Power lines carry electrical energy from power plants to homes, schools, and businesses over long distances. The main physics idea is electric power, which depends on both voltage and current. Transmission lines use very high voltage so the same power can travel with much less current.
This matters because lower current greatly reduces wasted energy as heat in the wires.
A generator at a power plant produces alternating current, and a step-up transformer raises the voltage before transmission. The electricity then travels through metal conductors, creating electric and magnetic fields around the lines as energy moves along the circuit. Near neighborhoods, step-down transformers lower the voltage to safer levels for local distribution.
The grid must constantly balance power generation and power use so voltage and frequency stay stable.
Key Facts
- Electric power is P = IV, where P is power, I is current, and V is voltage.
- Resistive heating loss in a wire is P_loss = I^2R, so reducing current strongly reduces wasted power.
- For the same power, increasing voltage lowers current because I = P/V.
- A transformer changes AC voltage by V_s/V_p = N_s/N_p, where N is the number of coil turns.
- Transmission lines often operate at tens to hundreds of kilovolts to reduce losses over long distances.
- Household electricity is delivered after step-down transformers lower voltage, often to about 120 V or 240 V depending on the system.
Vocabulary
- Voltage
- Voltage is electric potential difference, the energy transferred per unit charge between two points.
- Current
- Current is the rate at which electric charge flows through a conductor.
- Power
- Power is the rate at which electrical energy is transferred or converted.
- Transformer
- A transformer is a device that uses changing magnetic fields to raise or lower AC voltage.
- Transmission line
- A transmission line is a long conductor system designed to move electrical energy over large distances at high voltage.
Common Mistakes to Avoid
- Thinking high voltage means high current, which is wrong because for a fixed power, higher voltage means lower current.
- Ignoring wire resistance, which is wrong because real power lines lose energy as heat according to P_loss = I^2R.
- Using P = IV with mismatched units, which is wrong because voltage must be in volts and current must be in amperes to get power in watts.
- Assuming transformers work with steady DC, which is wrong because ordinary transformers need changing current to produce changing magnetic fields.
Practice Questions
- 1 A transmission line carries 50 MW at 250,000 V. What current flows in the line?
- 2 A wire has resistance 8.0 ohms and carries 100 A. How much power is lost as heat in the wire?
- 3 Explain why power companies use step-up transformers before long-distance transmission and step-down transformers near homes.