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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) Current I + V - Currents passing thru most conductors suffer a drop in voltage proportional to the current VI V=IR 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. Strategy: powerplant in countryside country side: high voltage lines cities: medium voltage lines neighborhoods: low voltage lines Step up transformers Step-down transformers 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 Bulb/Appliance P2=I2 V2 P1=I1 V1 Primary Coil AC (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 Primary AC Coil (N1 turns) A Step-up Transformer (N1< N2) V2/V1 = N2/N1 > 1