Category: 1. Current, Voltage, Resistance and Induction

The property of an inductor to get the voltage induced by the change of current flow, is defined as Inductance. Inductance is the ratio of voltage to the rate of change of current.

The rate of change of current produces change in the magnetic field, which induces an EMF in opposite direction to the voltage source. This property of induction of EMF is called as the Inductance.The formula for inductance is

Inductance = voltage / rate of change of current

Units −

• The unit of Inductance is Henry. It is indicated by L.
• The inductors are mostly available in mH milliHenrymilliHenry and μH microHenrymicroHenry.

A coil is said to have an inductance of one Henry when an EMF of one volt is self-induced in the coil where the current flowing changed at a rate of one ampere per second.

Self-Inductance

If a coil is considered in which some current flows, it has some magnetic field, perpendicular to the current flow. When this current keeps on varying, the magnetic field also changes and this changing magnetic field, induces an EMF, opposite to the source voltage. This opposing EMF produced is the self-induced voltage and this method is called as self-inductance.

The current i_s in the figure indicate the source current while i_ind indicates the induced current. The flux represents the magnetic flux created around the coil. With the application of voltage, the current i_s flows and flux gets created. When the current i_s varies, the flux gets varied producing i_ind.

This induced EMF across the coil is proportional to the rate of change in current. The higher the rate of change in current the higher the value of EMF induced.

We can write the above equation as

E α dI / dt

E = L dI / dt

Where,

• E is the EMF produced
• dI/dt indicates the rate of change of current
• L indicates the co-efficient of inductance.

Self-inductance or Co-efficient of Self-inductance can be termed as

L = E / dI /dt

The actual equation is written as

E = −L dI / dt

The minus in the above equation indicates that the EMF is induced in opposite direction to the voltage source according to Lenz’s law.

Mutual Inductance

As the current carrying coil produces some magnetic field around it, if another coil is brought near this coil, such that it is in the magnetic flux region of the primary, then the varying magnetic flux induces an EMF in the second coil. If this first coil is called as Primary coil, the second one can be called as a Secondary coil.

When the EMF is induced in the secondary coil due to the varying magnetic field of the primary coil, then such phenomenon is called as the Mutual Inductance.

The current i_s in the figure indicate the source current while i_ind indicates the induced current. The flux represents the magnetic flux created around the coil. This spreads to the secondary coil also.

With the application of voltage, the current i_s flows and flux gets created. When the current i_s varies, the flux gets varied producing i_ind in the secondary coil, due to the Mutual inductance property.

The change took place like this.

Vp Ip → B → Vs Is

Where,

• V_p i_p Indicate the Voltage and current in Primary coil respectively
• B Indicates Magnetic flux
• V_s i_s Indicate the Voltage and current in Secondary coil respectively

Mutual inductance M of the two circuits describes the amount of the voltage in the secondary induced by the changes in the current of the primary.

V(Secondary) = − M ΔI / Δt

Where ΔI / Δt the rate of change of current with time and M is the co-efficient of Mutual inductance. The minus sign indicates the direction of current being opposite to the source.

Units − = Henry(H)

Depending upon the number of turns of the primary and the secondary coils, the magnetic flux linkage and the amount of induced EMF varies. The number of turns in primary is denoted by N1 and secondary by N2. The co-efficient of coupling is the term that specifies the mutual inductance of the two coils.

Factors affecting Inductance

There are a few factors that affect the performance of an inductor. The major ones are discussed below.

Length of the coil

The length of the inductor coil is inversely proportional to the inductance of the coil. If the length of the coil is more, the inductance offered by that inductor gets less and vice versa.

Cross sectional area of the coil

The cross sectional area of the coil is directly proportional to the inductance of the coil. The higher the area of the coil, the higher the inductance will be.

Number of turns

With the number of turns, the coil affects the inductance directly. The value of inductance gets square to the number of turns the coil has. Hence the higher the number of turns, square of it will be the value of inductance of the coil.

Permeability of the core

The permeability μμ of the core material of inductor indicates the support the core provides for the formation of a magnetic field within itself. The higher the permeability of the core material, the higher will be the inductance.

Applications of Inductors

There are many applications of Inductors, such as −

• Inductors are used in filter circuits to sense high-frequency components and suppress noise signals
• To isolate the circuit from unwanted HF signals.
• Inductors are used in electrical circuits to form a transformer and isolate the circuits from spikes.
• Inductors are also used in motors.

Current is the aggregate flow of electrons through a wire and can be defined as the rate of electron flow. Resistance is defined as an opposition to current flow. An electrical circuit must have resistance in it in order to change electrical energy to light, heat or movement.

• Resistance is known that the directed movement of electrons constitutes a current flow.
• It is also known that the electrons do not move freely through a conductor’s crystalline structure.
• Resistance is an inherent opposition to the flow of electrons present in conductors.
• Inductance is the property of any circuit to oppose any change in current, whereas capacitance is the property of a circuit to oppose any change in voltage.

What is resistance?

Resistance is a measure of the opposition to current flow in an electrical circuit.

