Electricity
Charge in motion, and the three quantities that describe it.
Electricity is the presence and flow of electric charge. Almost all school electricity reduces to three quantities and the relationship between them.
| Quantity | Symbol | Unit | Is |
|---|---|---|---|
| Current | I | Ampere (A) | Rate of charge flow |
| Voltage | V | Volt (V) | Energy per unit charge; the push |
| Resistance | R | Ohm (Ω) | Opposition to flow |
Ohm's law
The water analogy is imperfect but useful: voltage is pressure, current is flow rate, resistance is pipe narrowness. It breaks down for anything involving fields or semiconductors, so retire it early rather than build on it.
Series and parallel
| Series | Parallel | |
|---|---|---|
| Path | One route | Multiple routes |
| Current | Same everywhere | Divides between branches |
| Voltage | Divides across components | Same across each branch |
| If one breaks | Everything stops | Others keep working |
| Total resistance | Sum of the parts | Less than the smallest |
Household wiring is parallel, which is why one failed bulb does not darken the house and why every socket sees the same voltage.
AC and DC
Direct current flows one way, as from a battery. Alternating current reverses periodically — 50 or 60 times a second depending on the country. AC won the distribution argument because transformers can step its voltage up for efficient long-distance transmission and back down for safe use, which is impossible with DC using the technology of the time.