The most common source of electric potential is a battery, which uses chemical reactions to separate charges. A battery consists of chemicals, called electrolytes, sandwiched between two electrodes made of different metals. Chemical reactions in the electrolytes transport ions (i.e., charges) from one electrode to the other. This chemical process pulls positive and negative charges apart, creating a potential difference between the terminals of the battery. When the chemicals are used up, the reactions cease and the battery is dead.

We can sidestep the chemistry details by introducing the charge escalator model of a battery.

MODEL 26.1 Charge escalator model of a battery

A battery uses chemical reactions to separate charges.

• The charge escalator “lifts” positive charges from the negative terminal to the positive terminal. This requires work, with the energy being supplied by the chemical reactions.

• The work done per charge is called the emf of the battery: $\epsilon =W_{\text{chem}}\text{/}q$.

• The charge separation creates a potential difference $\text{Δ}V{}_{\text{bat}}$ between the terminals. An ideal battery has $\text{Δ}{V}_{\text{bat}}=\epsilon$.

• Limitations: $\text{Δ}V{}_{\text{bat}}<\epsilon$ if current flows through the battery. In most cases, the difference is small and a battery can be considered ideal.

Full Alternative Text

Emf is pronounced as the sequence of letters e-m-f. The symbol for emf is $\epsilon$, a script E, and the units of emf are joules per coulomb, or volts. The rating of a battery, such as 1.5 V or 9 V, is the battery’s emf.

The key idea is that emf is work, specifically the work done per charge to pull positive and negative charges apart. This work can be done by mechanical forces, chemical reactions, or—as you’ll see later—magnetic forces. Because work is done, charges gain potential energy and their separation creates a potential difference $\text{Δ}V{}_{\text{bat}}$ between the positive and negative terminals of the battery. This is called the terminal voltage.

In an ideal battery, which has no internal energy losses, the work $W_{\text{chem}}$ done to move charge q from the negative to the positive terminal goes entirely to increasing the potential energy of the charge, and so $\text{Δ}{V}_{\text{bat}}=\epsilon$. In practice, the terminal voltage is slightly less than the emf when current flows through a battery—we’ll discuss this in Chapter 28—but the difference is usually small and in most cases we can model batteries as being ideal.