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AC DC
Not the awesome, long-may-theyreign, Australian kings of the power
chord ……..
Technology Wars
 Why is alternating current used to supply
electricity virtually everywhere in the
world??? And not direct current?
 Why iPod vs. Rio vs. Zune?
 Why PC vs. Mac?
Typical reasons why one ‘wins’ over
a competitor…
 Possess key ‘enabling’ technologies
 Better reliability, safety, ease of use, ability to
meets customers’ long term needs
 Public perception about reliability, ‘image’,
safety, ease of use
 More funding available or stronger backing
from scientific community
The War of the Currents
 Thomas Edison (proponent of direct
current) and Nicolai Tesla funded by
Westinghouse (of alternating current)
 Addressed these issues through
extensive research, public
demonstrations and lectures, ‘news’
reports via newspapers
Direct current
 Thought of as supplied by batteries
 Can also have DC powerplants
 Always flows in one direction
Alternating current
 Electrons move back and forth
 Net motion is zero
 BUT the transport of energy between the
electrons is the important thing
 Because it’s cyclic, there is another issue
 Can be transmitted at various
frequencies
 In US, 120V and 60Hz for homes
Residential and commercial
customer requirements
 electricity of various voltages
 Smooth delivery of power for running motors
 People would prefer not to live close to
powerplants, so electricity should be able to be
transmitted over relatively long distances
 Transmission should be efficient, not a lot of
power loss from powerplant to customer
 Might need both DC and AC
AC’s not perfect but it ‘won’
because….
 AC can be transformed from higher to lower
voltage and vice versa
 Enabling technology: the transformer
 AC provides ‘smoother’ flow
 AC can be transmitted over long distances, at
high voltages, therefore low power loss and
transformed to low voltages, closer to the user
 Easier to convert from AC to DC when needed
than vice versa (stuff that runs on batteries can
be plugged in to an outlet)
So, why the different types of plugs
on different appliances?
 Safety for appliances and wiring
 A way to match the appliance with its
required voltage and frequency
 No way you can mix up a square peg
(120 V plug) into a round hole (240 V)
Electrical Power
Transmission
 When transmitting electric power over long
distances, it is most economical to use high
voltage and low current
 Minimizes I2R power losses
 In practice, voltage is stepped up to about
230 000 V at the generating station and
stepped down to 20 000 V at the distribution
station and finally to 120 V at the customer’s
utility pole
Follow the flow!
How does it get to my
house?
Right outside your house,
Voltage is transformed to
120V
Household Circuits
 The utility company
distributes electric
power to individual
houses with a pair of
wires
 Electrical devices in the
house are connected in
parallel with those wires
 The potential difference
between the wires is
about 120V
How do you get 240v?
Household Circuits,
cont.
 A meter and a circuit breaker are connected
in series with the wire entering the house
 Wires and circuit breakers are selected to
meet the demands of the circuit
 If the current exceeds the rating of the circuit
breaker, the breaker acts as a switch and
opens the circuit
 Household circuits actually use alternating
current and voltage
Breaker panel
 Your house?
What’s with that third
prong? Ground Wire
 Electrical equipment
manufacturers use
electrical cords that
have a third wire,
called a case
ground
 Prevents shocks