Resistance is measured in ohms, symbolized by the Greek letter omega (Ω). Ohms are named after Georg Simon Ohm (1784-1854), a German physicist who studied the relationship between voltage, current and resistance. He is credited for formulating Ohm’s Law.

All materials resist current flow to some degree. They fall into one of two broad categories:

• Conductors: Materials that offer very little resistance where electrons can move easily. Examples: silver, copper, gold and aluminum.
• Insulators: Materials that present high resistance and restrict the flow of electrons. Examples: Rubber, paper, glass, wood and plastic.

Resistance measurements are normally taken to indicate the condition of a component or a circuit.

• The higher the resistance, the lower the current flow. If abnormally high, one possible cause (among many) could be damaged conductors due to burning or corrosion. All conductors give off some degree of heat, so overheating is an issue often associated with resistance.
• The lower the resistance, the higher the current flow. Possible causes: insulators damaged by moisture or overheating.

Many components, such as heating elements and resistors, have a fixed-resistance value. These values are often printed on the components’ nameplates or in manuals for reference.

When a tolerance is indicated, the measured resistance value should be within the specified resistance range. Any significant change in a fixed-resistance value usually indicates a problem.

“Resistance” may sound negative, but in electricity it can be used beneficially.

Examples: Current must struggle to flow through the small coils of a toaster, enough to generate heat that browns bread. Old-style incandescent light bulbs force current to flow through filaments so thin that light is generated.

Resistance cannot be measured in an operating circuit. Accordingly, troubleshooting technicians often determine resistance by taking voltage and current measurements and applying Ohm’s Law:

E = I x R

That is, volts = amps x ohms. R stands for resistance in this formula. If resistance is unknown, the formula can be converted to R = E/I (ohms = volts divided by amps).

Examples: In an electric heater circuit, as portrayed in the two illustrations below, resistance is determined by measuring circuit voltage and current, then applying Ohm’s Law.

In the first example, total normal circuit resistance, a known reference value, is 60 Ω (240 ÷ 4 = 60 Ω). The 60 Ω resistance can help determine the condition of a circuit.

In the second example, if circuit current is 3 amps instead of 4, circuit resistance has increased from 60 Ω to 80 Ω (240 ÷ 3 = 80 Ω). The 20 Ω gain in total resistance could be caused by a loose or dirty connection or an open-coil section. Open-coil sections increase the total circuit resistance, which decreased current.

Reference: Digital Multimeter Principles by Glen A. Mazur, American Technical Publishers.

As per voltage definition, it is the difference in the electric potential between two points. It is the work done in moving a charge from one pole to another through a wire.
To determine the voltage between any two points, both a static electric field and a dynamic electromagnetic field is considered. 

The mathematical representation of voltage is as follows:

V = IR

Where, 

• V is the voltage in volts
• I is the current in amperes
• R is the resistance in ohms

Symbol of voltage	V, ΔV
SI unit of voltage	Volt
Dimension of voltage	ML2T-3I-1

SI Unit of Voltage

After knowing the voltage definition and voltage formula, let us learn the SI unit of voltage. The standard unit of measurement used for the expression of voltage is volt which is represented by the symbol v. However, the volt is a derived SI unit of electric potential or electromotive force. For this reason, volt can further be defined in several ways.

Volt can also be defined as electric potential along a wire when an electric current of one ampere dissipates one watt (W) of power (W = J/s).

V = W/A

Volt can be expressed as the potential difference between two points in an electric circuit that imparts one joule (J) of energy per coulomb (C) of charge that passes through the circuit.

It can also be expressed as amperes times ohms, joules per coulomb (energy per unit charge), or watts per ampere (power per unit current).

And finally, volt can be stated in SI base units as 1 V = 1 kg m² s^-3 A ^-1 (one-kilogram meter squared per second cubed per ampere).
Read more : The difference between voltage and EMF

Other Electrical Units

Some of the other electrical units are given below.

Let us now define electric current and also know about conductors and insulators.

Electric Current is the rate of flow of electrons in a conductor. The SI Unit of electric current is the Ampere.

Electrons are minute particles that exist within the molecular structure of a substance. Sometimes, these electrons are tightly held, and the other times they are loosely held. When electrons are loosely held by the nucleus, they are able to travel freely within the limits of the body. Electrons are negatively charged particles hence when they move a number of charges moves and we call this movement of electrons as electric current. It should be noted that the number of electrons that are able to move governs the ability of a particular substance to conduct electricity. Some materials allow current to move better than others. Based on the ability of the material to conduct electricity, materials are classified into conductors and insulators.

Conductors: these materials allow the free flow of electrons from one particle to another. Conductors allow for charge transfer through the free movement of electrons. The flow of electrons inside the conducting material or conductor generates an electric current. The force that is required to drive the current flow through the conductor is known as voltage.

Examples of conductors: Human body, aqueous solutions of salts and metals like iron, silver and gold.

Insulators: Insulators are materials that restrict the free flow of electrons from one particle to another. The particles of the insulator do not allow the free flow of electrons; subsequently, charge is seldom distributed evenly across the surface of an insulator.

