Magnetic field and magnetic poles

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Godwin Allison
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2 Magnetic field and magnetic poles Magnetic Field B is analogically similar to Electric Field E Electric charges (+ and -)are in analogy to magnetic poles(north:n and South:S).

Paramagnetism, Diamagnetism, and Ferromagnetism Diamagnetic materials are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction,

4 Paramagnetism, Diamagnetism, and Ferromagnetism Diamagnetic materials are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. Pyrolytic carbon has one of the largest diamagnetic constants of any room temperature material. Here a pyrolytic carbon sheet is levitated by its repulsion from the strong magnetic field of neodymium magnets.

5 Paramagnetism is a form of magnetism whereby certain materials are attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field.

6 Ferromagnetism is the basic mechanism by which certain materials (such as iron) form permanent magnets, or are attracted to magnets.

11 An electric current can produces a magnetic field. The strength of this field is directly proportional to the value of the current. Thus a magnetic field produced in this way may be turned on and off, reversed, and varied in strength very simply. A magnetic field is a vector quantity

12 The flux pattern produced by a straight conductor can be adapted to provide a field pattern like a bar magnet. This is achieved by winding the conductor in the form of a coil. This arrangement is known as a solenoid

15 The flux must be directly proportional to the number of turns on the coil. The flux is also directly proportional to the value of current passed through the coil

16 The magnetic field strength is defined as the mmf per meter length of the magnetic circuit. The quantity symbol for magnetic field strength is H

qv B B is the magnetic force, q is the charge, is the

20 Force on a Charge Moving in a Magnetic Field The properties can be summarized in a vector equation: F qv B B is the magnetic force, q is the charge, is the velocity of the moving charge B F B is the magnetic field v

21 Charged Particle in a Magnetic Field Consider a particle moving in an external magnetic field with its velocity perpendicular to the field. The force is always directed toward the center of the circular path. The magnetic force causes a centripetal acceleration, changing the direction of the velocity of the particle.

22 Force on a Charged Particle Use the particle under a net force and a particle in uniform circular motion models. Equating the magnetic and centripetal forces: F B qvb mv r 2 r mv qb r is proportional to the linear momentum of the particle and inversely proportional to the magnetic field. The angular speed of the particle is v qb ω r m The angular speed, w, is also referred to as the cyclotron frequency. The period of the motion is 2πr 2π 2πm T v ω qb

23 Charged Particles Moving in Electric and Magnetic Fields In many applications, charged particles will move in the presence of both magnetic and electric fields. In that case, the total force is the sum of the forces due to the individual fields. The total force is called the Lorentz force. In general: F qe qv B

24 Mass Spectrometer, an example application A mass spectrometer separates ions according to their mass-to-charge ratio. In one design, a beam of ions passes through a velocity selector and enters a second magnetic field. After entering the second magnetic field, the ions move in a semicircle of radius r before striking a detector at P. If the ions are positively charged, they deflect to the left. If the ions are negatively charged, they deflect to the right.

25 Magnetic Force on a Current Carrying Conductor F IL B B

26 Torque on a Current Loop There is a force on sides 2 & 4 since they are perpendicular to the field. The magnitude of the magnetic force on these sides will be: F 2 = F 4 = I a B The direction of F 2 is out of the page. The direction of F 4 is into the page. Maximum Torque on a loop τ max = IAB

27 Magnetic Dipole Moment The product I Ais defined as the magnetic dipole moment,, of the loop. Often called the magnetic moment SI units: A m 2 Torque in terms of magnetic moment: B Analogous to p E for electric dipole

28 DC Electric Motors A DC motor is any of a class of rotary electrical machines that converts direct current electrical energy into mechanical energy. The most common types rely on the forces produced by magnetic fields.

Magnetic Fields. or I in the filed. ! F = q! E. ! F = q! v! B. q! v. Charge q as source. Current I as source. Gauss s Law. Ampere s Law.

Magnetic Fields. or I in the filed. ! F = q! E. ! F = q! v! B. q! v. Charge q as source. Current I as source. Gauss s Law. Ampere s Law. Magnetic Fields Charge q as source Gauss s Law Electric field E F = q E Faraday s Law Ampere-Maxwell Law Current I as source Magnetic field B Ampere s Law F = q v B Force on q in the field Force on q v