Basic physical quantities of circuits: current, voltage, potential, electromotive force, electrical power

Hello everyone, what I will talk about today is about learning physics, or learning basic knowledge of electronic technology, which can also be regarded as some common sense in life. Although it is simple knowledge, there are some things that need to be kept in mind. Whether you are preparing for exams or want to work in this industry in the future, this knowledge will be helpful to your problem solving and research and development.

(1) Current

The strength of the current is usually expressed by the amount of charge passing through the cross-section of the conductor per unit time, that is, the current intensity, or current for short. If Δq is used to represent the amount of electricity passing through the cross-section of the conductor within Δt time, the magnitude of the current can be expressed as:

If the magnitude and direction of the current do not change with time, it is called constant current. At this time, the magnitude of the current can be expressed as:

In the International System of Units, the unit of time is s (second), the unit of electricity is C (coulomb), and the unit of current is A (ampere).

In addition, the commonly used units for current are kA (kiloamps), mA (milliamps) and μA (microamps). The relationship between them is:

The physical quantity of current is an issue that needs to be considered when solving problems or designing circuits in the future. However, in the future, this formula will rarely be used to calculate current, and "Ohm's law" will be used more often.

Electric current not only has magnitude, but also direction. In electrical engineering, it is generally stipulated that the direction in which positive charges move is the direction of current. In metallic conductors, free electrons move in the opposite direction to the current.

The direction of electric current is also a headache for beginners. When I first learned Kirchhoff's and Thevenin's theorems, the direction of electric current kept me spinning. So readers please remember that the direction of current is from high potential to low potential, and the positive pole of the power supply flows to the negative pole. If you are unsure, you can assume the direction of the current. If the current value is negative, it means that the actual current direction is opposite to the assumed direction. If the current value is positive, it means that the actual direction is the same as the assumed direction.

(2) Voltage

Voltage, also known as potential difference, is a physical quantity that measures the strength of the electric field force. As shown in Figure 2-10, if the electric field force moves the positive charge q from point A to point B, the work done is WAB, then the ratio of the work to the charge q is called the voltage between the two points AB:

In the International System of Units, the unit of work is J (Joule), the unit of electricity is C (Coulomb), and the unit of voltage is V (Volt [Tes]). If the work done by the electric field force to move a 1C charge from point A to point B is 1J, then the voltage between two points AB is 1V.

Commonly used units for voltage include kV (kilovolts), mV (millivolts), etc. The relationship between them is:

(3) Potential

If B is taken as the reference point, the ratio of the work WA done by the electric field force to move the positive charge q from any point A to the reference point B and the amount of charge q is called the potential of point A to reference point B, and is recorded as:

The unit of electric potential is also V (volt [ter]). The potential difference between any two points in the circuit is the voltage between the two points, that is:

Potential is a physical quantity that represents the potential of a certain point in a circuit, and it is relative to a reference point. The potential of the reference point is usually stipulated as zero potential. The selection of the reference point is arbitrary in principle, but when actually studying problems, infinity or the earth is generally chosen as the zero potential reference point.

It should be noted that in the same circuit, when different reference points are selected, the potential of the same point is different. But once the reference point is determined, the potential of each point is also determined. When the potential is a positive value, it means that its potential is higher than the reference point potential; when the potential is a negative value, it means that its potential is lower than the reference point potential.

(4) Electromotive force

In the power supply, the work done by the non-electric field force to move the unit positive charge from the negative pole of the power supply to the positive pole of the power supply through the inside of the power supply is called electromotive force. For DC electromotive force, it is expressed as:

The unit of electromotive force is also V.

The electromotive force also has a direction. It is generally stipulated that the electromotive force points from the negative pole of the power supply to the positive pole inside the power supply.

For a power supply, there is both electromotive force and terminal voltage. The electromotive force only exists inside the power supply, and the terminal voltage exists at both ends of the external circuit when the power supply is applied, from the positive pole of the power supply to the negative pole. When the external circuit is open, the terminal voltage is numerically equal to the electromotive force.

Electromotive force and voltage are two different concepts, but they can both be used to express the potential difference between the positive and negative poles of the power supply. In the circuit shown in Figure 2-15, the reference directions of E and UAB are exactly opposite.

The figure below shows two commonly used representation methods of electromotive force.

During analysis and calculation, the reference direction is often selected for the electromotive force. When the true direction of the electromotive force is the same as the reference direction, the electromotive force is positive; when the true direction of the electromotive force is opposite to the reference direction, the electromotive force is negative. as the picture shows.

(5) Electric power

In the figure, one of the functions of the circuit where point charges move in the electric field is to transmit electrical energy. Calculations related to electrical power are often used when analyzing circuits.

In the International System of Units, the unit of voltage is V; the unit of current is A; and the unit of electrical power is W (watt [ter], referred to as watt). Commonly used units include kW (kilowatt) and mW (milliwatt). The relationship between them is:

In order to facilitate the discrimination, associated and non-associated reference directions are introduced. As shown in Figure 2-20(a), the voltage U across the selection element is consistent with the reference direction of the current I flowing through the element, which is called the associated reference direction. As shown in Figure 2-20(b), the voltage U across the selection element is opposite to the reference direction of the current I flowing through the element, which is called the non-correlated reference direction.

(6) Summary

a. The difference between voltage and potential

Connection: The units are all V, and voltage is equal to potential difference.

The difference: the potential is relative, and its size is related to the selection of the reference point; the voltage is an absolute quantity, and its size has nothing to do with the reference point.

b. The difference between electromotive force and voltage

The meaning is different. Electromotive force is a physical quantity that measures the power of a power source, while voltage is a physical quantity that measures the power of an electric field.

The location of existence is different. The electromotive force only exists inside the power supply, and the voltage exists not only inside the power supply but also outside the power supply.

The direction is different. The direction of electromotive force is from negative pole to positive pole, while the direction of voltage is from positive pole to negative pole.