1 CHAPTER 7 MAGNETISM AND ELECTROMAGNETISM

2 Objectives Explain the principle of the magnetic field Explain the principle of electromagnetism Describe the principle of operation for several types of electromagnetic devices Explain magnetic hysteresis Discuss the principle of electromagnetic induction

3 THE MAGNETIC FIELD A permanent magnet has a magnetic field surrounding it. All magnetic fields originate from charges in motion and the charges in motion are electron.

4 Attraction and Repulsion of Magnetic Poles

5 Altering a Magnetic Field

6 Magnetic flux ( ) The group of force lines going from the north pole to the south pole of a magnet is called the magnetic flux The unit of magnetic flux is the weber (Wb), 1Wb = 10 8 lines

7 Magnetic Flux Density The magnetic flux density is the amount of flux per unit area perpendicular to the magnetic field, and it unit is the tesla (T) One tesla equals to one waber per square meter (Wb/m 2 ) One tesla equals to one waber per square meter (Wb/m 2 ) The Gauss : Although the tesla (T) is the SI unit of flux density, another unit called the gauss (10 4 gauss = 1 T) Flux density (T)

8 How Materials Become Magnetized Ferromagnetic materials such as iron, nickel, and cobalt become magnetized when placed in the magnetic field of a magnet. Ferromagnetic materials such as iron, nickel, and cobalt become magnetized when placed in the magnetic field of a magnet.

9 Applications Operation of a magnetic switch Connection of a typical Perimeter alarm system

10 ELECTROMAGNETISM Current produces a magnetic field, called an electromagnetic field,around a conductor. Current produces a magnetic field, called an electromagnetic field,around a conductor. Magnetic Field around a current Carrying conductor Visible Effect of an electromagnetic field

11 Direction of the Line of Force Left-Hand Rule Left-Hand Rule Magnetic lines of force around a current-carrying conductor Illustration of left-hand rule

12 Electromagnetic Properties Permeability ( ) the ease with a magnetic field can be established in a given material is measured by the permeability of that material. Permeability ( ) the ease with a magnetic field can be established in a given material is measured by the permeability of that material. Permeability value varies depending on the type of material ( ) Permeability value varies depending on the type of material ( ) (weber/ampere-turn.meter) for a vacuum) relative permeability ( ) relative permeability ( )

13 Electromagnetic Properties Reluctance ( ) : The opposition to the establishment of a magnetic field in a material is called reluctance Reluctance ( ) : The opposition to the establishment of a magnetic field in a material is called reluctance

14 Magnetomotive Force (mmf) The force that produces the magnetic field is called the magnetomotive force

15 Electromagnet A basic electromagnet is simply a coil of wire wound around a core material that can be easily magnetized.

16 Applications Magnetic Disk/Tape Read/Write Head The Magneto- Optical Disk

17 ELECTROMAGNETIC DEVICES The Solenoid The Solenoid The Relay The Relay

18 ELECTROMAGNETIC DEVICES The Relay The Relay A Reed Relay A Reed Relay

19 ELECTROMAGNETIC DEVICES The Speaker The Speaker

20 ELECTROMAGNETIC DEVICES Meter movement Meter movement

21 MAGNETIC HYSTERESIS

22 The Hysteresis Curve and Retentivity Hysteresis is a characteristic of a magnetic material whereby a change in magnetization lags the application of a magnetizing force. Hysteresis is a characteristic of a magnetic material whereby a change in magnetization lags the application of a magnetizing force. Retentivity is an ability of material to maintain a magnetized state without the presence of a magnetizing force Retentivity is an ability of material to maintain a magnetized state without the presence of a magnetizing force

23 The Hysteresis Curve and Retentivity

24 ELECTROMAGNETIC INDUCTION Relative Motion results in an induced voltage (v ind ) in the wire. Relative Motion results in an induced voltage (v ind ) in the wire.

25 ELECTROMAGNETIC INDUCTION Polarity of the induced voltage

26 ELECTROMAGNETIC INDUCTION Induced current : the voltage induced by the relative motion in the magnetic field will cause a current in the load

27 Force on a Current-Carrying Conductor in a Magnetic Field (Motor Action) Upward and downward direction on the conductor Upward and downward direction on the conductor

28 Faraday’s Law The voltage induced across a coil of wire equals the number of turns in the coil times the rate of change of the magnetic flux The voltage induced across a coil of wire equals the number of turns in the coil times the rate of change of the magnetic flux

29 Lenz’s Law When the current through a coil changes, the polarity of the induced voltage created by the changing magnetic field is such that it always opposes the change in current that caused it When the current through a coil changes, the polarity of the induced voltage created by the changing magnetic field is such that it always opposes the change in current that caused it เมื่อทำการป้อนไฟฟ้ากระแสสลับเข้าไปยังขดลวดด้าน primary จะทำให้เกิดสนามแม่เหล็กที่มีการเปลี่ยนแปลงไปตัดกับ วงแหวนทางด้าน Secondary ซึ่งเป็นลูปปิด จะทำให้เกิดสนามแม่เหล็ก ชั่วขณะ มีทิศทางต้านกับสนามแม่เหล็กที่เกิดขึ้นทางด้าน primary จึงทำให้เกิดแรงผลักกัน ทำให้วงแหวนสามารถเคลื่อนที่ ออกไปได้ ดังรูป

30 Lenz’s Law

31 APPLICATIONS OF ELECTROMAGNETIC INDUCTION Automotive Crankshaft Position Sensor

32 APPLICATIONS OF ELECTROMAGNETIC INDUCTION DC Generator DC Generator

33 Summary Unlike magnetic poles attract each other, and like poles repel each other. Materials that can be magnetized are called ferromagnetic. When there is current through a conductor, it produces an electromagnetic lines of force around a conductor. An electromagnet is basically a coil of wire around a magnetic core.

34 Summary When a conductor moves within a magnetic field, or when a magnetic field moves relative to a conductor, a voltage is induced across the conductor The faster the relative motion between a conductor and a magnetic field, the greater is the induced voltage.