Final answer:
Electrons gain energy inside a battery due to the conversion of chemical potential energy, while a potential drop across the external circuit occurs because electrical energy is converted into other forms like heat or light. The movement of electrons from the negative terminal to the positive terminal inside a battery does not significantly reduce its voltage until the reactants are depleted.
Step-by-step explanation:
When electrons flow from the positive terminal to the negative terminal inside a battery, they do indeed gain energy but also experience a potential drop. This may seem contradictory, but the two phenomena are a result of how a battery operates and how potential energy and electrical potential (voltage) are related in a circuit. A battery stores chemical potential energy which is converted to electrical energy to move electrons through the circuit. As electrons flow through the circuit, they encounter resistance in circuit elements such as conductors and resistors, which converts some of the electrical energy into other forms, like heat or light.
In a battery, the negative terminal has an excess of electrons which are repelled by it and attracted to the positive terminal. This movement of electrons is essentially a flow from a lower potential (negative terminal) to a higher potential (positive terminal) inside the battery. Although it seems counterintuitive, this flow actually causes a potential drop across the external circuit, which is why there is a voltage decrease as electrons move through it. The initial gain in energy that the electrons receive from the negative terminal is due to the release of chemical potential energy inside the battery, while the subsequent 'loss' in voltage is due to energy conversion processes taking place in the external circuit. Even though the potential energy of the battery decreases, the reduction in internal energy doesn't significantly reduce the potential or the battery's ability to continue pushing electrons until its reactants are depleted.