Electrical Resistance and Its Measurement

Electrical resistance of an electrical component is the property of opposition to the passage of an electric current through that. It is a measure of the degree to which an object opposes an electric current through it. An object of uniform cross section has a resistance proportional to the resistivity and length and inversely proportional to its cross-sectional area. The voltage across the component drives an electric current through it and energy is used up which appears as heat. When a current passes through a conductor there is some resistance generated in the conductor to the passage of current. The electrical resistance is the reason that the conductor gives out heat when current passes through it. All materials have resistance. Resistance is measured in ohms. The instrument that measures resistance is called as an ohmmeter.

Resistors
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When a voltage is applied to a conductor, the voltage drop or voltage difference between one side of the conductor and the other, is causing the current to flow through it. The resistance of a conductor, wire, or an element is determined by 2 factors, the geometry and the material.

Types of Resistors
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Geometry

The resistance increases when the conductor is longer and it also increases when the area of the conductor is smaller. A long and thin conductor has a higher resistance than a short and thick conductor.

Material

The electrons and current can freely and easily flow through a copper wire than a steel wire of the same size and shape. The current cannot flow through an insulator with the same size or shape.

If the voltage is too high in an conductor or a component, very high current will pass through that and make it hot, or even sometimes burnout and explode. Components are designed to have a specific resistance so that they can dissipate electrical energy and modify how the circuit behaves, and these are called as resistors. The resistors are components made up of a wide range of materials depending on factors such as desired resistance, amount of energy needed to dissipate heat, precision, and cost.

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​Ohm’s Law

​Ohm’s law is an empirical law relating the voltage “V” across an element to the current “I” through it. V is directly proportional to I. The law is true in the case of wires and resistors. 

Ohm's law gives a relation between the current, voltage, and resistance. It states that the current measured in amperes flowing in any portion of an electrical circuit is equal to the applied voltage in volts divided by the resistance in ohms. When two values are known, the third value can be determined from the formula below.

I= V/R

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To determine the voltage when other 2 values are being known, the above mentioned formula can be rewritten by cross multiplying.

V=IR

When resistance is to be found out when the other 2 values are given, the formula will be

R=V/I

​Example:

When a 10 volt battery is connected to a 5 ohm resister in series, the current that flows through it is determined by

I = 10/5 = 2 A

Resistances in Series and in Parallel

In a circuit, resistances can be connected series or in parallel. Resistances in series or in parallel can occur in a circuit. It is essential to know the effective resistances when all the resistances are connected in series or all the resistances connected in parallel. When connecting resistances in series, the resistances will distribute away in series. So adding individual resistances in series will help to measure the effective resistance in the series circuit.

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When resistances R1, R2, R3, or RN are connected in series fashion in a circuit, the effective resistance of the circuit will be

R = R1 + R2 + R3 + RN

Example:

The resistances 5 ohm, 8 ohm, and 2 ohm are connected in series. So the effective resistance will be

R = 5 + 8 + 2 = 15 ohms.

When resistances R1, R2, R3, and RN are connected in parallel fashion, the effective resistance is measured by

1/R = 1/R1 + 1/R2 + 1/R3 + 1/RN

Example:

The resistances 5 ohms, 10 ohms, and 30 ohms are connected in parallel. So the effective resistances will be

1/R = 1/5 + 1/10 + 1/30

1/R = 6+3+1/30

R = 3 ohms

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Electrical Conductors and Insulators

All materials are made up of atoms and there are millions of them which form the substance. An atom consists of 3 basic particles the protons, neutrons, and the electrons. The center of an atom is a dense area called the nucleus. The nucleus contains the positively charged protons and electrically neutral neutrons that contains most mass of the atom. The electrons are the negatively charged particles that surrounds the nucleus as a cloud. The electrons are bound to the nucleus by an electromagnetic force. A group of atoms collectively bound together are called as a molecule. An atom that contains an equal number of protons and electrons are electrically neutral. If the atom contains more number of electrons then it will have a negative charge, and if the atoms contains less number number of electrons it will exhibit a positive charge. A positively or negatively charged atom is known as an ion. When the atom loses one of its electrons and becomes slightly positive, it is called as a positive ion. When the atom gains an electron by picking up from the neighboring atom it becomes slightly negative, and it is called as a negative ion.

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The electrons of different types of atoms possess varying degrees of freedom to move around their nucleus. Some materials have their outer electrons that are loosely bound to their nucleus and are influenced by heat or electrical voltage. These materials give up electrons and allow them to move about when electrically attracted. Those unbound electrons that moves freely and leave the atom to the adjacent one are called as fee electrons. When an electrical voltage is applied to those materials, the free electrons under the action of an electrical field move from a lower potential to the higher potential. Those materials that allows the free flow of electrons when an electrical voltage is applied are called as conductors.

Some examples of conductors are copper, silver, aluminum, gold, brass, bronze, iron, steel, mercury, graphite, concrete etc.

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​Certain materials do not allow the transfer of electrons from one atom to another as they are firmly bound to their nucleus. Electrical voltage does not produce any current to pass through those materials. Those materials that consist of atoms which does not allow the flow of electrons when applying electrical voltage across it are called as insulators. These materials offers high resistance to the flow of current through it.

Some examples of insulators are glass, rubber, asphalt, porcelain, fiberglass, Teflon, paper, wood, cotton, quartz, diamond, air, oil, etc.

All conductors do not have the same level of conductivity and all insulators do not have the same level of resistance to the movement of electrons. Materials such as silver are better conductors and allows the passage of electrons than graphite or concrete.

Some materials show changes in electrical properties under different conditions. Materials such as glass or air are very good insulators at room temperature but becomes a conductor when heated to high temperatures. Metals are good conductors when at room temperature but become less conductive when heated. Most metals become extremely conductive when cooled under extremely low temperatures.

Conductance is the ability of a material to conduct electricity. Conductance is inverse of resistance. Conductance  = 1/ resistance. The SI unit of conductance is siemens.

The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element. The unit used to measure the resistance of a conductor to the flow of current is called as ohm.

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