Slide 1

Facts about Power Distribution
• uses AC not DC currents
• power is transmitted by ‘high voltage’ (kilovolt) lines
but actual household voltage = 120 or 220 volts.
• appearance of power transformers periodically
along distribution lines
What is the voltage drop across a conductor* ?
(* wires, and other ‘ohmic’ devices)
Current I
Currents passing thru most conductors suffer a drop
in voltage proportional to the current
Ohm’s Law
proportionality constant
R = ‘resistance’, in ‘ohms ()’
But dissipated power P = I  V = I2R = V2/R
(Doubling current multiplies power dissipation 4x!)
Given that P = I  V = I2R = V2/R, how can we bring power to
your home w/out wasting power in heating transmission lines ?
Power plant
your home
• Power dissipated in distribution cables is minimized if
I is small or V is large; and if R is big.
We need high voltage, low current lines.
We need thick copper cables.
powerplant in countryside
country side: high voltage lines
cities: medium voltage lines
neighborhoods: low voltage lines
Step up
The Power Distribution Process
1. A generator at a power station produces AC electricity at
V = 25,000 volts, flowing at 8,000 A.
2. A step-up transformer raises V 16x to 400,000 volts
(decreases current by 16x to 500 A)
3. The current is still high but heating is minimized by
using thick cables (the ‘National Grid’):
• Underground Cu cables
• Aluminum cables suspended from pylons
4. At factories, step-down transformers reduce voltage
to 33,000 volts.
At homes, step-down transformers reduce this to 110 volts.
Power station
Step-down transformer
What’s a Transformer ?
• steps up or steps down voltages
• AC current in 1 circuit produces an induced current
in a second circuit
• transfers energy (voltages) between two separate circuits
P2=I2  V2
P1=I1 V1
Primary Coil
(N1 turns)
Iron Core
Secondary Coil (N2 turns)
where a larger current flows
A Step-Down
Transformer (N1>N2)
V2/V1 = N2/N1 < 1
Question: How are P1 and P2 related ?
Neon Light
Iron Core
Secondary Coil (N2 turns)
where smaller current flows
(N1 turns)
A Step-up
(N1< N2)
V2/V1 = N2/N1 > 1

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