phys magnetism review

Domain theory

• any sample of a metal is actually a large number of microscopic crystals

• Each crystal is a —— where all of the atoms align their magnetic fields 

• Each —— is a uncounted # of atoms aligned to for a microscopic magnet

Oersted's Discovery

• When you have a current in a wire, you have a huge # of charged particles (electrons) all moving the same direction. 

• Each produce a magnetic field, which compound into larger field that surround the entire wire

Faraday’s Discovery

1. Creating or stopping current builds up or collapses the magnetic field.

2. The change (build-up or collapse) in the magnetic field induces a current in the second coil.

• Turning current on/off in the first coil briefly induces current in the second coil.

• knew current creates a magnetic field (Oersted’s work).

• Induced current in the second coil is temporary—only while the magnetic field is changing.

• A moving magnet in a coil also induces current.

• Any change in magnetic field around a conductor induces current.

Right-Hand Rule for Straight Conductors

• Current flows from positive to negative

• Current = right hand

Right-Hand Rule for Coiled Conductors

• Place hand how current is flowing thumb points to north

Right-Hand Rule for Conductors in an External Magnetic Field (EMF)

• Thumb → Direction of the motion of the conductor (relative to the magnetic field)

• Fingers → Direction of the magnetic field (from North to South)

• Palm (or out of the palm) → Direction of the induced current (positive charge flow)

The Motor Principle

A current-carrying conductor placed in a magnetic field experiences a force, as long as the conductor is not parallel to the field.

The DC Motor

• Armature: the loop(s) of wire that spin inside the motor

• Commutator: transfers the current from the source to the armature

• Split Ring Commutator: allows the direction of the current to reverse to continue the spin of the armature

• Stator: the permanent magnet, usually an electro-magnet, often curved to follow the loop as it moves

The factors that affect the strength of a magnetic field in Straight conductors

• Current strength: More current = stronger magnetic field.

• Distance from conductor: Magnetic field strength decreases with distance from the wire.

The factors that affect the strength of a magnetic field in Coiled conductors

• Current strength: More current = stronger field.

• Number of coils (turns): More turns = stronger field.

• Spacing of coils: Tighter coils = stronger field.

• Presence of a core: Adding a ferromagnetic core (like iron) greatly increases field strength.

The factors that affect the strength of a magnetic field in Conductors in an external magnetic field

• Current strength: Affects the interaction with the external field.

• Orientation of the conductor: Maximum force (and field interaction) when the conductor is perpendicular to the field.

• Strength of the external magnetic field: Stronger external field = stronger overall magnetic effect.

• Length of conductor in the field: Longer conductor = greater interaction.

Electromagnetic Induction in transformer

• Transformers are used to change the voltage and current to suit the needs of different machines.

• They work based on Faraday’s discovery of electromagnetic induction using a primary and secondary coil.

• A current is only induced in the secondary coil when the magnetic field is changing.

• Therefore, transformers use alternating current (AC), as the constantly changing direction of current produces a changing magnetic field.

Lenz’s Law

If a changing magnetic field induces a current in a coil, the electric current is in such a direction that its own magnetic field opposes the change produced.

The AC Generator

• A coil of wire rotates in a magnetic field.

• As the coil spins, the magnetic field through the coil changes, inducing a current (Faraday’s Law).

• The direction of the current reverses every half-turn of the coil.

• This creates an alternating current that changes direction periodically.

• Rotor (rotating coil)

• Stator (stationary magnetic field)

• Slip rings and brushes to transfer current while allowing rotation.

The four principle ways of creating large scale electrical power and the advantages and

disadvantages of each:

1. Hydro

2. Thermal

a. Hydrocarbon

b. Nuclear

1. Wind

AC vs. DC

• Produced when a coil rotates in a magnetic field (generator).

• Faraday’s Law: Changing magnetic field induces current.

• Direction of current reverses every half-turn → creates ——.

• Uses slip rings and brushes to maintain contact while allowing rotation.

• Found in homes and power grids.

°°°°°°°°°°°°°°°°°°°°°°°°

• Current flows in one direction only.

• Produced by batteries or —— generators.

• In —— generators, a commutator replaces slip rings to keep current unidirectional.

• Common in batteries, electronics, and small devices.