces it and is commonly called the back electromotive force, or simply back emf.

29. PHY115 – Sault College – Bazlurslide 29 A common and dangerous effect Suppose that a coil with a large number of turns is used as an electromagnet and is powered with a dc source, perhaps a small battery. When the switch is opened, the current in the circuit falls rapidly to zero and the magnetic field in the coil undergoes a sudden decrease. What happens when a magnetic field suddenly changes in a coil - even if it is the same coil that produced it? The answer is that a voltage is induced. The rapidly collapsing magnetic field with its store of energy may induce an enormous voltage, large enough to develop a strong spark across the switch - or to you, if you are opening the switch!

30. PHY115 – Sault College – Bazlurslide 30 Questions From 100 V/100 primary turns = (?) V/200 secondary turns, what is the secondary voltage ? A load of 50 Ω is connected to the secondary, what is the secondary current? What is the Power consumption? What is the primary current? 100 50 

31. PHY115 – Sault College – Bazlurslide 31 Answers From 100 V/100 primary turns = (?) V/200 secondary turns, you can see that the secondary puts out 200 V. From Ohm's law, 200 V/50 Ω = 4 amps. Power = 200 V × 4 A = 800 W. By the law of conservation of energy, the power in the primary is the same, 800 W. 800 W = 100 V × (?) A, so you see the primary draws 8 A. (Note that the voltage is stepped up from primary to secondary and that the current is Ohm's law still holds in the secondary circuit. The voltage induced across the secondary circuit, divided by the load (resistance) of the secondary circuit, equals the current in the secondary circuit. In the primary circuit, on the other hand, there is no conventional resistance. What “resists” the current in the primary is the energy transfer to the secondary.

32. PHY115 – Sault College – Bazlurslide 32 Power Transmission Almost all electric energy sold today is in the form of ac, traditionally because of the ease with which it can be transformed from one voltage to another. Large currents in wires produce heat and energy losses, so power is transmitted great distances at high voltages and correspondingly low currents (power = voltage × current). Power is generated at 25,000 V or less and is stepped up near the power station to as much as 750,000 V for long-distance transmission, then stepped down in stages at substations and distribution points to voltages needed in industrial applications (often 440 V or more) and for the home (240 and 120 V).

33. PHY115 – Sault College – Bazlurslide 33 Field Induction Electromagnetic induction has been discussed in terms of the production of voltages and currents. Actually, the more fundamental way to look at it is in terms of the induction of electric fields. The electric fields, in turn, give rise to voltages and currents. Induction takes place whether or not a conducting wire or any material medium is present. In this more general sense, Faraday's law states: An electric field is created in any region of space in which a magnetic field is changing with time. The magnitude of the induced electric field is proportional to the rate at which the magnetic field changes. The direction of the induced electric field is at right angles to the changing magnetic field.

34. PHY115 – Sault College – Bazlurslide 34 Counterpart to Faraday's law A second effect, which is the counterpart to Faraday's law, is very similar to Faraday's law, with only the roles of electric and magnetic fields interchanged. The companion to Faraday's law, advanced by the British physicist James Clerk Maxwell in the 1860s, states: A magnetic field is created in any region of space in which an electric field is changing with time. The magnitude of the induced magnetic field is proportional to the rate at which the electric field changes. The direction of the induced magnetic field is at right angles to the changing electric field.

35. PHY115 – Sault College – Bazlurslide 35 Summary Electromagnetic induction Faraday's law Generator Transformer Maxwell's counterpart to Faraday's law

36. PHY115 – Sault College – Bazlurslide 36 Summary Electromagnetic induction The induction of voltage when a magnetic field changes with time. If the magnetic field within a closed loop changes in any way, a voltage is induced in the loop: This is a statement of Faraday's law. The induction of voltage is actually the result of a more fundamental phenomenon: the induction of an electric field, as defined for the more general case below. Faraday's law An electric field is created in any region of space in which a magnetic field is changing with time. The magnitude of the induced electric field is proportional to the rate at which the magnetic field changes. The direction of the induced field is at right angles to the changing magnetic field. Generator An electromagnetic induction device that produces electric current by rotating a coil within a stationary magnetic field. A generator converts mechanical energy to electrical energy. Transformer A device for transferring electric power from one coil of wire to another by means of electromagnetic induction, for the purpose of transforming one value of voltage to another. Maxwell's counterpart to Faraday's law A magnetic field is created in any region of space in which an electric field is changing with time. The magnitude of the induced magnetic field is proportional to the rate at which the electric field changes. The direction of the induced magnetic field is at right angles to the changing electric field.