AP® Physics 2: Algebra-Based · Unit 11 · Topic 11.2
The Complete Loop and the Battery
A circuit needs a closed loop because charge has nowhere to go without a return path; the battery doesn't supply electrons, it supplies the chemical push that moves the electrons already sitting in the wire.
A bulb needs two wires because current is charge flowing in a complete loop, not charge disappearing into a dead end. One wire alone would let charge pile up at the bulb with nowhere to go, and the flow would stop almost instantly. With a return wire to the battery, the same charge can keep circulating, out one terminal, through the bulb, back into the other terminal.
The wire itself is already packed with free electrons, long before any battery is attached. The battery's job is not to hand out electrons but to do work on the ones already there, using a chemical reaction inside it to push electrons out the negative terminal, through the external circuit, and pull them back in at the positive terminal. That reaction is the 'push' that keeps charge moving instead of stalling.
Follow one electron around a simple loop: inside the battery, the chemical reaction does work on it, raising its electric potential energy. As it travels through the external wire and bulb, it gives up that energy as light and heat, losing potential energy as it goes. By the time it reaches the far terminal, its energy is back down, ready for the battery to lift it again. This is why a battery eventually dies: the chemical reaction runs out, not the electrons, which just keep circulating the same loop the whole time.
Key terms
- Complete loop
- An unbroken circular path of conductors that lets charge return to its source instead of dead-ending.
- Chemical reaction (in a battery)
- The internal process that does work on electrons, raising their potential energy to drive them around the circuit.
- Internal resistance
- The small resistance inside the battery itself, which limits current even when external resistance is nearly zero.
- Short circuit
- A connection with very low external resistance across a battery's terminals, causing a dangerously large current.
Watch it explained
- How it worksHow does a battery keep pushing charge around a loop instead of running out of push instantly?
- How it worksWalk me through the energy a single electron gains and loses on one trip around a simple circuit.
- Common mistakeWhy isn't a battery a source of electrons that eventually empties itself of charge?
- Going furtherWhat happens to the current when a wire shorts straight across the battery terminals?
Check yourself
End of topic.