Resistors are devices used to control the passage of electric current in electric circuits through the joule effect, which converts electricity in Thermal energy.
Most resistors are made of high-quality materials. electrical resistance, and these are known as dielectrics. Those with a constant electrical resistance are known as ohmic resistors.
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What are resistors?
They are electronic components that resist at the passage of chainelectric. When we insert a resistor into an electrical circuit, a decrease in electric current intensity, moreover, its presence along a wire entails reduction or fall of electric potential.
Some resistors can keep their resistanceelectricconstant, even over a wide range of electrical voltages, they are known as ohmic resistors.
1st law of Ohm
According to the 1st Ohm's law, a reason between the potentialelectric and the
chainelectric that forms in an ohmic resistor is everconstant. In ohmic resistors, the electrical current is directly proportional to the applied electrical voltage, and inversely proportional to the electrical resistance, as shown in this figure, which brings the formula of the 1st law from Ohm:
R – electrical resistance (Ω)
U – electrical potential difference (V)
i – electric current (A)
You resistorsreal they are not ohmic for any voltage or current measurements, however, they have a constant electrical resistance for a wide range of these measurements, as shown in the graph:
2nd law of Ohm
The ability of resistors to control the flow of electrical current is related to their resistance. It, in turn, depends on geometric factors, such as the length and the areacross of the resistor, and also of a characteristic quantity of each material known as resistivity.
In other words, the resistance modulus of an ohmic resistor does not depend on the electrical potential applied to its terminals, but on its shape and the material used in its manufacture. Understand the subject better by accessing our text: 2nd law of Ohm.
The formula we use for calculate electrical resistance, as a function of geometric parameters such as the cross-sectional area and length of the resistor, known as the 2nd ohm's law, is this:
R – resistance (Ω)
ρ – resistivity (Ω.m)
L – length (m)
THE – cross-sectional area (m²)
Resistivity
The resistivity (ρ) is a physical quantity that depends on microscopic factors and is related to each type of material. The resistivity of materials conductors, like silver or copper, is very low since they offer little resistance to the passage of electrical current. Other materials, such as rubber, glass and plastic, have very high resistivity measures.
Types of resistors
They can be different according to the material they were produced, in addition, there are resistors that change resistance when subjected to different external agents.
Some of them are sensitive to temperature variations and known as thermoresistors. Besides them, there are those that respond to variations in luminosity, known as photoresistors. There are also resistors that change resistance when subjected to magnetic fields, they are the magnetoresistors.
Color code of resistors
This code is used for visually represent the electrical resistance of the resistors, as shown in the figure:
K – 10³
M – 106
According to the color code, the twofirsttracks indicate the twodigitsinitials of resistance, while the thirdrange indicates the multiple (1, 10, 1000) that we should multiply by the first two digits. THE lastrange indicates the purity or the degree of tolerance, in percentage, that the resistance measurement may deviate from the theoretical value assigned to that resistor.
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Resistor association
It's about the ways how resistors can be connected to each other inside an electrical circuit. There are three types of association: serial association, parallel association, and mixed association, which contains resistors connected both in series and in parallel.
serial association
In it the resistors are connected sequentially. In this configuration, the chainelectric suffers a reduction in its intensity, however, the electrical current that runs through each of the resistors is equal. Below, we show the formula used to calculate the equivalent strength of the series association:
parallel association
When connected in parallel, the resistors are subjected to the same electrical potential. Furthermore, the electrical current that passes through each of them varies according to their resistance. In this type of association, the resistanceequivalent, calculated by the next formula, will always be less than the smallest resistance.
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When there are only two resistors associated in parallel, it will be possible to calculate the equivalent resistance using this formula:
If you want to go deeper into the relationships of resistors, read our text: Resistor Associations.
Resistor exercises
Question 1) (UFPA) In the Amazon River, an inexperienced fisherman tries to catch a poraque holding the fish's head with one hand and the tail with the other. The why is an electric fish, capable of generating, between the head and the tail, a potential difference of up to 1500 V. For this potential difference, the electrical resistance of the human body, measured between the two hands, is approximately 1000 Ω. In general, 500 mA of direct current passing through a person's chest is sufficient to cause ventricular fibrillation and death from cardiorespiratory arrest. Using the values mentioned, we calculate that the current passing through the fisherman's chest, in relation to the current sufficient to cause ventricular fibrillation, is:
a) one third
b) half
c) equal
d) double
e) the triple
Template: Letter e
Resolution:
Based on Ohm's first law, we will calculate the electric current, note:
So we find that the electric current has an intensity of 1500 mA, which is three times greater than the current of 500 mA, so the correct alternative is the letter e.
Question 2) (Enem PPL) Electric shock is a sensation caused by the passage of electric current through the body. The consequences of a shock range from a simple scare to death. The circulation of electrical charges depends on the material's resistance. For the human body, this resistance ranges from 1000 Ω when the skin is wet, to 100,000 Ω when the skin is dry. A barefoot person, washing his house with water, got his feet wet and accidentally stepped on a bare wire, suffering an electrical discharge at a voltage of 120 V.
What is the maximum intensity of electrical current that passes through the person's body?
a) 1.2 mA
b) 120 mA
c) 8.3 mA
d) 833 A
e) 120 kA
Template: Letter B
Resolution:
To solve the exercise, we will make use of Ohm's first law, as we did in the previous exercise, in order to find the intensity of the electric current:
In the calculation we made the division between the electrical voltage and the electrical resistance, finally, we rewritten the value obtained according to the rules of cientific notation and also according to the prefixes of SI. By doing this we find that the correct alternative is letter b.
Question 3) (EEar) Knowing that the potential difference between a cloud and the Earth, for a lightning strike to happen, is around 3.108 V and that the electric current produced in this case is approximately 1.105 A, what is the average air resistance, in ohms (Ω)?
a) 1,000
b) 2000
c) 3,000
d) 4,000
Template: Letter C
Resolution:
Using the formula of the first Ohm's law, it is possible to obtain the value of the electrical resistance by dividing the voltage by the electrical current:
The calculation indicates that the correct alternative is the letter c.
By Rafael Hellerbrock
Physics teacher
