17.2. Ohm’s Law http://www.ck12.org
FIGURE 17.4
Alessandro Volta, inventor of the first batteryIt was once assumed (incorrectly) that an electrical current in metals was a flow of positive charge. In spite
of discovering the error, scientists have maintained that assumption. A positive flow of charge is referred to as
conventional current flow. Diagrams showing conventional current have an arrow directed from the positive to the
negative terminal of the battery (or whatever source is used to create a potential difference) as shown inFigure17.5.
Figure17.5 shows a simple electrical circuit with conventional current flow. An electrical circuit is a closed path
that can conduct an electric current. An electrical component of some sort is located between the two terminals of
the battery in an electrical circuit. In the case ofFigure17.5, there is a light bulb.
The left side of the figure is shown in red, indicating that this terminal of the circuit is at a potential of 6V. The
right terminal of the figure is shown in blue, indicating that this side of the circuit is at a potential of 0V. It is only
the potential difference of the battery that is meaningful. Assigning one terminal a potential of 6Vand the other
terminal is 0Vis arbitrary. We could just as well, for example, assign potentials of 20Vand 26Vto the negative
and positive terminals of the battery, respectively (though this would be needlessly confusing).
Check Your Understanding
a. A small light bulb has a 6Vpotential difference applied to it, as shown inFigure17.5. If the wire at pointa
in the figure is disconnected from the positive terminal of the battery and reconnected to the negative terminal of
the battery, and the wire at pointbis disconnected from the negative terminal of the battery and reconnected to the
positive terminal of the battery (that is, the wires are reversed), will the bulb remain lighted?
Answer: Yes. As long as a potential difference remains across the bulb, the bulb remains lit.
b. Is the current flowing through the bulb in the same direction?
Answer: No. If the wires are reversed, the direction of current is reversed.
Figure17.6 shows a toy motor with the red wire connected to the positive terminal of the battery and the yellow
wire connected to the negative terminal of the battery. When the wires on the battery are reversed as inFigure17.7,
the motor will spin in the opposite direction. If the motor were connected to the axle of a car, the wheels on the car
would turn in the opposite direction.
The potential difference, which is responsible for the current, is often likened to water pressure.
The pressure in a water storage tank makes it possible for the water to be transferred to connecting pipes and
eventually to its destination. A battery puts forth “electrical pressure” called potential difference, or voltage. The
voltage is responsible for conveying an electrical current.
Care should be taken in extending mechanical analogies, like flowing water, to a “flow” of electricity, as explained
below.