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DMI 50B ELECTRICITY

Kyle Thornton

Why Study Electricity?

Electric energy is supplied to the x-ray tube in order for it to function

The x-ray tube converts electric energy into x-rays

This is the primary function of the x-ray unit

Electrical charge

Electron - smallest unit of electric charge A mobile charge carrier Can move from one point

to another Carries a negative

charge Proton - Equal in mass and

weight to electron Bound in the nucleus Carries a positive charge

Terminology Neutral

object has equal number of protons and electrons Ion pair

Electron removed from atom Atom becomes a positive ion

More positive charges Positive atom + free electron = ion pair Radiation passing through matter causes ion pairs -

potentially harmful Ground

A conduit for excess electric charges to move through

Divisions Of Matter

Insulator Does not allow an electrical charge to move freely

Conductor Allows a charge to move freely Either through it or on its surface

Semiconductor A charge moves through easier than an insulator

but not as easy as a conductor

Electrostatics And Electrification

Electrostatics Non-moving electrical charges Static electricity

Electrification Removal or addition of electrons An excess or deficiency of electrons will

determine if positive or negative

Methods Of Electrification

Friction Removal of electrons from one object to another Usually by rubbing the objects together Simplest form of electrification

Contact A body charged by friction touches another object Confers the same type of charge

More Methods Of Electrification

Induction All charged objects have an electric field Uncharged metallic object will experience an

electron shift if brought into an electric field Only the electrons move

Laws Of Electrostatics 1. Like charges repel; opposites attract 2. Electrostatic force - Coulomb’s law

Inversely proportional to square of the distance between two charged objects

3. Electric charges reside only on the surface of conductors

4. Concentration of charges on curved surface is greater where curvature is greatest

5. Only negative charges can move in solid conductors

Electrodynamics

Charges in motion Current - Flow of charged particles 1. In a vacuum 2. In a gas 3. In an ionic solution 4. In a metallic conductor Current is measured in amperes - A

Sources of Current

Cells or batteries Converts chemical to

electrical energy

Dynamo or generator Mechanical to electrical

energy

Solar Atomic Wind Geothermal

How does electric current flow? There must be a path on

which to flow Circuit Generally controlled by

switches that open or close the circuit

There must be a potential difference between two points within the path Voltage or V This is usually supplied by a

battery, power supply, or electrical outlet

Resistance

Acts to oppose current flow Resistance is generally present in all

materials Resistance is measured in Ohm’s or Ω Resistance is determined by:

The material of the conductor The length of the conductor The cross-section of the conductor

More on Resistance

http://micro.magnet.fsu.edu/electromag/electricity/resistance.html#CODE

Ohm’s Law

The relationship of current, voltage, and resistance in a circuit

If voltage is increased, and electrons are available, the electrons will move faster

Current flow is affected by resistance Resisitors may be added to the circuit to regulate

and direct the current flowing through the circuit file:///C:/DOCUME~1/kthornto/LOCALS~1/Temp/

phet-ohms-law/ohms-law_en.html

Ohm’s Law, Continued

Voltage = Current X Resistance or V = I X R or R = V/I or I = V/R

Work The Following Problem

A 1.5 battery produces 300 mA. Determine the resistance of the circuit.

Given: V = 1.5 I = 300 mA = 0.3 A R = ? Ohm’s law = V = IR or R = V/I Answer: 1.5/0.3 = 5 ohms

Types Of Circuits

Series All resistors are

connected in a continuous line

If a resistor is damaged so that current cannot flow, it won’t flow through the entire circuit

Types of Circuits

Parallel All resistors are

connected side by side If one resistor is broken,

there is still a path on which to flow

Ohm’s Law In A Series Circuit

Rule #1 I(T) = I(1) = I(2) = I(3) The current in each circuit

component is constant Rule #2

V(T) = V(1) + V(2) + V(3) The total voltage is equal to

the sum of the voltage in each component

Rule #3 R(T) = R(1) + R(2) + R(3) The total resistance is equal to

the sum of the resistance in each component

Ohm’s Law Website

http://micro.magnet.fsu.edu/electromag/java/ohmslaw/index.html

More Practice With Ohm’s Law There are three resistors in a circuit. They

have the following magnitudes R(1) = 8, R(2) = 10, R(3) = 6 What is the total resistance? Answer = 8 + 10 + 6 = 24 Voltage = 12 Find the total current. Current = V/R Answer = Amperes = 12/24 = 0.5 A

Ohm’s Law In A Parallel Circuit

Rule #1 V(T) = V(1) = V(2) = V(3)

Rule #2 I(T) = I(1) + I(2) + I(3)

Rule #3 1/R(T) = 1/R(1) + 1/R(2)

+ 1/R(3) The total resistance is

inversely proportional to the sum of the reciprocals of each individual resistor

Total Resistance In A Parallel Circuit

R(T) will always be less than the lowest individual resistance in the circuit

Find the total resistance for the following: R(1) = 4, R(2) = 3, R(3) = 12 1/4 + 1/3 + 1/12 3/12 + 4/12 + 1/12 = 8/12 R(T) = 12/8 = 1.5 ohms

Series and Parallel Circuit Diagram

http://www.ndt-ed.org/EducationResources/HighSchool/Electricity/seriescircuit.htm

Charge Flow Direction: Direct Current vs. Alternating Current

Direct current - charges move in only one direction The type that is supplied by a battery or DC power supply

Alternating current Alternates its direction of motion

It flows one direction during the first half Reverses direction in the second half

Divided into cycles 60 cycles per second in the US One full cycle is 1/60 second One half cycle is 1/120 second There are 60 cycles/second This is called a 60 Hertz AC

Examples of DC and AC

http://www.pbs.org/wgbh/amex/edison/sfeature/acdc.html

http://www.ndt-ed.org/EducationResources/HighSchool/Electricity/directcurrent.htm

http://www.youtube.com/watch?v=JZjMuIHoBeg&feature=player_detailpage#t=8s

http://www.youtube.com/watch?v=g17f9J1-r-k&feature=related

Other Circuit Components Capacitor

A device that stores electrical charge until needed Generally consists of two metal plates

http://micro.magnet.fsu.edu/electromag/java/capacitance/index.html

http://micro.magnet.fsu.edu/electromag/java/capacitor/index.html

Rheostat A device to vary resistance Produces gradual changes in voltage or current flow

http://micro.magnet.fsu.edu/electromag/java/filamentresistance/index.html

Electrical Power

Measured in Watts Watts = Volts X

Amperes