Ohm's law and power

Voltage is the push that moves charge, current is how much charge flows, and resistance opposes the flow. Ohm's law ties them together: V = I x R. Power, the rate a part turns energy into heat or work, is P = V x I.

These two formulas pick almost every resistor, confirm a pin can drive a load and estimate battery life. Work in consistent units: volts, amps and ohms, or volts, milliamps and kilohms, which also works out. Buy a basic multimeter early and measure a few resistors and a battery so the numbers feel real.