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produced by a greater gravitational potential diff erence. Th e the following corrections: (1) in an electric current, electrons

rate of water flow is thus directly proportional to the diff erence do not move through a wire just as water flows through a pipe;

in gravitational potential energy. In an electric circuit, the rate (2) electrons are not pushed out one end of the wire as more
of current (coulombs/second, or amps) is directly proportional electrons are pushed in the other end; and (3) electrons do not

to the difference of electrical potential (joules/coulombs, or move through a wire at the speed of light since a power plant
volts) between two parts of the circuit, I ∝ V. failure hundreds of miles away results in an instantaneous loss
of power. Perhaps you have held one or more of these miscon-
ceptions from fl uid analogies.
THE NATURE OF CURRENT What is the nature of an electric current? First, consider
There are two ways to describe the current that fl ows outside the nature of a metal conductor without a current. Th e atoms

the power source in a circuit: (1) a historically based descrip- making up the metal have unattached electrons that are free to
tion called conventional current and (2) a description based move about, much as the molecules of a gas do in a container.

on a flow of charges called electron current. Th e conventional They randomly move at high speed in all directions, oft en col-

current describes current as positive charges moving from the liding with one another and with stationary positive ions of the

positive to the negative terminal of a battery. Th is description metal. This motion is chaotic, and there is no net movement in
has been used by convention ever since Ben Franklin fi rst mis- any one direction, but the motion does increase with increases
named the charge of an object based on an accumulation, or in the absolute temperature of the conductor.

a positive amount, of “electrical fluid.” Conventional current When a potential difference is applied to the wire in a cir-

is still used in circuit diagrams. Th e electron current descrip- cuit, an electric field is established everywhere in the circuit.
tion is in an opposite direction to the conventional current. Th e electric fi eld travels through the conductor at nearly the

The electron current describes current as the drift of negative speed of light as it is established. A force is exerted on each

charges that fl ow from the negative to the positive terminal of electron by the field, which accelerates the free electrons in

a battery. Today, scientists understand the role of electrons in the direction of the force. The resulting increased velocity of
a current, something that was unknown to Franklin. But con- the electrons is superimposed on their existing random, cha-

ventional current is still used by tradition. It actually does not otic movement. This added motion is called the drift velocity

make any difference which description is used, since positive of the electrons. Th e drift velocity of the electrons is a result of

charges moving from the positive terminal are mathematically the imposed electric fi eld. The electrons do not drift straight
equivalent to negative charges moving from the negative termi- through the conductor, however, because they undergo count-
nal (Figure 6.12). less collisions with other electrons and stationary positive ions.

The description of an electron current also retains his- This results in a random zigzag motion with a net motion in

torical traces of the earlier fluid theories of electricity. Today, one direction. This net motion constitutes a current, a fl ow of
people understand that electricity is not a fluid but still speak charge (Figure 6.14).

of current, rate of flow, and resistance to flow (Figure 6.13). When the voltage across a conductor is zero, the drift veloc-

Fluid analogies can be helpful because they describe the overall ity is zero, and there is no current. The current that occurs when
electrical effects. But they can also lead to bad concepts such as there is a voltage depends on (1) the number of free electrons

per unit volume of the conducting material, (2) the charge on
each electron (the fundamental charge), (3) the drift velocity,
which depends on the electronic structure of the conducting
Conventional current
material and the temperature, and (4) the cross-sectional area
Electron current of the conducting wire.
The relationship between the number of free electrons,

(–) (–)

charge, drift velocity, area, and current can be used to deter-
mine the drift velocity when a certain current fl ows in a certain
(–) (+) size wire made of copper. A 1.0 amp current in copper bell wire
(#18), for example, has an average drift velocity on the order

of 0.01 cm/s. At that rate, it would take over 5 h for an elec-
tron to travel the 200 cm from your car battery to the brake
light of your car (Figure 6.15). Thus, it seems clear that it is the

electric fi eld, not electrons, that causes your brake light to come
Voltage
Voltage drop on almost instantaneously when you apply the brake. Th e elec-
source tric field accelerates the electrons already in the filament of the

(–) (–) brake lightbulb. Collisions between the electrons in the fi lament
cause the bulb to glow.
FIGURE 6.12 A conventional current describes positive charges
moving from the positive terminal (+) to the negative terminal (−). Conclusions about the nature of an electric current are

An electron current describes negative charges (−) moving from the that (1) an electric potential difference establishes, at nearly the

negative terminal (−) to the positive terminal (+). speed of light, an electric field throughout a circuit, (2) the fi eld
148 CHAPTER 6 Electricity 6-10