-magnetic dipoles are largely analogous to electric dipole moments -both types of dipoles

Transcription

1 Student Name Date Manipulating Magnetization Electric dipole moment: Magnetic dipole moment: -magnetic dipoles are largely analogous to electric dipole moments -both types of dipoles -physical separation of positive and negative charge in electric dipole moments, but polarization: magnetization: -magnetic moment of an atom or material is the combination of two sources of magnetism, both arising due to the angular momentum of charged particles: - - -two types of magnetic fields 1

2 1. H field: 2. B field: -B field lines must close! B = µ 0 (H+M) µ 0 *H represents response from µ 0 *M represents response from demagnetizing field (stray field): -less stray field, less -leads to formation of in all but the smallest magnets -acts to reduce Draw the H field and B field lines resulting from the uniformly magnetized block shown below. Magnetization H Field B Field S N S N isotropic - in every direction 2

3 anisotropic - in every direction -in small magnets, demagnetization energy leads to crystalline materials (single crystal, monocrystalline): polycrystalline materials: amorphous materials: Draw the arrangement of atoms in crystalline, polycrystalline, and amorphous materials. Crystalline Polycrystalline Amorphous magnetocrystalline anisotropy - property of magnets whereby -this cancels out in materials with Zeeman energy: 3

4 E = -µ 0 M H cosθ -energy is minimized when magnetization and applied field are Draw the evolution of the magnetic domain structure as an increasingly large magnetic field is applied to a small permanent magnet. -(major) types of magnetic materials: 1. ferromagnet (e.g. ) - magnetization induced in direction as applied field, material magnetization after removal of applied field -what people call 2. paramagnet (e.g. ) - magnetization induced in direction as applied field, material magnetization after removal of applied field 3. diamagnet (e.g. ) - magnetization induced in direction as applied field, material magnetization after removal of applied field magnetic hysteresis - a memory effect resulting from -enabling phenomenon behind 4

5 Label the individual magnetic domains of states a through d with realistic distributions of magnetization direction. (Still need to cover blue arrows with white boxes) (a)/(d) saturation magnetization: (b)/(e) remanence: (c)/(f) coercivity: -thermal energy -low temperature - moments -medium temperature - moments -high temperature - moments 5

6 Name a few real-life applications of motors (in groups): Electric Magnetic (EM) Motors: a. EM motors converts energy into energy. There are mainly two types of motors based on the passing through: motor and motor. b. DC motors: Operation principle: The force experienced by a moving charge in the magnetic field: 6

7 i) When θ = 90 o, ii) When θ = 0 o, a charge moves parallel to the magnetic field, the magnetic force. iii) If we have a stationary charge, the magnetic force. iv) Identify the direction of force on charge: Current carrying wire in magnetic field: If we bend the current-carrying loop, what would happen? 7

8 a. Components of a DC motor: Stator: Rotor: Windings Commutator: Brushes: b. The picture below shows the basic components of a motor. On the diagram, indicate Direction of the magnetic field around the rotor Polarity of the magnets How to calculate the electromagnetic force experienced by the wire c. Torque & Torque variation in a DC motor: 8

9 When passes through a coil placed in a field, is produced by the force, thus the DC motor turns. Q: How to keep the torque in the same direction for unidirectional rotation? A: In our motor experiment, we used a slightly different design, where we scratch off some parts of the copper wire. In that case, torque vs. angle is: Relationship between generated power and torque: 9

10 Question: Q: What would happen to the force when the field direction is reversed? What about when both the field direction and current flow direction are reversed? A: Q: Do motors have a high or low inductance? A: 10

11 Electromagnetic Motor Design Select three design variables to study. Indicate your selections with a check mark. # of loops of copper wire size/shape of rotor applied voltage (# of AA batteries) magnetic field strength (proximity of rotor to magnet) other: First Design Variable: Design Variables Held Constant Value Values of Design Variable Under Study Resulting RPM Second Design Variable: Design Variables Held Constant Value 11

12 Values of Design Variable Under Study Resulting RPM Third Design Variable: Design Variables Held Constant Value Values of Design Variable Under Study Resulting RPM Fourth Design Variable (optional): Design Variables Held Constant Value Values of Design Variable Under Study Resulting RPM 12

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14 Strain-Mediated Magnetoelectric Coupling magnetostriction - property of ferromagnetic materials that leads to: most (but not all) magnetostrictive materials expand in the direction as an applied magnetic field piezoelectricity - analogous to magnetostriction, a property of ferroelectric materials (which have after removal of an applied ) that leads to: magnetoelectric effect - induction of via an applied field converse magnetoelectric effect - induction of via an applied field -factors that influence how well strain is coupled at the interface: in general, much better coupling is achieved in 14

15 Q: Do motors have a high or low inductance? A: Review of Lenz s law and Inductance Lenz s Law: Inductance: (Let s also derive the relationship between number of loops N in coil and inductance L) 15

16 Losses in DC motors: Generator and Motor: First invented by Nicola Tesla, AC generator converts energy into energy. The working principle of AC generator is based on : as we turn, the passing through the changes, and the current changes in the coil to oppose the change, so that is produced. Hence, a voltage is induced in the coil. (Figures adapted from: [Brushless motor] Advantages compared to motors with brush: Mechanism: 16

17 Another example of a simple brushless motor that employs Hall effect: Brushless motor components: 1) Rotor, cooling magnet 2) Stator with core arrangements. 2) Apply DC to pairs of coils which will become electro-magnets [Reference: Electrical Engineering: concept and applications, Seyed A Zekavat] Servo motor (Stepper motor) 17

18 Notes on Nanomotors: 18

19 Image source: 19

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