Chapter 3: Electrical Theory and Practice

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Transcript Chapter 3: Electrical Theory and Practice

Notes taken from Gillette’s Designing
with Light

Atoms are the smallest
complete building
block in nature

Atoms are made up of:
 Protons - positive charge
 Electrons - negative charge
 Neutrons - neutral charge

A stable atom
# of electrons in orbit
around the nucleus =
# of protons in the
nucleus
 Ie. Hyrdrogen

Electron
Nucleus

Law of Charges

Like charges repel and unlike charges attract
 Two protons would defy attempts to be close together
Same with electrons
 An electron and proton would stick together

Valence shell: the outermost plane of
orbiting electrons in the structure of an
atom


Weak force holds electron in orbit
Free electrons: an electron that has broken
away from its “home” atom to float free

Copper

Electrical current

The flow or movement of electrons through a
conductor



Ampere: the unit of measurement of electrical
current
Potential: the difference in electrical charge
between two bodies; measured in volts
Volt: the unit of measurement of electrical
potential

There are two types of electrical current:

Direct current (DC)
 The flow of electrons in one direction. AKA: Batteries
 Short distances

There are two types of electrical current:

Alternating current (AC)
 Transmitted over long distances
 The flow of electrons is reversed on a periodic basis.
 Utility companies
http://www.answers.com/topic/alternating-current

3 parts

Source: the origin of electrical potential
 battery or 120-volt wall outlet

Load: a device that converts electrical energy into
another form of energy
 Lamp or motor

Circuit: a conductive path through which electricity
flows
 Wire or cable
Series

In a series circuit all of the electricity flows through
every element of the circuit

If any of the loads don’t work then the circuit will be
broken, the electricity won’t flow, and the remaining
loads won’t work either
Series
www.berkeleypoint.com/images/series.jpg
Parallel


In a parallel circuit only a portion of the electricity
flows through each of the branches of the circuit
If one load on a circuit doesn’t work then the
electricity will continue to flow in the rest of the
circuit and the other loads will continue to work
Parallel
www.berkeleypoint.com/images/series.jpg
The Combination Circuit


Combines the principles of the two primary circuits.
Any electrical circuit that uses a switch to control a
load is an example of a combination circuit.

Ohm’s Law


As voltage increases, current increases; as resistance
increases, current decreases
Resistance
 Opposition to electron flow within a conductor
 Depends on chemical makeup
 Measured in Ohm’s

Low-voltage systems

Ohm’s Law






I=current in amperes
E=voltage in volts
R=resistance in ohms
I=E/R
E=IR
R=E/I

Power Formula


Higher voltage
Watts
 Unit of measurement of power required to do work
 Amount of electrical energy converted or consumed
 Converted into light, heat or mechanical energy

Power Formula
“PIE”
 P=power in watts
 I=current in amperes
 E=voltage in volts



Power Formula
“PIE”
P=power in watts
 I=current in amperes
 E=voltage in volts


P=IE

West Virginia
W=power in watts
 V=voltage in volts
 A=current in amperes


W=VA



Dimmer Voltage = 120VAC
Dimmer can handle 20 amperes of current
What is the maximum safe load that can be
placed on this dimmer

How many 575W instruments can you plug
into a 2400W dimmer with cable that can carry
20 amperes of current?

How many 750W instruments can you plug
into a 2400W dimmer with cable that can carry
20 amperes of current?



System Voltage = 120VAC
14 gauge cable connecting the instruments can
carry 15 amperes
How many 500 watt instruments can be loaded
onto a dimmer?

Conductor



Any material with an abundance of free electrons
Water, copper, silver, gold, aluminum
Insulator


Any material with few free electrons
Air, glass, paper, rubber, most plastics

Created when a large SURGE of current causes
a portion of the conductor to explosively melt

Provides a low resistance path for the
electricity to follow in case of a short circuit
between the hot wire and the devices metallic
housing
http://sol.sci.uop.edu/~jfalward/physics17/chapter8/plugwithground.jpg


Electrical cable containing
hot, neutral and ground
wires
Connects lighting
instrument to the power
source

Plug


Male portion of a connecting device
Receptacle

Female portion of a connecting device


Many hand tools do not have ground pins
Instead they have an outer plastic insulation
that protects you from a short circuit


The grounding point
Usually a metal rod driven into the ground or
an underground metal water pipe
http://en.wikipedia.org/wiki/Image:HomeEarthRodAustralia1.jpg



HOT = BLACK, occasionally RED
GROUND = GREEN
NEUTRAL = WHITE


Greater current in system than it was designed
to handle
Fuse and circuit breaker protects from overload

Must have matching amperage as system

Fuse


Contains a soft metallic strip that melts when the
current exceeds what the system is designed for
Must be replaced when “blown”
http://www.m99.co.uk/Car_Audio/Car_Fuses/gold
_fuse.jpg
images.orgill.com/200x200/4180923.jpg
images/jpegs/fuse30a.jpg

Circuit Breaker


Like a switch
Bimetal strip flexes to trip circuit if overloaded
cache.smarthome.com/images/7103.jpg

1. If you don’t know what you are
doing, don’t do it. Ask for help.

2. Use tools covered with plastic or
rubber insulation

3. Use wooden or fiberglass ladders

4. Disconnect device from circuit
before you work on it.

5. Use common sense
 Don’t touch bare wires
 Don’t work in damp locations or
put drink where it could spill
 Don’t intentionally overload circuit
 Don’t try to bypass fuse or circuit
breaker

6. Maintain ground circuits

7. Check cables and connectors
periodically. Replace cracked,
chipped or deteriorating equipment

8. Keep cables and connectors clean.

9. Store cables neatly with ends
plugged together and tied

10. Disconnect plug by pulling on
body of plug.

11. Ensure all elements have same
electrical rating.