# How Things Work - How Everything Works Electric Power Distribution Question: Electric power reaches the University via high voltage transmission lines. What fraction of the electric charges traveling on those transmission lines pass through this room? 1. About 1% 2. About 0.01%

3. Exactly 0.0% Observations About Power Distr.

Household power is AC (alternating current) Power comes in voltages like 120V & 240V Power is transmitted at high voltage Power transformers are visible everywhere Power substations are visible on occasion Ohms Law The currents passing through most wires and other devices experience voltage drops

In an ohmic device, the voltage drop is proportional the current: voltage drop = resistance current where resistance is constant for the device Power in Ohmic Devices A calculation: power consumption = voltage drop current voltage drop = resistance current

power consumption = resistance current2 Impact of the calculation: Wires waste power as heat Doubling current quadruples wasted power Power Transmission Power delivered to a city is: power delivered = current voltage drop

Power wasted in transmission wires is: power wasted = resistance current2 For efficient power transmission: Use low-resistance wires (thick, short copper) Use low current and high voltage drop Voltage Hierarchy High voltage is dangerous High current is wasteful

Use the following hierarchy: low voltage circuits in neighborhoods medium voltage circuits in cities high voltage circuits across the countryside Use transformers to transfer power Transformer Alternating current in

one circuit induces an alternating current in a second circuit Transfers power between the two circuits Doesnt transfer charge between the two circuits Current and Voltage

Power arriving in the primary circuit must equal power leaving the secondary circuit Power is the product of voltage current A transformer can change the voltage and current while keeping power unchanged! Step Down Transformer Fewer turns in secondary circuit so

charge is pushed a shorter distance Smaller voltage rise A larger current at low voltage flows in the secondary circuit Step Up Transformer More turns in

secondary circuit so charge is pushed a longer distance Larger voltage rise A smaller current at high voltage flows in the secondary circuit Question:

Electric power reaches the University via high voltage transmission lines. What fraction of the electric charges traveling on those transmission lines pass through this room? 1. About 1% 2. About 0.01% 3. Exactly 0.0%