Basic laws and concepts in electronics: part 2 – In this section, we will discuss the following issues:
Electric Current, Electric Voltage, Electric Power, Resistance.
Ohm’s Law, Kirchhoff’s Laws.
Electric charge – there are two types of electrical charges: positively (protons) and negatively charged (neutrons). Like charges repel and unlike ones are attracted to each other. The unit of electric charge is called coulomb [C]. The orderly movement of electrical charges is called the electric current. Virtually every material object is able to adopt a certain electrical charge, becoming positively charged (excess amount of protons) or negatively (excess amount of electrons).
Electric current – directed and orderly movement of electrical charges. 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 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 source to the receiver,
in the second half of period T: from the receiver to the source.
3. Electric Voltage – defined as the difference between the electrical potential between two points of 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 (voltage), the greater the force. The voltage, similarly to electric current may be direct or alternating. It is important to distinguish between the voltage on the 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 of voltage source, then power is positive and the source is doing the work. The opposite: the power will be negative and the source will „absorb” the job (accumulate energy). Electronic devices lose power in quantities that depend on their systems and their applications. This phenomenon may be desired e.g. in case of heaters, and harmful in amplifiers or integrated circuits – systems, that require additional cooling solutions.
5. Resistance – is a measurement of how difficult it is to flow electric current across e.g. a particular element or system resulting from its physical properties. It is marked with “R” and its unit is ohm [Ω]. The conductance “G” in Simens unit [S] is the inverse of the resistance.
6. Ohm’s Law (for DC) – the proportionality of the current „I” in the circuit component and the voltage „V” between its terminals is given by:
After a simple mathematical transformation we get two more equations:
The illustration that helped me in learning Ohm’s Law, I’m sure it will be helpful for you too:
Materials or components complying with Ohm’s Law are called linear.
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 in 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. Otherwise: across elements connected in parallel the same voltage is present.