Fundamental Laws

From its beginnings in the late nineteenth century, electrical engineering has blossomed from focusing on electrical circuits for power, telegraphy and telephony to focusing on a much broader range of disciplines. However, the underlying themes are relevant today: Power creation and transmission and information have been the underlying themes of electrical engineering for a century and a half. The fundamental laws governing Electrical Engineering are as follows:-
  • Ohm's Law
  • Kirchhoff's Laws
  • Lenz's Law
  • Faraday's Law
  • Coulomb's Law
  • Oersted's Law
  • Ampere's Circuital Law
  • Biot-Savart's Law
  • Lorentz's Force Law
  • Gauss's Law
  • Fleming's Right hand and Left hand Rule

Ohm's Law

According to this law, if there is no change in the physical state of a conductor then the ratio of the potential difference applied at its ends and the current flowing through it is constant. Thus if the potential difference at the ends of the conductor be V and the current flowing through it be i, then according to Ohm's law we have
R = V/i
A graph drawn between the applied potential difference which V and the current i flown through the conductor is straight line. Ohm's law is true for metallic conductors only.

Kirchhoff's Law

Ohm's law is not sufficient to give current in complicated circuits.There are two laws given by Kirchhoff in 1842 namely Kirchhoff's Current Law or KCL and Kirchhoff's Voltage Law or KVL

  • Kirchhoff's Current Law(KCL)

It is also called as junction law. In an open circuit the algebraic sum of currents meeting at a point is zero. This is also called as point rule.
Kirchhoff's Current law or KCL

Let I1 and I2 are meeting at O in AO and BO direction and I3, I4 and I5 currents are leaving the point O in OC, OD and OE directions. Assuming that current reaching at junction O are positive and current leaving junction O are negative then,
I1 + I2 + [-I3] + [-I4] + [-I5] = 0
i.e., I1 + I2 = I3 + I4 + I5

  • Kirchhoff's Voltage Law(KVL)

In a closed circuit the algebraic sum of the product of currents and resistances of different parts of loop is equal to the algebraic sum of the e.m.f. in the loop.
Kirchhoff's Voltage law or KVL

I1R1 + I2R2 - I3R3 = E

Lenz's Law

The direction of emf induced in a conductor or coil is governed by Lenz's Law which states that the direction of induced emf would be in such a direction that it would try to oppose the very cause for which it is due.
Induced emf, e = - NdΦ/dt 

Faraday's Laws of Electromagnetic Induction

  • Faraday's First Law:-

This law states that, "when the flux linking with the coil or circuit changes an emf is induced in it or whenever the magnetic flux is cut by the conductor an emf is induced in the conductor."

  • Faraday's Second Law:-

This law states that magnitude of emf induced is directly proportional to the rate of change of flux linking the coil.
i.e., induced emf, e  NdΦ/dt 
where NdΦ/dt is product of number of turns and rate of change of linking flux and is called rate of change of flux linkage.

Coulomb's Law

The force of attraction between the two charges Q1 and Q2 is directly proportional to the product of the two charges and inversely proportional to the square of the distance between the two charges.
F = k Q1Q2/R2  
where k is proportionality constant.

Oersted's Law

On April 21, 1820 Danish Physicist Hans Christian Oersted discovered the magnetic field produced by an electric field.

Ampere's Circuital Law

The line integral of vector H over the whole contour depends only on the algebraic sum of the currents intersecting the surfaces, and is equal to that sum i.e., 
∮H.dl = ∑ I
The above equation is known as Ampere's circuital law or sometimes referred to as Ampere's work law.

Biot - Savart's Law

It states that the magnetic flux density dB is directly proportional to the length of the element dl, the current I and the sine of angle Θ between the direction f the current and vector joining a given point of the field and the current element, and is inversely proportional to the square of the distance of the given point from the current element r,
i.e., dB   IdlsinΘ/r2
dB = K IdlsinΘ/r2
where K is proportionality constant and is dependent upon the magnetic properties of the medium and the system of units employed.

Lorentz's Force Law

Lorentz's Force, the force exerted on charged particle q moving with velocity v in an Electric field E and Magnetic field B. The entire force on the charged particle is called the Lorentz force and is given by
F = qE + qv × B.

Gauss's Law

According to this theorem, the total electric Ψ flux emanating from a closed surface is equal to the total charge enclosed by the surface.
Ψ = ∫ D. ds

Fleming's Rule

  • Fleming's Right Hand Rule

According to Fleming's right hand rule if the thumb, fore-finger and middle finger of the right hand are held mutually perpendicular to each other, fore-finger pointing in the direction of the field and thumb in the direction of motion of conductor then middle finger will point in the direction of induced emf.  

  • Fleming's Left Hand Rule

When a wire carrying electric current is moved in a magnetic field of a magnet, the magnetic field induced by the wire reacts with the magnetic field of the magnet causing the wire to move outwards. Fleming's left hand rule helps to predict the movement. According to Fleming's right hand rule if the thumb, fore-finger and middle finger of the left hand are held mutually perpendicular to each other, the fore-finger points in the direction of magnetic field, thumb in the direction of movement of wire then the middle finger will point in the direction of current.

Note:- This above explanation of various fundamental laws of electrical engineering are just an introduction to them and they are briefly described in various in our website.

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