Examples of Insulator: Plastic, Wood and Glass.

Prerequisites for the Current to Flow in a Conductor

Some of the prerequisites for the electric current to flow in a conductor are discussed here. The circuit includes an energy source (a battery, for instance) that produces voltage. Without voltage, electrons move randomly and are undirected; hence current cannot flow. Voltage creates pressure on the electrons which channelizes it to flow in a single direction.

The circuit forms a closed conducting loop through which electrons can flow. A circuit is said to be closed or complete when a switch is turned ON.

What is an Electromotive Force?

The motion of free electrons is normally haphazard. If a force acts on electrons to make them move in a particular direction, then up to some extent random motion of the electrons will be eliminated. An overall movement in one direction is achieved. The force that acts on the electrons to make them move in a certain direction is known as electromotive force and its quantity is known as voltage and is measured in volts.

Unit of Electric Current

Let us know what is current and the unit to measure it.

The magnitude of electric current is measured in coulombs per second. The SI unit of electric current is Ampere and is denoted by the letter A. Ampere is defined as one coulomb of charge moving past a point in one second. If there are 6.241 x 10¹⁸ electrons flowing through our frame in one second then the electrical current flowing through it is ‘One Ampere.’

The unit Ampere is widely used within electrical and electronic technology along with the multipliers like milliamp (0.001A), microamp (0.000001A), and so forth.

Visualizing Electric Current

To gain a deeper understanding of what an electric current is and how it behaves in a conductor, we can use the water pipe analogy of electricity. Certainly, there are some limitations but they serve as a very basic illustration of current and current flow.

We can compare the electric current to the water flowing through the pipe. When pressure is applied to one end of the pipe, the water is forced to flow through the pipe in one direction. The amount of water flow is proportional to the pressure placed on the end. This pressure can be compared to the electromotive force.

Conventional Current flow Vs Electron Flow

There is a lot of confusion around conventional current flow and electron flow. In this section, let us understand their differences.

Conventional Current Flow 

The conventional current flow is from positive to the negative terminal and indicates the direction that positive charges would flow.

Electron Flow

The electron flow is from negative to positive terminal. Electrons are negatively charged and are therefore attracted to the positive terminal as unlike charges attract.

Properties of Electric Current

After we define electric current, let us learn the properties of electric current. Electric current is an important quantity in electronic circuits. We have adapted electricity in our lives so much that it becomes impossible to imagine life without it. Therefore, it is important to know what is current and the properties of the electric current.

• We know that electric current is the result of the flow of electrons. The work done in moving the electron stream is known as electrical energy. The electrical energy can be converted into other forms of energy such as heat energy, light energy, etc. For example, in an iron box, electric energy is converted to heat energy. Likewise, the electric energy in a bulb is converted into light energy.
• There are two types of electric current known as alternating current (AC) and direct current (DC). The direct current can flow only in one direction, whereas the alternating direction flows in two directions. Direct current is seldom used as a primary energy source in industries. It is mostly used in low voltage applications such as charging batteries, aircraft applications, etc. Alternating current is used to operate appliances for both household and industrial and commercial use.
• The electric current is measured in ampere. One ampere of current represents one coulomb of electric charge moving past a specific point in one second.

1 ampere = 1 coulomb / 1 second

• The conventional direction of an electric current is the direction in which a positive charge would move. Henceforth, the current flowing in the external circuit is directed away from the positive terminal and toward the negative terminal of the battery.

Effects of Electric Current

After defining electric current, let us learn various effects of electric current. When a current flows through a conductor, there are a number of signs which tell if a current is flowing or not. Following are the most prominent signs:

Heating Effect of Electric Current

When our clothes are crumpled, we use the iron box to make our clothes crisp and neat. Iron box works on the principle of heating effect of current. There are many such devices that work on the heating effect.

When an electric current flows through a conductor, heat is generated in the conductor.

The heating effect is given by the following equation

H=I²RT

The heating effect depends on the following factor:

• The time ‘t‘ for which the current flows. The longer the current flows in a conductor more heat is generated.
• The electrical resistance of the conductor. Higher the resistance, the higher the heat produced.
• The amount of current. The larger the amount of current higher the heat produced.

If the current is small then the amount of heat generated is likely to be very small and may not be noticed. However, if the current is larger then it is possible that a noticeable amount of heat is generated.

Magnetic Effect of Electric Current

Another prominent effect that is noticeable when an electric current flows through the conductor is the build-up of the magnetic field. We can observe this when we place a compass close to a wire carrying a reasonably large direct current, the compass needle deflects. The magnetic field generated by a current is put to good use in a number of areas. By winding a wire into a coil, the effect can be increased, and an electromagnet can be made.

Chemical Effect of Electric Current

When an electric current passes through a solution, the solution ionizes and breaks down into ions. This is because a chemical reaction takes place when an electric current passes through the solution. Depending on the nature of the solution and the electrodes used, the following effects can be observed in the solution:

• change in the colour of the solution
• metallic deposits on the electrodes
• a release of gas or production of bubbles in the solution