Ch. 4: Electromagnetism

Magnetism

Force exerted by a magnet when attract/repal each other

• Is a fundamental force

• Results from the motion of charged atomic particles

Magnetic Dipole

North and south poles

Magetic Domains

Collection of aligned magnetic dipoles

Direction of flow

• Outside magnet

• Inside magnet

• Lines of force NEVER intersect

• 3D force field around/through magnetic material

Classification of Magnets (Types)

• Natural Permanent

• Artificial Permanent

• Electromagnets

Natural Permanent Magnets

• Naturally found

• Lodestones (mineral magnetite)

Artificial Permanent Magnet

• Formed/human made

• Alnico (a type of iron alloy with aluminum + cobalt and nickel)

Electromagnets

• Temporary due to moving electric current

• EX: when current ceases, magnetic field collapses

Laws of Magnetism

• Repulsion/Attraction

• Inverse Square Law

• Magnetic Poles

Inverse Square Law

Force between two magnetic fields directly porportional to product of magnitudes and incersely propertional to square of distance between them

Inverse square law (also applies to)

• Magnetism

• Electric field

• Gravity

Magnetic Poles

• Regions of magnetism always exist as a dipole (N and S)(no matter the size)

Oersted’s experiment

• Demonstrated relationship between moving electric charge and magnetism

• A charge in motion will form a magnetic field

• Magnetic field always perpendicular to dicrection of moving charge

Flemings’ right-hand thumb rule for straight conductor

Hold the solid conductor in the right hand

• Wrap fingers around the conductor, thumb adjacent to wire

Right-hand thumb rule

• Thumb indicates direction of current

• Giners show the direction of the magnetic field surrounding an electrically conducting wire

• Related to Lenz’s law

Flemings right hand thumb rule for solenoid

Solenoid - type of electromagnet

• coiled, helix of wire carrying an electrical current

Two primary laws of electromagnetics

• Faraday’s Law

• Lenz’s Law

Faraday’s Law

Magnetic lines of flux and the wire must have motion relative to one another to induce electric current

Faraday’s Law (Required Components)

• Move conductor

• Move magnetic lines of force

• Vary magnetic flux

Faraday’s Law (four factors)

1) Strength of magnetic field

2) Speed of motion between lines of force and conductor

3) Angle between lines of flux and conductor (increase or decrease area)

4) Number of turns in conductor coil

Lenz’s Law

Induced current flows from a magnetic field opposing the action that produced the original current

Self-Induction

• Single coil

• Always present in coils supplied with alternating current

• Constantly changing current polarity (Changing magnetic fields, induced voltage opposing the original current)

• Inductive reactance (Tendency of alternating current)(Dependent on current supplied to the coil)

Mutual Induction

• Two coils

• Varying current supplied to the primary coil and therefore varying EMF

• Induces current in the secondary coil

• Two coils are electrically insulated from each other

Self Induction (steps)

1) A current in the coil produces a magnetic field directed to the left

2) If the current increases, the increasing magnetic flux creates an induced emf in the coil the polarity shown by the dashed bettery

3) The polarity of the induced emf reverses if the current decreases.

Generations

• Convert mechanical energy to electrical energy

• Produce direct current/alternating current

Generations (Components)

• Armature (coil of wire)

• Magnets

• slip rings

• Brushes

Motors

• Convert electrical to mechanical energy

• Similar components to generations

• Armatire supplied with current

Devices controlling electrical current

• Transformers

• Autotransformers

• Capacitors

NOW: 3 devices are now miniaturized into chip technology

Transformers (AC current)

• Electrons move in one direction and reverse

• Allows variation of voltage levels

• All electric power is transmitted this way from generating facilities to end users

Transformers (Two coils)

• Current supplied to the primary coil

• Induced voltage on the secondary coil

• Coils electrically isolated from each other

Transformer law

Step-up transformer

• Voltage increases from primary to the secondary coil

Step-down transformer

• Voltage decreases from primary to the secondary coil

Transformer types (mutual induction)

• Air core

• Open core

• Closed core

• Shell type (most common/efficient)(used by x-ray generation)

Autotransformers (variable transformers)

2 designs

• Primary/secondary coils connected in series (insulating them like mutual induction

• Single coil on central core

• Both considered self induction

Basic X-ray circuit (3 divisions)

1) Low-voltage circuit (main circuit)

2) High-voltage circuit (main circuit) (high-tension circuit)

3) Filament circuit

Capacitors

• Accumulates/store electrical charge/energy

• Charge with direction current (DC) voltage (Electrons move in one direction only in direct stream

• Simple design

• Useful in mobile x-ray units for small body parts (can’t handle thinck body parts due to voltage drop)

Rectification

• Process of changing AC to DC

• Half-wave rectification

• Full-wave rectification

• Located in high voltage section

Rectifiers

• Solid-state diodes

• Vacuum tube rectifier

Solid-state diodes (rectifier used in x-ray machines)

• Solid state design acts like a one way gate

• Uses p-n junction semiconductors

Solid-state diodes (p-n junctions)

• N-type material (loosely bound electrons relatively free to move)

• P-type material (holes, or spaces where there are no electrons)

Electrons from from n(-) side to p(+) side

Need for rectification in X-ray circuits

• X-ray tube requires DC

• Rectification ensures inly DC applied to xray tube with 4 soild state diodes (electrons travel from cathode(-) to anode(+)

• Current passing from anode to cathed to very damaging to x-ray tube because it create way too mach heat

• The X-ray circuit is built to protect from current that can damage equipment.

Thermionic Emission

• Electrical process of liberating electrons from wire filaments

• Filament is heated to very high temp due to creating resistance with small diameter (Filament is typically tungsten)

• Boiling off electrions creat an electron cloud around filament

• Principle of an incandescent light bulb

• Very important to X-ray tube design.