Magnetism 2

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  1. 1. Prepared by Md. Amirul Islam Lecturer Department of Applied Physics & Electronics Bangabandhu Sheikh Mujibur Rahman Science & Technology University, Gopalganj 8100
  2. 2. An electric current can be induced in a circuit by a changing magnetic field. Reference: Physics II by Robert Resnick and David Halliday, Topic 31.1, Page 980 (a) When a magnet is moved toward a loop of wire connected to a galvanometer, the galvanometer deflects as shown, indicating that a current is induced in the loop. (b) When the magnet is held stationary, there is no induced current in the loop, even when the magnet is inside the loop. (c) When the magnet is moved away from the loop, the galvanometer deflects in the opposite direction. Law
  3. 3. Faradays Experiment: Reference: Physics II by Robert Resnick and David Halliday, Topic 31.1, Page 980 A primary coil is connected to a switch and a battery. The coil is wrapped around a ring, and a current in the coil produces a magnetic field when the switch is closed. A secondary coil also is wrapped around the ring and is connected to a galvanometer.
  4. 4. Reference: Physics II by Robert Resnick and David Halliday, Topic 31.1, Page 980 No battery is present in the secondary circuit, and the secondary coil is not connected to the primary coil. Any current detected in the secondary circuit must be induced by some external agent. At the instant the switch is closed, the galvanometer needle deflects in one direction and then returns to zero. At the instant the switch is opened, the needle deflects in the opposite direction
  5. 5. Reference: Physics II by Robert Resnick and David Halliday, Topic 31.1, Page 980 and again returns to zero. Finally, the galvanometer reads zero when there is either a steady current or no current in the primary circuit. when the switch is closed, the current in the primary circuit produces a magnetic field in the region of the circuit, and it is this magnetic field that penetrates the secondary circuit.
  6. 6. Reference: Physics II by Robert Resnick and David Halliday, Topic 31.1, Page 980 Furthermore, when the switch is closed, the magnetic field produced by the current in the primary circuit changes from zero to some value over some finite time, and it is this changing field that induces a current in the secondary circuit. This experiment conducted by Michael Faraday proves that a changing electric field can induce electric current.
  7. 7. Mathematical Expression of Faradays Law: Reference: Physics II by Robert Resnick and David Halliday, Topic 31.1, Page 982 The emf induced in a circuit is directly proportional to the time rate of change of the magnetic flux through the circuit. Here, B = B.dA is the magnetic flux through the circuit. Fig: Conducting loop that encloses an area A in the presence of a uniform magnetic field B. The angle between B and the normal to the loop is . ** We will get the explanation of the negative sign in the equation, in Lenzs Law.
  8. 8. Reference: Physics II by Robert Resnick and David Halliday, Topic 31.1, Page 982 If the circuit is a coil consisting of N loops all of the same area and if B is the flux through one loop, an emf is induced in every loop; thus, the total induced emf in the coil is given by the expression, According to the figure, B = BAcos then the equation can be expressed as,
  9. 9. Reference: Physics II by Robert Resnick and David Halliday, Example 30.6, Page 943 Right hand rule: Put your four fingers on the direction of loop current I. The direction of thumb will indicate to the magnetic field B. Screw rule: Put a screw on the center of the loop. Move the screw on the direction of current. The motion of the screw will indicate the direction of magnetic field.
  10. 10. The polarity of the induced emf is such that it tends to produce a current that creates a magnetic flux to oppose the change in magnetic flux through the area enclosed by the current loop. Reference: Physics II by Robert Resnick and David Halliday, Topic 31.3, Page 988 Law Example I: (a) When the bar moves to the right, by Lenzs law, the induced current must be counterclockwise so as to produce a counteracting magnetic flux directed out of the page. (b) When the bar moves to the left, the induced current must be clockwise.
  11. 11. Explanation: Reference: Physics II by Robert Resnick and David Halliday, Topic 31.3, Page 988 As the bar moves to the right, the magnetic flux through the area enclosed by the circuit increases with time because the area increases. According to Lenzs law the induced current must be directed so that the magnetic flux it produces opposes the change in the external magnetic flux. As the external magnetic field is inward to the board, the magnetic field produced by the induced current will be outward direction. According to the right hand rule (or screw rule), the direction of the induced current will be counterclockwise direction for a outward magnetic field. In second case, the current direction will be clockwise because in this case the magnetic field due to current I should be inward direction to minimize the decreasing magnetic field.
  12. 12. Reference: Physics II by Robert Resnick and David Halliday, Topic 31.3, Page 988 Example II: (a) When the magnet is moved toward the stationary conducting loop, a current is induced in the direction shown. (b) According to right hand rule this induced current produces its own magnetic flux that is directed to the left and so counteracts the increasing external flux to the right.
  13. 13. Reference: Physics II by Robert Resnick and David Halliday, Topic 31.3, Page 988 (c) When the magnet is moved away from the stationary conducting loop, a current is induced in the direction shown. (d) According to right hand rule this induced current produces a magnetic flux that is directed to the right and so counteracts the decreasing external flux to the right.
  14. 14. Later Reference: Law