To keep the system going, there must be a net flow of positive ions from the negative
terminal to the positive terminal.
As in any manufacturing process, there are raw materials (the electrodes, and the acid
solution), there is a product (the electrons), and there are waste products (the and water).
A battery runs out when its raw materials are used up, or when enough waste products build up to
inhibit the reactions. In a rechargeable battery, the battery is recharged by running the chemical
reactions in the opposite direction, re-creating the electrodes and removing waste products.
Fuel cells use a similar process as batteries but, whereas a battery is a closed system in
which its raw materials and waste products are sealed in a container, in a fuel cell everything is
open so that raw materials can be continually fed into the system and waste products removed. A
number of manufacturers are researching ways to run portable electronic devices, such as laptops
and cell phones, from fuel cells instead of from batteries. The advantage offered by the fuel cell is
that you could run the device for significantly longer than you could run it off a battery. Also,
instead of plugging the device into an electrical outlet for a few hours to re-charge it, you could
just take a few seconds to top it up with, say, methanol, and the device would be good to go again.
Figure 18.1 has three views of a circuit involving a battery, two wires, and a light bulb.
Figure 18.1(a) shows conventional current, where the charges flowing are positive. Figure 18.1(b)
shows the actual situation, showing that the charges flowing through the wires and the light-bulb
filament are actually electrons, while positive ions flow within the battery itself. The two
situations look different, but the light bulb would be equally bright in either case. Figure 18.1(c)
shows the circuit diagram. The current I is in the direction of conventional current.
Every battery has an associated potential difference: for instance, a 9-volt battery
provides a potential difference of around 9 volts. This is the potential difference between the
battery terminals when there is no current, and is known as the battery emf, (emf stands for
electromotive force, and you say emf as it is spelled, e-m-f).
Related End-of-Chapter Exercises: 13, 15.
Essential Question 18.1: Which is closer to the truth – a battery is a source of constant potential
difference, or a battery is a source of constant current? Explain.
Chapter 18 – DC (Direct Current) Circuits Page 18 - 3
Figure 18.1: Three views of a battery-powered circuit. Figure (a) shows conventional current, in
which the charge that flows is always positive. Figure (b) shows the actual situation, in which
electrons flow through the circuit and positive ions flow within the battery. Figure (c) shows a
circuit diagram for this circuit. R stands for resistor, which we cover in the next section. The
arrow under the I shows the direction of the conventional current. The two parallel lines, the
shorter one marked with a – and the longer one with a plus, represent the standard symbol for a
battery in a circuit diagram. The + is the positive terminal, and the – is the negative terminal.