Transcript Electricity
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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 xrays
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
Conductor
Does not allow an electrical charge to move freely
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
Dynamo or generator
Converts chemical to
electrical energy
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/electr
icity/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/phe
t-ohms-law/ohms-law_en.html
Ohm’s Law, Continued
Voltage = Current X
Resistance
or
V=IXR
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
Rule #2
I(T) = I(1) = I(2) = I(3)
The current in each circuit
component is constant
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
Rule #2
V(T) = V(1) = V(2) = V(3)
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.ndted.org/EducationResources/HighSchool/Elect
ricity/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/s
feature/acdc.html
http://www.ndted.org/EducationResources/HighSchool/Electrici
ty/directcurrent.htm
http://www.youtube.com/watch?v=JZjMuIHoB
eg&feature=player_detailpage#t=8s
http://www.youtube.com/watch?v=g17f9J1-rk&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/in
dex.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/filamentresista
nce/index.html
Electrical Power
Measured in Watts
Watts = Volts X
Amperes