Transcript Basic_Electricity

Basic Electricity

All material is made of atoms
(we think)
– Atoms are comprised of
 Nucleus: protons (+) and
neutrons
– Fixed
 Electrons (-) orbit outside the
nucleus
Basic Electricity
– Electrons and protons are
charged
 Opposite charges attract
 Like charges repel
 In other words, charges
desire to be “zeroed” or
“neutralized” or
“balanced”
Basic Electricity

How tightly electrons are bound depends
on material
– Tightly bound electrons: insulator
 Does not conduct electricity (or heat) well
 glass, plastic, rubber
– Loosely bound (“free”) electrons: conductor
 Conducts electricity (and heat) well
 metals
– In between: semiconductors
 Important for computer chip production
 silicon, germanium, carbon
Basic Electricity
The copper atom has one lonely electron in
the valence shell, just hanging out, waiting
to be stripped away. It’s a good conductor.
Voltage

• Charge imbalance
caused by chemical
reactions 
What causes electrons to flow?
battery

+++++
   
+++++

+++++

+++++











Protons fixed; Electrons “free”
Electrons move!


• Charges desire to
be neutralized 
• Charges flow!
Voltage
battery


+++++
   
+++++

+++++

+++++










There must be a complete path between excess +
and excess – charges
 Otherwise + and – charges don’t “see” each other

Voltage
A battery has regions of excess negative
and positive charges caused by chemical
reactions
 The amount of “push” exerted on the
electrons is called “electromotive force”

– unit: volt (V)
– Voltage is relative, only is meaningful when
talking about voltage difference between 2
points
Voltage

Do batteries run out of charge?
battery

+++++
   
+++++

+++++

+++++












What happens to total # of electrons? Nothing!
Electrons do not get used up – energy does!!
Current

Number of electrons that pass a certain
point in a certain time: current

wire

 






Count ‘um passing here!
– Unit: Ampere (amp) = 6.25  1018 electrons/s
– Or Amp = 1 Coulomb/second
Current
Historically, current discussed in terms of
positive charge flow: “conventional”
current flow – actually it’s wrong!
 Electrons (-) actually flow to the (+)
 Symbol for battery:



+
+
+

 Equivalent 

+
Time Delay?

Does it take time for electrons to flow
from a switch to a bulb?
10 miles
- - - -- - - - -
Light Bulb
Time Delay?

Does it take time for electrons to flow
from a switch to a bulb?
10 miles
Light Bulb
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - -- - - - -

NO!! Wire is already full of electrons!
Current

How much current will flow?
– Wire is broken? Zero (need a complete circuit)
– Large voltage – large current
– Small voltage – small current
– All other things being equal!

For a given V, current depends on how
much of the electron’s energy is used up:
– Wire: electron’s energy  overcomes friction
– Light bulb: electron’s energy  light and heat
– Motor: electron’s energy  turns motor
Resistance

Measure of energy “used up”
–
–
–
–
–
Depends on the material (and temperature)
Resistance: opposition to electron movement
Unit: Ohm ();
Wires: very low resistance (often neglected)
Insulators: very high resistance (often assumed to be
infinite)
– As resistance increases, it takes more “push” (voltage)
to cause a current
I=V/R
Ohm’s Law
Resistance

SPEED of electron’s is NOT effected by voltage!
Currents transfer energy essentially
instantaneously!
– Why? Wires are already full of “free” electrons!!
– First electron into wire causes one to pop out
Symbol of resistor:
 Often called “loads”

AC/DC

Batteries produce “direct current” (DC)
– Current is steady
– One direction

“Wall outlets” provide “alternating current”
(AC)
– Current oscillates back and forth

More on this later…(old movie)
Power Law

Power is the rate at which energy is being
delivered or consumed
Power = (Current)(Voltage)
P = IV
– Units: Watt (W)
– So if 2 A of current is flowing through a load
at 120 V, the Power used by the load is
P = IV = (2A)(120V) = 240 W
Power Law

How much current flows through a
household (120 V) 60 W light bulb?
P = IV
I = P/V = 60 W / 120 V = 0.5 A
Power vs. Energy

What’s the difference?
– Power (Watts): Rate at which energy is used
or delivered. Doesn’t accumulate, and cannot
be stored in a bucket
– Energy (kWhrs): A quantity that accumulates,
or can be stored in a bucket
Energy = Power x Time
How to measure voltage and
current
Think “voltage across” and “current through”
 When measuring voltage, + and - probes are
inserted in the circuit where you want to
measure the voltage difference.

V
How to measure voltage and
current

When measuring current, the circuit must be
broken so the current flows through the
meter
A
In Summary
Electricity is the flow of electrons through
a conductor driven by an electromotive
force
 V = IR (Ohms Law)
 P = IV (Power Law)
 Energy = Power x Time

Flux Magic
Rectified to DC
(one phase shown)
An electrically charged particle moving in a
magnetic field will experience a force
(known as the Lorentz force) pushing it in
a direction perpendicular to the magnetic
field and the direction of motion:
Source:
http://www.wondermagnet.com/magfaq.ht
ml#q18
Most of the electric power in the world is 3 phase. The
concept was originally conceived by Nikola Tesla and
was proven that 3 phase was far superior to single
phase power. 3 phase power is typically 150% more
efficient than single phase in the same power range. In
a single phase unit the power falls to zero three times
during each cycle, in 3 phase it never drops to
zero. The power delivered to the load is the same at
any instant. Also, in 3 phase the conductors need only
be 75% the size of conductors for single phase for the
same power output.
source: www.windstuffnow.com/ main/3_phase_basics.htm