Basic Electrical Theory: The Fundamental Laws of Electricity

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BASIC LAWS OF ELECTRICAL ENGINEERING

In this article, we will study about the basic laws of Electrical Engineering. A strong foundation for any electrical worker is built on a thorough knowledge of the laws that govern the operation of electricity.

* Ohm’s law.

* Lenz law.

* Fleming’s left hand rule.

* Fleming’s right hand rule.

* Faraday’s law of Electromagnetic Induction.

These are some basic and simple laws. These laws forms the basis of many big machines like motor, generator, transformers etc. We will study these laws one by one.

OHM’S LAW

Ohm’s law deals with the relationship between potential difference in the circuit and the current flowing in the circuit.This law was discovered by Georg Simon Ohm. Ohm law states that:

Potential difference (Voltage) applied across conductor is directly proportional to the current flowing through it.

   V ∝ I

where, V = potential difference across conductor.

I = current flowing through the conductor.

Here, the constant of proportionality is called RESISTANCE  ‘R’. 

So,  V = I × R.

Ohm’s law is used to solve complex electric circuits.

If we plot a graph between Voltage V and current I , then the curve will be a straight line passing through origin. This figure shows the graph between Voltage and Current.

Graph of Voltage vs Current.
Slope of the line represents the resistance of the conductor/circuit.

Higher the slope, higher will be the resistance.
Lower the slope, lower will be the resistance.

Note that , Ohm’s law is not an universal law. Materials which obeys Ohm law are called linear materials or ohmic materials.

LENZ LAW 

This law states that the direction of induced emf is such that the current produced by it sets up a magnetic field which opposes the cause producing it.

Lenz law indicates the direction of induced current.

Formula

\epsilon = -N \frac{\partial \Phi_{B}}{\partial t}

\epsilon

=

induced emf

N

=

number of turns in coil

\partial \Phi_{B}

=

change in magnetic flux

\partial t

=

change in time

For example : As shown in this figure, a bar magnet with the North Pole towards coil is moved towards the coil. A current will be induce in the coil. The direction of current in the coil will be such that it will oppose the motion of the magnet.

The direction of the current in the coil will be as shown in the figure. This direction of current will produce a north pole towards the bar magnet. Since north pole and north pole will cause repulsion between them. Hence it opposes the cause producing the current in the coil. In this case, the cause is the motion of bar magnet.

FLEMING’S LEFT HAND RULE.

This rule helps to find the direction of the motion of the conductor placed in magnetic field with current flowing through it. This law states that:

If a conductor is placed in magnetic field and current is flowing through it, then it will experience a force which is mutually perpendicular to the field and current.

If we stretch the fingers of left hand as shown in figure, then

Left Hand Rule

Index finger will represent the direction of magnetic field.

Middle finger shows the direction of current.

Thumb represents the direction of force.

This principle is used in the working of DC MOTOR.

FLEMING’S RIGHT HAND RULE.

This law helps to find the direction of induced current in the conductor when a conductor moves in the magnetic field.

This rule is used in finding the direction of induced current in generator windings.

If we strech the fingers of right hand as shown in figure, then

Right Hand Rule

Thumb represents the direction of motion.

Index finger represents the direction of magnetic field.

Middle finger represents the direction of induced current in the conductor.

FARADAY’S LAW OF ELECTROMAGNETIC INDUCTION.

Faraday formulated two laws related to electromagnetic induction.

Faraday’s first law. 

This law states that when the flux linking with the coil changes, an emf is induced in it  or whenever the conductor cuts the magnetic flux then an emf us induced in it.

Methods to change magnetic field/magnetic flux.

1. By moving the magnet towards or away from the coil.

2. By moving the coil towards or away from the magnetic field.

3. By changing the area of the coil placed in magnetic field.

4. By the relative motion of the coil with the magnetic field.

Faraday’s second law.

This law states that the magnitude of emf induced in the coil is directly proportional to the rate of change of flux and number of turns in the coil.

Result from Faraday’s second law:-

E = -N (dΦ/dt)   

The negative sign shows that, the direction of the induced emf and the direction of change in magnetic fields are opposite to each other.

READ HERE  Ohm's Law - Statement, Formula, Solved Examples

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