Section Summary

Ohm’s law

• Direct current is constant over time; alternating current alternates smoothly back and forth over time.
• Electrical resistance causes materials to extract work from the current that flows through them.
• In ohmic materials, voltage drop along a path is proportional to the current that runs through the path.

Series Circuits

• Circuit diagrams are schematic representations of electric circuits.
• Resistors in series are resistors that are connected head to tail.
• The same current runs through all resistors in series; however, the voltage drop across each resistor can be different.
• The voltage is the same at every point in a given wire.

Parallel Circuits

• The equivalent resistance of a group of N identical resistors R connected in parallel is R/N.
• Connecting resistors in parallel provides more paths for the current to go through, so the equivalent resistance is always less than the smallest resistance of the parallel resistors.
• The same voltage drop occurs across all resistors in parallel; however, the current through each resistor can differ.

Electric Power

• Electric power is dissipated in the resistances of a circuit. Capacitors do not dissipate electric power.
• Electric power is proportional to the voltage and the current in a circuit.
• Ohm’s law provides two extra expressions for electric power: one that does not involve current and one that does not involve voltage.