Physics Midterm

Electricity

Concerns distribution/movement of electrons, little to do with positively charge protons locked within atomic nucleus

Electrostatics

Electric charge at rest

Electrification

Electric charge in motion (Can be positive or negative)

Laws of electostatics

1) Repulsion and attraction

2) Inverse Square Law

3) Distribution

4) Concentration

5) Movement

Inverse Square Law

Force between 2 charges is directly proportional to the product of magnitudes and inversely proportional to the square of distance between them.

• Describe the relationship between the radiation intensity/distance from the source.

Concentration (Law of electrostatics)

• The greatest of charge gather at sharpest area curvatire

Movement (law of electrostatics)

• Only negative charges move along solid conductors

• Charges move along the surface of solid conductors

Distribution (Law of electrostatics)

• Charge on the outside surface of solid conductors

• Attempt to repel

Methods of electrifications

• Friction

• Contact

• Induction

Friction

Abrasion 2 conductive matierials transfer charges between them

Contact

Touching (physically) a charged body with a neutral body causes a transfer of charge

• Static discharge; release excess energy as light photons

• Permits electrons to move from 1 object to another

Induction

• brings charge object close to neutral one

• Used: operation electronic devices

• Electroc fields acting on another

Electrodynamics

• Study of electric charges in motion

• Movement of electrons (Neg to pos)

Condition that permits/encourage electron movement

• Vacuum

• Gasses

• Ionic solutions

• Metallic conductors

Conventional Current Flow

Positive to Negative nomenclature (NOT TRUE DIRECTION)

Actual flow of electricity

• Negative to positive

Quantity of electrons flowing

• Current

• Measurement: Ampere (A)

Force which they travel

• Potential difference

• V (voltage)

Alternating Current

• AC

• Can switch

• Decreases temp.

Direct Current

• DC

• All electrons go in the same direction

• Can over heat

This is the one used in x-ray

Resistance

• Opposition to the current

• In Ohms

• Measured in siemens

Resistance affected by

• Materials conductivity

• Length

• Diameter

• Temp.

Materials conductivity

• Conductors

• Semi-conductors

• Super-conductors

• Insulators

Conductors

Valance band and conduction band overlap

Semi-conductors

Valance band and conduction band in close proximity

Super-conductors

Valance band and conduction band overlap greatly

Insulators

Valence band and conduction band far apart

Length (Resistance)

Directly related to resistance

• Length of solid conductor increases, resistance increases propotionally

Diameter (Resistance)

• Inversely related to resistance

• As cross sectional diameter of solid conductor increases, resistance decreases

Thicked wires offer less resistance

Temp. (Resistance)

• Directly related to resistance

• Increased atomic motion, therefore prevents electrons from moving freely

Circuits

• Series Ciruits

• Parallel Ciruits

Series Circuits

• Current flows in a single loop (single pathway)

Parallel Ciruits

Current flow is divided between branches

Add Resistance in Parallel

• Total resistance decreases

• Total amt increases

• Total voltage unchanged

Add Resistance in Series

• Total resistance increases

• Total amp. decreases

• Total voltage depends (voltage drops)

Resistance in parallel circuits

Disadvantage:

• Adding resistance: increases circuit temp

Devices added to circuits for protection

• Circuit breaker

• Fuse

Matter

Has mass and occupies space

Energy

Is a force to do work

Radiation

Emitted energy is when matter is struck by radiant energy (taking an x-ray = taking an exposure)

Law of conservation of matter and energy

The sum of matter and the sum of energy is constant

E=mc²

Substances

• Simple

• Complex

• Atom

• Molecule

Simple substance

Element/pure subtance (cannot be broken down)

Complex Substance

Compound, 2+ elements united (different elements)(can be visible)

• EX: H2O

Atom

Includes protons, neutons, elements

Molecule substance

2+ atoms united (can be some or different)

Atomuc Theory: Historical Overview (include)

• Ancient Greeks

• Dalton (early 1800s)

• Rutherford (1911)

• Bohr (1913)

• Schrodinger

Ancient Greeks (Atomic theory: historical overview)

Air, water, earth, fire

Dalton (early 1800s)(Atomic theory: historical overview)

• Separted elements based on mass

• Lead to mendeleev’s development of periodic table of elements

Rutherford (1911)(Atomic theory: historical overview)

Dense, positive nucleus surrounded by cloud of negative electrons

Bohr (1913)(Atomic theory: historical overview)

Miniature solar system (expanded daltons work)

Schrodinger (Atomic theory: historical overview)

• The foundation of modern physics

• Ouatum physics or wave mechanics

Quantum Physics

• Study of matter/energy at most fundamental level (Subatomic particles)

• Protons/neutrons made up of quarks

• M theory (string theory)

M Theory (String Theory)

• Links quantum physics and relativity

• How matter responds to stimuli and environmental factors on quantim levl

• Five string theories

Basic atomic particles

• Proton (+ charge)

• Neutron (no charge)

• Electron (- charge)

Ionization

• X-ray photons can interact with an atom, resulting in the ejection of an electron

• Change charge between atoms

• Formed by energy/interacting with matter

DOES NOT change element

Orbital Electrons

Defines location where an electron might be at any given time in an atom. Shells include:

• K(1),L(2),M(3),N(4),O(5),P(6),Q(7)

Electron Binding energy (Eb)

Energy needed to eject from atom

• Related to closeness of electron to nucleus

• Eb increases as zX increases

Types of energy

• Mechanical energy

  • Potential

  • Kinetic

• Chemical Energy

• Heat Energy (Thermal energy)

• Nuclear energy

• Electrical energy

• Electromagnetic (EM) energy

Mechanical Energy (includes)

• Potential energy

• Kinetic

Heat Energy (thermal energy)

• Movement of molecules

Nuclear Energy

Obtained by breaking bonds between particles (nucleons) within nucleus

Electrical Energy

Movement of electrons in conductor

Electromagnetic (EM) energy

• Both electic + magnetic fields traveling through space

• Produced by acceleration of charged particles

• Travel in medium or vacuum

• Excitation/ionization

• Energies arranged in spectrum (Wavelength/frquency)

• X-ray only type photon EM energy

Characteristics (EM Radiation)

• Wavelength

• Energy

• Frequency

Particle Theory

• High frequency, high energy EM Radiation

• Interacts like particles when in contact with matter

E=hv (E: photon energy)(v: photon energy)(h: planks constant)

Discovery of first X-ray

Wilhelm Condrad Rontgen (1895)

Wilhelm Conrad Rontgen

• First Nobel Prize winner (physics)

• Occurred while studying electrons travelling through gas tube

• First: X-ray image was the left hand (with a ring)

• Barium platino cyanide as a fluorescent

X-Ray Properties

• Penetrating and invisible form of EM radiation

• Electrically neutral

• Polyenergetic or heterogeneous energies

• Release heat when passing through matter

• Travel in straight lines

• Travel at the speed of light in a vacuum

• Can ionize matter

• Cause fluorecence (emission of light) of certain crystals

• Cannot be focused by the lens

• Affect photographic film

• Produce chemical and biological changes in matter through ionization and excitation

• Produce secondary and scatter radiation

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