Ch. 3: Electricity

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