Basics of Electricity and Magnetism
Basics of Electricity
and Magnetism
Many fundamental, or subatomic, particles
of matter have the property of electric charge. For example,
electrons have negative charge and protons have positive charge, but
neutrons have zero charge.
Definition :
Electric charge is the property of
subatomic particles that causes it to experience a force when placed in an
electromagnetic field.
Explanation:
Electric charge is the basic physical
property of matter that causes it to experience a force when kept in an
electric or magnetic field.
Properties:
1. There are two kinds of charge, positive and negative
2. Like charges repel, unlike charges attract
3. Positive charge comes from having more protons than
electrons; negative charge comes from having more electrons than protons
4. Charge is quantize, meaning that charge comes in integer
multiples of the elementary charge e
5. Charge is conserved. The algebraic sum of the fundamental
charges remains the same.
Electric Charge:
The unit of electric charge in SI systems is
the coulomb or C.
Definition :
When two charges having equal strengths are placed in air 1 m
apart and exert a force of 9 x 109 N, then each charge is said to be
unit charge or 1 coulomb charge .
Electric charge is
given by, Q = I . t
where, Q is Electric
charge, I is Electric current, t is
time. One coulomb consists of 6.24 × 1018 natural units of
electric charge, such as individual electrons or protons. From the definition
of the ampere, the electron itself has a negative charge of 1.602176634 × 10−19 coulomb.
Usually
it is taken as 1.6 × 10−19 C.
Electric charge is a scalar quantity.
Symbol : Q Q = ne
S.I. Unit : coulomb or C
One coulomb is the quantity
of charge transferred in one second.
Types : 1. Positive
charge
2. Negative
charge
Coulomb's Law:
Coulomb’s
law provides a means to calculate the strength of the force between two point
charges.
Statement:
The magnitude of the electrostatic force
of attraction or repulsion between two charges is directly proportional to the
product of the magnitudes of charges and inversely proportional to the square
of the distance between them.
The formula that measures the electrical forces between two
objects is
The constant
of the equation is k.
k = 1/ 4 πϵ0 = 9 x 109 Nm2 / C2
ϵ0
Absolute Permittivity = 8.854 x 10-12 C2/ Nm2
The permittivity of an insulating, or dielectric, material is
commonly symbolized by the Greek letter epsilon, ε; the permittivity of
a vacuum, or free space, is symbolized ε0;
and their ratio ε/ε0,
called the dielectric constant is symbolized by the Greek letter kappa, κ.
Relative
permittivity κ or εr = ε/ε0
In SI systems, the magnitude of the permittivity of a vacuum
ε0 =
8.854 × 10−12 C2 /
N m2
Permittivity of medium
ε = κε0 or εr ε0
In the cgs system, the value of the permittivity of free space ε0 is
chosen arbitrarily to be 1.
Electric Field:
Concept : It is an electric
property associated with each point in space when charge is present in any
form.
Definition: The space around an
electric charge in which its influence can be felt is known as the electric field.
Explanation : It is the physical field that surrounds each electric charge
and exerts force on all other charges in the field.
Symbol : E
Formula : E =F/q where
F is force, q is charge
S. I. Unit : V/m or N/C Vector quantity
Electric
field Intensity:
Concept : Whenever a unit test charge is placed in the electric field it
will experience the force emitted by the source particle.
The amount of force experienced by a unit charged particle when
it is placed in the electric field is known as Electric field intensity.
Definition: The electric field
intensity at any location is the force that would be experienced by unit test
charge placed at the location.
OR
It is defined as the force per unit charge when placed in
the electric field.
It is a vector quantity.
Formula :
From Coulomb’s law
Electric field Intensity is given by,
E = k Q/r2 = Q
/ 4 πϵ0 r2
Where, k is
proportionality constant and ϵ0 absolute permittivity
Q is source charge, r is the distance between the charges
Electric lines of force:
Electric lines of force are an excellent
way of visualizing electric field. Electrical lines of force can be described as a way or path, it
can be straight or curved so that the tangent gives the direction of the
intensity of the electric field at that point at any point.
If we place a positive unit
charge near a positively charged object, the positive unit charge will
experience a repulsive force. Due to this force, the positive unit charge will
move away from the said charged object. The imaginary line, through which the
unit positive charge moves, is known as Electric lines of force.
But for a negatively charged object, these electric lines of force come into this negatively charged object.
Properties of electric lines
of force
1)The electric lines of force start from a positive charge and
ends on a negative charge.
2) The electric lines of force always enter or leave the charged surface
normally.
3) Electric lines of force can never intersect each other.
4) The electric lines of force cannot pass through a conductor.
5) When two opposite charges are placed close to each other, the electric lines
of force present between them will become shorten in length.
