What is current and what is voltage?

Original title: What is the difference between voltage and current?

1. A few small concepts.

Before answering the difference between voltage and current, let’s talk about several physical quantities related to voltage and current. Try to be as easy to understand as possible here. The amount of charge. Although we can't see it or touch it, charge exists objectively. There are positive and negative charges. The amount of charge is called charge quantity, referred to as electric quantity, represented by Q. The basic unit is Coulomb, and the symbol is C. There are special instruments that can measure it, just like using a scale to measure the mass (weight) of an object. There is a special substance around the charge, called an electric field, which arises and disappears with the generation of the charge. An important property of the electric field is that the charge placed in the electric field is acted upon by a force. This force is called the electric field force. Under the action of the electric field force, the charge will move in a direction. We say that the electric field force does work on the charge, which is represented by W, and the basic unit is Joule (J).

2. The concept of voltage.

In the electric field, the work W (J) done by the electric field force to move the unit positive charge Q (C) from point a to point b is called the voltage Uab between two points ab, also called The mathematical expression for the potential difference between two points ab is: Uab=W(J)/Q(C). The basic unit of voltage is volts, referred to as volts, and is represented by V. This concept is not easy to understand for netizens. Examples can help to understand it better. Taking mobile phone charging as an example, assuming that the positive charge of 1 Coulomb (C) moves from the pole through the mobile phone to the - pole under the action of the charger's output electric field force, and the work done is 5 Joules (5J), then - pole The voltage between them is U=W(J)/Q(C)=5(J)/1(C)=5(V) ①Equation. If W=10J, Q=2C, then U=W(J)/Q(C)=10(J)/2(C)=5(V) ②Equation. From equations ①② we can see that U=W/Q=5/1=10/2 =5 (Ⅴ) That is, the W/Q ratio remains unchanged, which means that the output voltage of the charger has nothing to do with the amount of charge being moved or the charging time. , it only determines the charger itself, so voltage is a physical quantity that describes the work done by the electric field force.

In an actual circuit (such as a circuit board), the voltage between a certain point and the reference point (ground) is also called the potential of that point. Voltage is also the potential difference between any two points.

3. The concept of electric current.

Under the action of electric field force, the directional movement of charges forms a current. The direction of movement of positive charges is usually specified as the direction of current (free electrons in metal conductors are negatively charged and move in the opposite direction to the specified direction). But this is not the definition of current. It is defined as: The amount of electricity Q(C) passing through the cross-section of the conductor within unit time t(s) is called the current intensity passing through the conductor, referred to as current, represented by I, and the mathematical expression is I=Q(C)/t( s), the basic unit of current is ampere, abbreviated as ampere, symbol A. It is a physical quantity that describes how quickly charges pass through a conductor. Let's take the charger charging a mobile phone as an example. In the previous example, the amount of charge being moved was 1 coulomb, that is, Q=1C. So how long did it take? It took 1 second (1s) and 2 seconds (2s). Obviously The speed is different. Assume Q=1C, t=1s, then I=Q(C)/t(s)=1(C)/1(s)=1(A), if Q=2C, t=1s, then I= Q /t=2/1=2(A). If Q=1C, t=0.5s, then I=Q/t=1/0.5=2A.

From the above descriptions of 1 and 2, it can be seen that voltage and current are two physical quantities with completely different concepts. Voltage is the cause of the formation of current. In a closed loop, only voltage can form current. But what ties them together is the charge quantity Q, which is used in both defining mathematical expressions. Ohm's law I(A)=U(V)/R(Ω) describes the relationship between voltage, current, and resistance. I won’t go into details here.

In actual circuits, voltage and current analysis, measurement, calculation, etc. are relatively simple, convenient, and intuitive. So the charge power is rarely used.

4. Unit conversion.

Voltage: 1Ⅴ=1000mV, 1mV=1000uⅤ, 1kⅤ=1000Ⅴ.

Current: 1A=1000mA, 1mA=1000uA.

Electric energy (work): 1kWh=1000Wh=3.6×10^6(J), 1kWh=1 kilowatt hour

Personal point of view, for reference only, inappropriate points, criticism Correction.

Published on 2018.01.27 5:16