Basic laws and concepts in electronics: part 2 – In this section, we will discuss the following concepts:
Electric Current, Electric Voltage, Electric Power, Resistance.
Ohm’s Law, Kirchhoff’s Laws.
Electric charge – there are two different types of electric charges: positive and negative. Particles that carry the same type of charge repel each other, but the opposite ones are attracted to each other. The same phenomenon can be observed in magnets. The unit of electric charge is called coulomb [C]. Virtually every material object is able to adopt a certain electric charge, becoming positively or negatively charged.
Electric current – orderly movement of many electrical charges in the same direction through the conductor. However, there must be external electric voltage applied to that conductor to “push” these charges in desired direction. Current intensity is expressed in amperes [A], which means the rate of charge flow from point to point and is indicated by the letter “I”. We distinguish between two types of electric current:
Direct Current (DC) – is characterized by the same arrow and direction of flow of electric charge in the circuit (flows always from the power source to the receiver),
Alternating Current (AC) – the instantaneous current values change in a repetitive, periodic (amplitude V0) with a specified frequency “f”. It can be transmitted, e.g. by a sine wave, rectangular wave etc. (more in the article about signals). However, it is required for this function to be periodically variable. This current flows bi-directionally (unlike DC):
in the first half of period T: from power source to the receiver,
in the second half of period T: from the receiver to the power source.
3. Electric Voltage – defined as the difference of the electrical potential between two points in the circuit, measured in volts [V] with “V” or “U” symbol. Simply put: it is the force with which the carriers of the electrical charge “want to get closer to each other”, so the higher the tension (electric voltage), the greater the force. It is important to distinguish between the voltage at the electromotive force (EMF: battery, power supply) terminals and the voltage drops across components in electrical circuits.
4. Electric Power – defined as the amount of work done, collected or given per unit of time. It is expressed in watts [W] with “P” symbol. It can be calculated mathematically as the product of voltage and current:
If the direction of current is the same as the direction of the voltage supplied by the voltage source, then power is positive and the source is “doing the work”. In opposite scenario, the power will be negative and the source will „absorb the work” (accumulate energy). Electronic devices lose power in quantities that depend on their systems and their applications. This phenomenon may be smart used e.g. in heaters, but is deadly to amplifiers or integrated circuits – systems, that require additional cooling to prevent overheating.
5. Resistance – is a measurement of how difficult it is to flow electric current “through” e.g. a particular electronic component or system, what is the result of its physical properties. It has its own symbol “R” and its unit is called ohm [Ω]. The conductance “G” in Simens unit [S] is the “inverted” resistance.
6. Ohm’s Law (for Direct Current) – the proportionality of the current „I” in the component of the circuit and the voltage „V” between its terminals is given by:
After a simple mathematical transformation we obtain two more equations:
This illustration has helped me in learning Ohm’s Law during my studies, I hope that it will be hepful for you too:
Ohm’s Law only applies to linear electronic circuits.
7. 1st Kirchhoff’s Law (current related) – the sum of the currents „flowing in” to a given node of a circuit equals the sum of currents „flowing out” of it. This is illustrated by the following example:
8. 2nd Kirchoff’s Law (voltage related) – the sum of voltage drops and electromotive forces across components connected in a closed circuit equals zero. So, in other words: the same value of the electric voltage is present across all of the components connected in series in this circuit.