1.4.1 Representation of Electromotive Force, Voltage, and Current Directions

The direction of an electromotive force, such as that of a battery, and the direction of the voltage across a resistor are indicated by arrows pointing from the lower-potential side toward the higher-potential side.

Because conventional current flows from a point of higher potential (the positive terminal) toward a point of lower potential (the negative terminal), its arrow begins in the direction in which the potential is raised by the electromotive force and points toward the direction in which the potential decreases due to voltage drop.

As shown in the figure below, the arrows representing the electromotive force and current point in the same direction, while the arrow representing the voltage across the resistor points in the opposite direction.
Because this voltage acts in a direction that opposes the current, it can be regarded as a counter electromotive force.

Figure 1.4-1 Representation of Electromotive Force, Voltage, and Current Directions

1.4.2 Voltage, Electric Potential, and Potential Difference

Electric circuits are often explained using an analogy with flowing water.

As shown in the figure, when water is pumped upward, areas with higher and lower water levels are created, resulting in a difference in water level.
When the valve is opened, this difference causes water to flow through the pipe and rotate the waterwheel.

This can be related to an electric circuit as follows.

First, the pump corresponds to a battery (electromotive force).
The side into which water enters the pump can be regarded as the negative terminal, while the side on which the pumped water is stored can be regarded as the positive terminal.

As the pump moves water to a higher position, a difference in water level is created between the higher and lower points.
In an electric circuit, the quantity corresponding to this difference in water level is called the potential difference, which is also referred to as voltage.

The valve corresponds to a switch. Just as opening the valve allows water to flow, closing the switch allows electric current to flow.

In this way, just as the energy produced by the difference in water level causes the waterwheel to rotate, the energy associated with the potential difference (voltage) can be supplied to a load in an electric circuit.

Figure 1.4-2 Electric Circuit and Water Flow

【Note】
When a water channel becomes narrower, the flow of water may appear to change. However, under steady-flow conditions, the volume of water flowing per unit time remains unchanged.

The flow velocity increases in the narrower section, thereby maintaining the overall flow rate.
The same principle applies to an electric circuit. Even if the thickness of the conductor changes, the magnitude of the current remains constant along the path as long as it forms a single unbranched closed circuit.


This property is called continuity of electric current.


However, when a conductor becomes thinner, its resistance increases.
Consequently, the voltage drop across that section becomes greater, making it more susceptible to heat generation.

About This Article

Reference
・Fundamentals of Electricity, Volume I, Corona Publishing Co., Ltd., by Toshio Utsunomiya, Hiroshi Takahashi, and Isao Izumi
※This article was prepared with reference to the source listed above and organized based on the author’s understanding.