6) When two like charges are placed closer to each other, the electric lines of
force present between them will become enlarged in length.
Electric
potential:
The physical quantity that drives electric charge in
an electric field is called electric potential.
Definition:
Electric
potential at a point inside an electric field is defined as the
amount of work done in bringing unit positive charge from infinity to that
point .
Electric potential, V = W / q
Electric potential --
SI unit is J / C or volt or V.
Electric potential at a point said to be 1 volt, if one joule of
work is to be done in moving one coulomb of charge from infinity to that point
against the electric field.
It is a scalar quantity.
Electric potential V due
to a point charge at a distance r is given by
V = Q / 4π εo
r
Where Q is charge, r is distant of point from charge Q, εo
is absolute permittivity .
Electric Potential Difference The electric potential
difference between two points in an electric field is defined as the amount of
work done in moving a unit positive test charge from one point to the other
point against of electrostatic force without any acceleration (i.e. the
difference of electric potentials of the two points in the electric field).
SI unit is volt or V.
VB - VA
= WAB / q0
where,
WAB is work done in taking
charge q0 from A to B against of electrostatic force.
Potential Gradient:
The rate of change of potential with distance in electric field
is called potential gradient.
Potential
gradient = dV / dr
Its unit is V / m.
Relation between potential gradient and electric field intensity is
given by
E = – (dV / dr
Magnetism :
Introduction:
A magnet is
a material or object that produces a magnetic field.
This magnetic field is invisible but is responsible for the most
notable property of a magnet: a force that pulls on other materials, such
as iron, steel, nickel, cobalt, etc. and attracts or repels other magnets.
Magnetic Poles:
A
region on a magnet which produces magnetic forces is called as Magnetic pole.
The poles
of a suspended magnet will align themselves to the poles of the Earth.
Fundamental Rule:
- Like poles repel; opposite
poles attract.
- Magnetic poles behave
similarly to electric charges .
- Magnetic poles cannot be
isolated.
Magnetic Field:
Concept
: A magnet is an object or a device that
gives off an external magnetic field.
Basically, it applies a force over a
distance on other magnets, electrical currents, beams of charge, circuits, or
magnetic materials. Magnetism can even be caused by electric
current.
Definition:
Magnetic field, a
vector field in the neighborhood of a magnet, electric current,
or changing electric field, in which magnetic forces are
observable.
Explanation : Magnetic fields are caused by the motion of
electric charges. Every spinning electron is a tiny magnet.
Magnetic field Intensity:
Concept
:The magnetic field strength is known as magnetic field intensity H.
Definition:
A vector quantity pertaining to the
condition at any point under magnetic influence (as of a magnet,
an electric current, or an electromagnetic wave) measured by the force exerted
in a vacuum upon a free unit north pole placed at the point is called
magnetic field intensity.
Or
The ratio of
magnetic field in vacuum ( B0 ) to the absolute permeability ( μ0 ).
Magnetic field intensity is a vector quantity.
Formula :
H
= B / μ0
Where
μ0 = 4 π x 10-7 tesla and
B is Magnetic Induction or Magnetic Flux Density
SI unit of magnetic field
intensity is A / m or N / Wb
Magnetic lines of force:
Magnetic lines of force are an excellent way of
visualizing magnetic field. Magnetic
lines of force can be described as a way or path, it can be straight or
curved so that the tangent gives the direction of the intensity of
the magnetic field at that point at any point.
Definition :
The path or curve
along which the unit north pole moves in magnetic field is called Magnetic
lines of force.
Properties of magnetic lines
of force:
1)The magnetic lines of
force start from a north pole and ends on a south pole.
2) The direction of magnetic field B at any point is along the tangent to the
magnetic lines of force at that point.
3)All Magnetic lines of force have same strength.
4) The magnetic lines of force are crowded near the
poles where the magnetic field is strong .
5) Closely spaced magnetic field lines represents strong magnetic field but
widely spaced magnetic field lines represents weak magnetic field.
Magnetic Flux: Concept:
The magnetic flux φ provides the
measurement of the total magnetic field that passes through a given
surface area.
Definition:
Magnetic
flux is defined as the number of magnetic field lines passing
normally through the surface area.
φ = B S cos ϴ
Where B is magnetic field, ϴ is angle between magnetic field and normal to surface S.
SI unit of Magnetic flux is weber or Wb
Magnetic flux density B also can be understood as
the density of magnetic lines of force, or magnetic
flux lines, passing through a specific area.
Also called as magnetic induction B.
SI units of magnetic flux density is tesla or Wb / m2.
The value of the field intensity, H, is directly
proportional to the value of induced flux density, B (or B-field). B
α H
B = μ H
where μ is absolute permeability = μ0 μr = permeability of
air x relative permeability
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