Batteries

Electrical cells

An electrical cell is an object that holds chemical energy. This can be done in numerous ways, none of which are important to understand to grasp the electrical behaviour of cells. It suffices to know that some chemical interaction causes the cell to have a difference in electric potential (potential difference or voltage) between two points of the battery, which we call its poles. The positive pole is defined as the pole that positive charges are repelled by, and the negative pole is the pole that is not the positive pole.

Definition of batteries

The definition of a battery is entirely based on that of an electrical cell. In general, a battery is any system of one or more cells joined in a combination of parallel and serial joints, but at the level of this article, the following definition is adequate.

As a result, a battery has a positive and a negative pole, and the potential difference of a battery is the sum of the potential differences of the individual cells inside the battery, given that the cells are joined facing the same way (meaning the negative end of one cell is connected to the positive cell of another cell).

Batteries with an indication of the positive pole by a + sign.

How do batteries work?

Batteries contain cells that contain chemical energy, which causes a voltage between its poles. If we put a particle with a positive charge near the positive pole of a battery, it will be pushed to the negative pole of the battery by the voltage (see the figure below). This means that the battery does work on the particle (because it exerts a force over a distance), so the battery loses energy in this process. This energy came from the chemical energy inside the battery: the battery converted its chemical energy into work. Thus, after this process, the battery contains less chemical energy.

A battery exerting a force on a positive charge, pushing it from the positive pole to the negative pole in the process, StudySmarter Originals.

Properties of batteries

The chemical design and composition of a battery will dictate how much chemical energy it contains, and thus how much electrical energy the battery can provide. It will also dictate how large the potential difference is between the poles of the battery.

Remember that the voltage determines how much energy a unit charge gains when travelling through the voltage difference. The work$W$done on a particle with a charge$q$that travels through a voltage$V$is given by the product of the charge and the voltage,

$W=qV$

(because voltage is energy gained divided by the charge of a particle). A battery with a higher voltage will thus put a larger force on a charge than a battery with a lower voltage will put on the same charge.

The battery capacity is the total charge it can displace from one pole to the other. This is measured in SI-units in$\mathrm{C}$(coulomb), but it is commonly measured in$\mathrm{Ah}$(ampere-hours). The total chemical energy$E$is the total amount of work the battery can do (assuming a 100% efficiency for simplicity), so the total charge$Q$that the battery can displace from one pole to the other pole is given by

$Q=E/V$.

In short, batteries have properties such as battery capacity, voltage, and energy capacity.

Battery symbol in physics

In physics and electrical engineering, there exist symbols for every element in an electrical circuit, and cells and batteries are no exception. See the figure below for the symbols for an electrical cell and a battery.

Above is the symbol of an electrical cell, and below is the symbol of a battery, Wikimedia Commons.

We see that the symbol for a battery is a graphical representation of what a battery is, namely a couple of electrical cells joined in series.

Functions and examples of batteries

Batteries are commonly used in electrical devices, examples of which are phones, digital watches, laptops, and cars. In all these examples, the function of a battery is to provide electrical energy to a system. What purpose this energy has differs from case to case. In phones, watches, and laptops, this energy is mainly used to operate the system and make a screen light up, while in cars, this energy is used to power the starter motor, headlights, and indicators, among other things. In electric vehicles, the energy from the battery is used mainly to power the electric motors to make the car move: this is an example in which the energy from the battery is mainly converted to kinetic energy.

Types of batteries

The chemical energy inside a battery can come from a wide variety of chemical compositions, and the transfer of chemical to electrical energy can happen via a lot of different chemical or physical reactions inside the battery. This means that there are a lot of different types of batteries.