Download Alternating current

13.3 Alternating Current (AC)
Electromagnetic induction requires a changing magnetic field to produce an
electric current. If you were to push a permanent magnet through a coil, current
flows for only as long as you move the magnet in one direction.
It is not possible to move the magnet in the same direction indefinitely. At some
point the magnet will need to be pulled in the opposite direction.
As soon as you reverse the direction of the magnet being pulled through the coil,
the electric current produced in the coil will also reverse direction.
Alternating current is current that periodically reverses direction.
13.3 What is Alternating Current?
Alternating current is the back and forth motion
of charges. Both voltage and current continuously
increase to a positive maximum and decrease to
a negative minimum.
This process repeats itself at a frequency of 60
Hz. This means that the current is flowing in a
positive direction, reverses, then flows in a
negative direction 60 cycles every second.
Note the graphs go through zero twice per cycle
as the current changes direction. This implies that
the circuit shuts off for an instant, yet we do not
notice this effect because the cycles occur too
quickly for our eyes to detect.
13.3 Edison vs. Tesla
Today’s electrical power grids do not rely on direct (DC) current because it is
limited to how far it can be transferred without significant energy losses, due to
friction, in the form of thermal energy.
Direct current was the standard used in the first electrical power grids. In 1882,
Thomas Edison built the Pearl Street power station in Manhattan , but he was only
able to provide energy to 193 homes.
Nikola Tesla, an electrical engineer who at one point worked for Edison, developed
a competing system using alternating current. The AC system could transfer
energy from a power plant more efficiently than the DC system.
Edison and Tesla were bitter rivals who both fought to have their respective
systems accepted, but in the end, it was Tesla and his AC system that became the
favoured choice.
13.3 Household Circuits
Your home is designed for a voltage of 240 V.
Most household appliances run on 120 V, yet
larger appliances, such as an electric stove or
clothes dryer run on 240 V.
A three-wire system is used, with red and black
being ‘hot’ and white being neutral. Electricians
wire homes mainly using two wires; a white and
a ‘hot’ for 120 V. For appliances that require 240
V, the red and black are used.
For safety purposes, a fourth wire called a
ground is also used to direct stray currents into
the ground, preventing them from reaching you
when you touch a circuit.
13.3 Safety Systems
Many safety systems exist in your home to prevent harm to
you, damage to your appliances, or electrical fires.
Fuses are devices that placed in series with one of the
branches in your home to prevent excess current. If the current
exceeds the maximum rated value, a fine wire inside the fuse
melts, opening the circuit to prevent any more current.
Circuit breakers also prevent excess current in a wire for
extended periods of time, potentially causing a fire. A
bimetallic strip heats up and bends, causing the circuit to trip,
turning it off. This may occur if too many loads are connected
to a branch and require more current.
13.3 Safety Systems
Ground Fault Circuit Interrupters (GFCIs) are installed
in bathroom outlets to detect any small difference in
current going into the a circuit compared to going out.
This prevents electrocution if a wet hand touches the
outlet. GFCIs are designed to trip if a current
difference of 0.006 A is detected, as opposed to 15 A
to trip a circuit breaker.
Arc Fault Circuit Interrupters (AFCIs) prevent sparking
or arcing, which could start a fire. If insulation around
a wire becomes frayed, the bare wire could move
close to another metal part causing arcing to occur.
AFCIs prevent current flow if an arc is detected.
13.3 Homework
Questions # 1-3, 6 p.598