ELECTRICAL ENGINEERING-IMPORTANT ELECTRICAL LAWS

Maxwell’s right hand grip rule and right handed cork screw rule
We know that a current carrying
conductor creates a magnetic field around it. The nature
of magnetic field around a straight current carrying conductor
is
like concentric circles having their center at the axis of the conductor (as
shown in the figure at right). The direction of these circular magnetic lines
is dependent upon the direction of current. The density of the induced magnetic
field is directly proportional to the magnitude of the current.
Direction of the circular magnetic field lines can be given by Maxwell’s
right hand grip rule
or Right handed cork screw rule.
Maxwell’s right hand grip rule
Assume that the current carrying conductor is held in the
right hand so that the fingers wrap around the conductor and the thumb is
stretched (as shown in the figure at left). If the thumb is along the direction
of current, wrapped fingers will show the direction of circular magnetic field
lines.
 
Right handed cork screw rule

If
a right handed cork screw is assumed to be held along the conductor, and the
screw is rotated such that it moves in the direction of the current, direction
of magnetic field is same as that of the rotation of the screw.

Fleming’s left hand rule and right hand rule

If a current carrying conductor placed in a magnetic field,
it experiences a force due to the magnetic field. On the other hand, if a
conductor moved in a magnetic field, an emf gets induced across the conductor (Faraday’s law of electromagnetic
induction).
John Ambrose Fleming introduced two rules to determine the direction of motion
(in motors) or the
direction of induced current (in generators).
The rules are called as Fleming’s left hand rule (for motors) and Fleming’s
right hand rule
(for generators).

Fleming’s left hand rule

Whenever a current carrying conductor is placed in a
magnetic field, the conductor experiences a force which is perpendicular to
both the magnetic field and the direction of current. According to Fleming’s
left hand rule
, if the thumb, fore-finger and middle finger of
the left hand are stretched to be perpendicular to each other as shown in the
illustration at left, and if the fore finger represents the direction of
magnetic field, the middle finger represents the direction of current, then the
thumb represents the direction of force. Fleming’s left hand rule is applicable
for motors.

How to remember Fleming’s left hand rule?

Method 1: Relate the thumb with thrust, fore finger
with field and center-finger with current as explained below.

1.The Thumb
represents the direction of Thrust on the conductor (force on the
conductor).

2.The Fore
finger represents the direction of the magnetic Field.

3.The Center
finger (middle finger) the direction of the Current.

Method 2:
Relate the Fleming’s
left-hand rule
with FBI. Here, F for Force, B is the symbol of magnetic flux
density and I is the symbol of Current. Attribute these letters F,B,I to the
thumb, first finger and middle finger respectively.

Fleming’s right hand rule

Fleming’s right hand rule is applicable for electrical
generators. As per Faraday’s
law of electromagnetic induction, whenever a conductor is forcefully
moved in an electromagnetic field, an emf gets induced across the conductor. If
the conductor is provided a closed path, then the induced emf causes a current
to flow. According to the Fleming’s right hand rule, the thumb, fore
finger and middle finger of the right hand are stretched to be perpendicular to
each other as shown in the illustration at right, and if the thumb represents
the direction of the movement of conductor, fore-finger represents direction of
the magnetic field, then the middle finger represents direction of the induced
current.

How to remember Fleming’s right hand rule?

You can follow the same methods mentioned above for
Fleming’s left hand rule. In this case, you just have to consider your right
hand instead of the left hand.

Faraday’s law and Lenz’s law of electromagnetic induction

Faraday’s laws of of
electromagnetic induction
explains the relationship between electric
circuit and magnetic field. This law is the basic working principle of the most
of the electrical motors,
generators, transformers, inductors etc.

Faraday’s first law:

Whenever a conductor is placed
in a varying magnetic field an EMF gets induced across the conductor (called as
induced emf), and if the conductor is a closed circuit then induced current
flows through it.
Magnetic field can be varied by various methods –
1. By moving magnet
2. By moving the coil
3. By rotating the coil relative to magnetic field

Faraday’s second law:

Faraday’s second law of
electromagnetic induction
states that,  the magnitude of
induced emf is equal to the rate of change of flux linkages with the coil. The
flux linkages is the product of number of turns and the flux
associated with the coil.

Formula of Faraday’s law:

Consider the conductor is moving
in magnetic field, then
flux linkage with the coil at initial position of the conductor = NΦ1 
(Wb) (N is speed of the motor and Φ is flux)
flux linkage with the coil at final position of the conductor = NΦ2
  (Wb)
change in the flux linkage from initial to final = N(Φ1 – Φ2)

let  Φ1 – Φ2 = Φ
therefore, change in the flux linkage = NΦ
and, rate of change in the flux linkage = NΦ/t
taking the derivative of RHS
rate of change of flux linkages = N (dΦ/dt)
According to Faraday’s law of electromagnetic induction, rate of change
of flux linkages is equal to the induced emf
So, E = N (dΦ/dt)    (volts) 
Phenomenon of Mutual Induction

Alternating current flowing in a coil produces alternating
magnetic field around it. When two or more coils are magnetically linked to
each other, then an alternating current flowing through one  coil causes
an induced emf across the other linked coils. This phenomenon is called as
mutual induction.

Lenz’s law

Lenz’s  law of electromagnetic induction states
that, when an emf is induced according to Faraday’s law, the polarity
(direction) of that induced emf is such that it opposes the cause of its
production.

Thus, considering Lenz’s law
E = -N (dΦ/dt)   (volts)

The negative sign shows that, the direction of the induced
emf and the direction of change in magnetic fields have opposite signs.

ELECTRICAL ENGINEERING-THREE PHASE A.C. CIRCUITS (PART ONE)
RAJASTHAN RAJYA VIDYUT UTPADAN NIGAM LTD AE EXAM DATE ,ADMIT CARD AND INFORMATION ABOUT EXAM PATTERN AND SAMPLE QUESTIONS

2 Comments

  1. Amit sharma December 28, 2017
    • Harikesh Yadav December 28, 2017

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