27.7: DC Battery
A conductor needs to be a component of a path that creates a closed loop or full circuit to have a continuous current flowing through it. A current starts to flow if an electric field is created inside an isolated conductor that is not part of a full circuit. The conductor quickly develops a net positive charge at one end and a net negative charge at the other. These charges generate an electric field opposite the direction of the applied electric field, which reduces the current. Eventually, the total electric field and the current inside the conductor become zero. Therefore, an open circuit cannot have a consistent flow of charge.
The potential energy must be the same at the start and end of a charge's complete circuit round trip if it is to return to its starting place. Charges always experience a loss of potential energy while passing through a typical conducting substance. If the charge needs to continue following the path from the higher to the lower potential, an external agent must bring it back from the lower to the higher potential. This component functions like a water pump in a water fountain and is called a battery.
Inside the battery, even though the electrostatic force attempts to push the charge from higher to lower potential energy, the battery pushes the charge "uphill" from lower to higher potential energy. Hence, the direction of the current is from lower to higher potential, which is the exact opposite of what occurs in a typical conductor.
Electromotive force (EMF) is the work done per unit charge that causes current to flow from a lower to a higher potential. Every fully functional circuit with a constant current must contain an emf-producing component. Examples of emf sources include batteries, electric generators, solar cells, thermocouples, and fuel cells. The energy from some source (mechanical, chemical, thermal, etc.) is converted into electric potential energy by all these devices, which transfers it into the circuit.
