Facts about Power Distribution uses AC not DC

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 V I V=I R Ohm’s Law proportionality constant R = ‘resistance’, in ‘ohms ( )’ But dissipated power P = I V = I 2 R = V 2/R (Doubling current multiplies power dissipation 4 x!)

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 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 16 x to 400, 000 volts (decreases current by 16 x 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 P 2=I 2 V 2 P 1=I 1 V 1 Primary Coil AC (N 1 turns) Iron Core Secondary Coil (N 2 turns) where a larger current flows A Step-Down Transformer (N 1>N 2) V 2/V 1 = N 2/N 1 < 1 Question: How are P 1 and P 2 related ?

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