 # Facts about Power Distribution uses AC not DC currents power is transmitted by ‘high voltage’ (kilovolt) lines but actual household voltage = 120 or 220.

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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) Currents passing thru most conductors suffer a drop in voltage proportional to the current V  I V = I  R Current I +-V proportionality constant R = ‘resistance’, in ‘ohms (  )’ But dissipated power P = I  V = I 2 R = V 2/ R Ohm’s Law (Doubling current multiplies power dissipation 4x!)

Given that P = I  V = I 2 R = V 2 /R, how can we bring power to your home w/out wasting power in heating transmission lines ? Power plantyour 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. Strategy: powerplant in countryside country side: high voltage lines cities: medium voltage lines neighborhoods: low voltage lines Step-down transformers Step up transformers

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. The Power Distribution Process

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 Iron Core AC Primary Coil (N 1 turns) Secondary Coil (N 2 turns) where a larger current flows Bulb/Appliance A Step-Down Transformer (N 1 >N 2 ) P 2 =I 2  V 2 P 1 =I 1  V 1 V 2 /V 1 = N 2 /N 1 < 1 Question: How are P 1 and P 2 related ?

Iron Core AC Primary Coil (N 1 turns) Secondary Coil (N 2 turns) where smaller current flows Neon Light A Step-up Transformer (N 1 < N 2 ) V 2 /V 1 = N 2 /N 1 > 1

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