is conserved, meaning the electric charge is not created nor destroyed, it can only be transferred
from one object to another.
You may already know that in order for water to flow through a pipe, a pressure differ-
ence must exist. Moreover, the water flows from the high-pressure region to the lower-pressure
region. In Chapter 11, we also explained that whenever there is a temperature difference in a
medium, or between bodies, thermal energy flows from the high-temperature region to the
low-temperature region. In a similar way, whenever there exists a difference in electric poten-
tial between two bodies, electric charge will flow from the higher electric potential to the lower
potential region. The flow of charge will occur when the two bodies are connected by an elec-
trical conductor such as a copper wire. The flow of electric charge is calledelectric current
or simply,current. The electric current, or the flow of charge, is measured in amperes. One
ampere or “amp” (A) is defined as the flow of 1 unit of charge per second. For example, a
toaster that draws 6 amps has 6 units of charge flowing through the heating element each
second. The amount of current that flows through an electrical element depends on the elec-
trical potential, or voltage, available across the element and the resistance the element offers
to the flow of charge.12.2 Voltage
Voltagerepresents the amount of work required to move charge between two points, and the
amount of charge that is moving between the two points per unit time is calledcurrent.
Electromotive force (emf)represents the electric potential difference between an area with an
excess of free electrons (negative charge) and an area with an electron deficit (positive charge).
The voltage, or the electromotive force, induces current to flow in a circuit. The most common
sources of electricity are chemical reaction, light, and magnetism.Batteries
All of you have used batteries for different purposes at one time or another. In all batteries,
electricity is produced by the chemical reaction that takes place within the battery. When a
device that uses batteries is on, its circuits create paths for the electrons to flow through.
When the device is turned off, there is no path for the electrons to flow, thus the chemical
reaction stops.
A battery cell consists of chemical compounds, internal conductors, positive and nega-
tive connections, and the casing. Examples of cells include sizes N, AA, AAA, C, and D. A
cell that cannot be recharged is called aprimary cell. An alkaline battery is an example of a
primary cell. On the other hand, asecondary cellis a cell that can be recharged. The recharg-
ing is accomplished by reversing the current flow from the positive to the negative areas.
Lead acid cells in your car battery and nickel-cadmium (NiCd), and nickel-metal hydride
(NiMH) cells are examples of secondary cells. The NiCd batteries are some of the most com-
mon rechargeable batteries used in cordless phones, toys, and some cellular phones. The
NiMH batteries, which are smaller, are used in many smaller cellular phones because of their
size and capacity.12.2 Voltage 347
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