Why is resistance used in circuits

Voltage and current are the first two, and the third is resistance. As electrical resistance is such a basic concept in electrical and electronic circuits it is necessary to answer some questions: what is resistance, what are resistors, and how resistance affects circuits. Before looking at what resistance is, it is necessary to understand a little about current and what it is. Essentially a flow of current in a material consists of a movement of electrons in one direction.

In many materials there are free electrons moving about randomly within the structure. While these move randomly there is no current flow, because the number moving in one direction will be equal to the number moving in the other. Only when a potential causes a drift in a particular direction can a current be said to flow. Resistance is the hindrance to the flow of electrons in material.

While a potential difference across the conductor encourages the flow of electrons, resistance discourages it. The rate at which charge flows between two terminals is a combination of these two factors.

If two different conductors are placed in a circuit, then the amount of current found to flow in each may not be the same. There are a number of reasons for this:. In most cases conductors are required to carry current with as little resistance as possible. As a result copper is widely used because current flows easily within its structure. Also its cross sectional area is made wide enough to carry the current without any undue resistance.

In some instances it is necessary to have elements which resist the flow of current. These items are called resistors and they are made out of materials which do not conduct electricity as well as materials like copper or other metals. The concept of resistance is not always easy to understand because it is not possible to visibly see the quantities involved: voltage, current and resistance itself are all rather invisible quantities to the naked eye, although they can be detected and measured in a variety of ways.

Basically the function of a resistor is always to oppose the flow of current through it and the strength of this opposition is termed as its resistance.

German physicist, Sir G. Ohms was able to discover a definite relationship between voltage, current and resistance.

According to him a potential difference or a voltage V across a resistor R is proportional to the instantaneous current I flowing through it and is given as:. Transistor Biasing: Through one of my previous articles you must have acquired good knowledge regarding transistors. But the base of a transistor is quite vulnerable to high currents, so a resistor is incorporated to limit the current and provide a safe biasing voltage.

The value of the base resistor of a transistor may be calculated through the below-given formula:. A resistor when placed in series with the LEDs regulates a proper flow of current through them. To calculate the value of a series LED resistor, the following formula may be used:. In Timing Circuits: The timing components used in timer and oscillator circuits always incorporate a resistor and a capacitor.

Here the time taken to charge or discharge a capacitor constitutes the basic time pulse or trigger for the circuit. A resistor is effectively used to control this charging and discharging process and its value is varied to obtain different time intervals. Surge Protection: The initial switch ON of a power supply may at times inflict a dangerous voltage surge into an electronic circuit, damaging its critical components. A resistor when introduced in series with the supply terminals of the circuit helps in checking the sudden rise in voltage and averting possible harm.

These resistors are generally of low values so that the overall performance of the circuit is not affected. The above basic examples must have provided you sufficient knowledge regarding the use of resistors in electronic circuits and helped you to understand what is the function of a resistor.

The role of the resistor in an electrical circuit: NOTE: Diagrams, links and practice questions in this document are still to be added: In direct current DC or alternating current AC electrical circuits that have a resistor, as its name implies, resists the flow of electrons. It is one of the most basic of electrical components. It can be used to reduce the available voltage or current present in a circuit. While there are differences in how a resistor affects the two different types of current sources DC or AC , depending on how the resistor is constructed and the AC frequency involved, it can be assumed that the following applies equally to both.

For AC circuits it may be necessary to indicate how the voltage is represented, the average value, the peak value or the root mean square RMS. As discussed in the Ohm's Law section, in an electrical circuit, voltage measured in volts and denoted by the letter V equals current measured in amps and denoted by the letter I multiplied by the resistance measured in ohms and denoted by the letter R present in the circuit.

This is represented by the following formula. An electrical circuit may include many resistors. Resistors may be arranged in series or in parallel with the voltage supply source. Refer to the example below. Figure 1 represents an electrical circuit with two resistors in a series arrangement. For current to complete the electrical circuit it must flow from the voltage source B1 and pass through both resistor 1 R1 and resistor 2 R2 and then back to B1.