Notes

Outline

Technical Physics 2 Magnetism

Theory of magnetism results from the movement of electrically charged particles within atoms magnetic substances do NOT have individual magnetic particles such as electrons in electrically charged objects. the magnetic polarity of atoms is believed to be related to the spin of electrons

Observations of magnetism poles exist within the smallest part of a magnetic substance (domains) like poles - repel opposite poles - attract

Magnetic fields magnetic field direction - always away from the north pole and toward the south pole magnetic field intensity (H) - due to the alignment of domains. May be accomplished with another magnet (induction) or electric current. Independent of the material surrounding the magnet permanent magnets retentivity saturation

Magnetic flux density may also be known as magnetic induction measured in Tesla (T) = Weber/m determined by the permeability of the material around the magnet Permeability(m)- the ability of the material to establish a magnetic field Relative permeability(mr  ) - compares the permeability of the material to that of a vacuum(mo) - diamagnetic mr < 1 slightly repelled - paramagnetic mr  > 1 slightly attracted - ferromagnetic  mr  >>1 strongly attracted (Fe, Co, Ni )

Magnetic force a moving charge will experience a magnetic force when moved through a magnetic field

Magnetic force Force on a current carrying wire

Calculating flux density (b) Next to a current carrying wire

At the center of a single loop

At the center of a coil of wire

In a solenoid

Slide 12

Force and Torque on a Loop The force on a current-carrying conductor is directed perpendicular to the magnetic field.

current-carrying loop For a current-carrying loop, the force is directed upward for the left hand side and downward for the right hand side of the diagram below, thus creating torque on the loop.

Magnetic Torque on a Solenoid

Faraday’s Law Moving a conductor through magnetic flux lines causes an emf (voltage) to be induced in the conductor. Relative motion between a conductor and a magnetic field induces an emf in the conductor.  The direction of the induced emf depends on the direction of motion of the conductor with respect to the field.  The magnitude of the emf is directly proportional to the rate at which magnetic flux lines are cut by the conductor.  The magnitude of the emf is directly proportional to the number of turns of the conductor crossing the flux lines.

Lenz's Law Lenz’s Law: An induced current will flow in such a direction that it will oppose by its magnetic field the motion of the magnetic field that is producing it. Fleming’s Rule: If the thumb, forefinger, and middle finger of the right hand are held at right angles to each other, with the thumb pointing in the direction in which the wire is moving and the forefinger pointing in the field direction (N to S), the middle finger will point in the direction of induced conventional current.

The DC Motor An electric motor converts electrical energy to mechanical energy. The key component of the DC motor is the split-ring commutator which reverses the current through the loop every 180°. This allows for continuous rotation.

The DC Generator An electric generator converts mechanical energy to electrical energy. The input torque rotates a loop of wire through a magnetic field producing an induced current.

The AC Generator The change from a DC generator to an AC generator is completed by removing the split-ring commutator with a pair of slip rings.

Back EMF in a Motor In an electric motor, a magnetic torque turns a current-carrying loop in a constant magnetic field. A coil rotating in a magnetic field induces an emf that opposes the cause that gave rise to it.Thus, every motor is also a generator.

Types of Motors DC motors are classified according to how the field coils and the armature are connected. When the armature coils and the field coils are connected in series, the motor is said to be series-wound. When the armature windings are connected in parallel, the motor is said to be shunt-wound.

The Transformer The basic parts of a simple transformer are: A primary coil connected to an ac source. A secondary coil connected to a load. A soft iron core.

The Transformer The induced voltage (emf) in a transformer is in direct proportion to the number of turns.

Transformer Efficiency Efficiency is the ratio of power output (secondary) to power input (primary).

Terms to know induced emf Lenz’s law ac generator dc generator field magnet armature slip rings commutator electromagnetic induction back emf motor step-up transformer step-down transformer transformer efficiency