One of the most fundamental terms in electrical engineering is the term “electrical voltage”. In this article, we will explain what it is and how to calculate it.
Definition and Meaning
Voltage U is the very cause that “makes” electric current I flow. Voltage always occurs when the charges are separated from each other, that is, all the negative charges are on one side and all the positive charges are on the other side. If you connect these two sides with an electrically conductive material, electric current will flow.
The generally accepted definition of the term “electrical voltage”.
Electrical voltage (or simply voltage) is the potential difference between two points in an electric field. It is the driving force for an electric charge.
A potential in an electric field is the energy of a charged body, independent of its electric charge. For clarification, you can look at the comparison to the water circuit just below in the article.
There is another definition (from a physics book):
Voltage is a physical quantity characterizing an electric field. The electric voltage between two points in an electric field is numerically equal to the work done by the transfer of 1 C of charge between them by the electric field forces.
Note: 1 C = 1 Coulomb.
Comparison using the water flow model.
A good analogy to help you imagine electrical voltage and potential is a water circuit. In this diagram you have two pools at different heights, which are connected by a pipe. In this pipe, water can flow from the upper pool to the lower pool. The water is then pumped back to the upper pool with a pump, as shown in the picture below.
In your thinking, you can now easily compare a pump to a source of electric voltage. You can also compare the flow of water to an electric current. The pump transports the water from the lower pool to the upper pool. From there, it flows back into the lower pool on its own. In this example, the pump is the driver for the flow. The greater the difference in height, the greater the flow. The deciding factor is the potential energy of the upper pool. You can compare the difference in energy of the two pools to the difference in electrical potential. Simply put, a larger height difference corresponds to a larger electrical voltage.
In Europe and most parts of the world, 230 V outlets are available. However, in the United States, Japan, and some South American countries, 110 V is the standard. These differences (as well as different plug standards) cause problems especially for tourists.
The formula for the electrical voltage U, according to Ohm’s law for a circuit section, is:
U = R * I.
As you can see from this formula, if the electrical voltage remains unchanged, the greater the electrical resistance (R), the lower the electric current (I).
Another formula for calculating the electrical voltage is as follows:
U = P / I .
That is, the electrical voltage U equals the electric power P divided by the electric current I.
What is Voltage Measured In (Voltage Units)?
The SI unit for voltage is the Volt, abbreviated V (after the Italian physicist Alessandro Volta).
1 volt (1 V) is the voltage between two points in an electric field that transfers 1 C of charge between them, and work is done by 1 J.
Now you can explain the meaning of the inscription 4.5 V or 9 V on a round or flat battery. The meaning is that when a charge of 1 Coulomb is transferred from one pole of the source to another (through the coil of a light bulb or other conductor) by the electric field forces, the work of 4.5 J or 9 J can be done, respectively.
In electrical engineering, voltages can range from microvolts (1 µV = 1 * 10-6 V) and milivolts (1 mV = 10-3 V), to kilovolts (1 kV = 1 * 103 V) and megavolts (1 MV = 106 V).
You can convert individual measurement units as follows:
1 V = 1000 mV, 1 mV = 1000 µV, 1 MV = 1000 kV, 1 kV = 1000 V.
Electrical Voltage in the Circuit
One of the following symbols is usually used for voltage sources in circuits.
A voltage source always has two connections/poles. A plus pole and a minus pole. The voltage itself is indicated by the voltage arrow (UQ). For sources it is always displayed from plus to minus.
The electrical voltage falling across the resistor can also be indicated by a voltage arrow (indicated in the diagram as red arrow UR ). This indicates the technical direction of the electric current.
You may also often hear the term “idle voltage” or “source voltage”. This is the output voltage of an unloaded source, i.e., a source to which nothing is connected. If the circuit is closed with a load, you can only measure the voltage across the poles of the source.
Electrical Voltage in Series and Parallel Circuits
We already have an article on series and parallel connection of conductors, in which we discuss this topic in more detail. So here we will only look at some of the basics.
When connected in series, the components are connected in a row.
Here the electrical voltage of the source is divided by the resistors. This point is also described by the second Kirchhoff law. The following applies here:
UQ = U1 + U2 + U3
that is, the source voltage is equal to the sum of the electrical voltages on the individual resistors. The source voltage is distributed differently across the different resistors.
In an electrical circuit with a parallel connection the components are respectively arranged in parallel with respect to each other. This can be seen in the following diagram.
Here it is much easier to determine the electrical voltages on the resistors, as in parallel connection:
UQ = U1 = U2 = U3
Therefore, the electrical voltage on the resistors is as high as the electrical voltage of the source.
Measuring Electrical Voltage
Voltage measuring instruments, also called voltmeters, are always connected in parallel to the consumer where the electrical voltage is to be measured.
One of the most commonly used voltmeters is the digital multimeter (DMM), so we will show you the procedure for measuring voltage with a DMM. First you need to set the type of electrical voltage (DC – direct current or AC – alternating current).
For DC you have to pay attention to the correct polarity, i.e. connect the plus to the positive pole. The next step is to select the correct measuring range. If you can’t estimate how big the measurement value is, set the highest possible range and move down from there until you find the right one. Finally, you only need to “read” the electrical voltage with the instrument.
Examples of Typical Electrical Voltage Values
For some applications, the appropriate electrical voltage can be found in the table below.
|LED||1,2 – 1,5 V|
|USB charger||5 V|
|Car battery voltage||12, 4 – 12,8 V|
|Socket voltage (rms or effective value)||110 V (USA), 230 V (Europe)|
|High Voltage Power Lines (HVL)||60 kV – 1 MV|
You can see that there are voltages of up to megavolts on high voltage lines. Such large electrical voltages are used in order to reduce losses in long lines.
The deciding factor for the consumer is the electric power P, which can be calculated for a constant voltage using a formula:
P = U * I
This means that the electric current I is just as important to the consumer as the electric voltage. According to Ohm’s law, the relationship between current and voltage is as follows:
U = R * I .
If the voltage remains unchanged, the resistance determines the magnitude of the current. To illustrate this, imagine the following. You have three different pools which are filled with the same amount of water. Each pool has a drain that varies in cross-section, i.e. one pool has a very small drain pipe and the other has a very large drain pipe.
A constant electrical voltage can be determined by making sure that all the pools are filled to the same height. If the drain is narrow at the bottom, it represents a large resistance. Current can only flow slowly here. If the cross-section of the drain pipe is larger, there is less resistance and therefore more current can flow.