Basics of Electromagnetism: Easy learning

by Dr. Anjana Sen

Part 1: Electricity

Part 2: Magnetism

Part 3: Electromagnetism

Part 1: Electricity

fundamental sub-atomic particles

++

─

──

── ─

electron (e─)

proton (p+)

neutron (n0)

nucleus

Electric charge occurs when, the number of protons in the nucleus ≠ the number of electrons in the surrounding orbitals. more protons than electrons cation, positive charge less protons than electrons anion, negative charge

useful definitions

Electricity: a physical phenomenon associated with presence of electric charge

(stationary / moving).

static electricity: accumulated electric charge on non-conducting material, results from

imbalance of positive and negative charges between object.

“valence electron”

• is located in the outermost shell (valence shell),

• participates in forming of chemical bonds,

• can escape from atomic orbital and become free.

free electron = flowing charge

conductor permits very mobile electrons to flow freely and easily

releases electrons, such as metal

semiconductor intermediate conductivity, increases with temperature,

such as silicon

insulator extremely high resistance to flow of charge, such as

rubber, wood, plastic

voltage: difference in charge between two points.

electric current: flow of electric charge / electrons through a conducting material.

Why does electric current flow in the direction from the positive charge towards the negative charge?

+ ++++ + + ++

― ― ― ― ― ― ― ― ―

elec

tric

pote

ntia

l

high potential energy

low potential energy

flow from high to low,just like water

What is electric potential energy? Electric potential energy of a charge describes the amount of its stored energy.

Electrostatic force converts electric potential energy to kinetic energy (charge flow).

Voltage is measured as electric potential energy per unit charge (Joules per

Coulomb = Volt). electrical resistance: material’s tendency to resist the flow of charge (current).

electrical conductance: material’s ability to conduct the flow of charge (current).

Electrical resistance is measured as the ratio of voltage and current.

Ohm’s Law

V = I.R V = voltage (Volt), I = current (Amp), R = resistance (Ohm)

• length of the wire

factors affecting electrical resistance

• material of the wire

• cross-sectional area of the wire

electric circuit: a closed path/line/loop of conducting material that allows flow of current.

electric field: a region associated with the existence of a force generated by a distribution

of electric charge.

Electric field is a vector quantity (force) exerted by a charged particle on the surrounding

charged particles.

Electric field is measured as electric force per unit charge

examples of electric fields

Inductance: a property of an electric circuit of inducing electromotive force by variation of

current.

It’s measured as the ratio of the induced electromotive force and the rate of variation of the

inducing current.

Capacitance: the ability of a device to store electric charge.

It’s expressed as charge difference per potential difference (voltage)

direct current (dc) electricity

The flow of electricity is continuous in one direction.

DC electricity can be created by battery or DC generator.

Plot of DC voltage is just simple.

The terminals constantly switch polarity, the direction of the current alternates periodically.

alternating current (ac) electricity

time

ampl

itude

+

―

timeampl

itude

+

―

timeampl

itude

+

―

sine wave

square wave

triangle wave

ac waveforms

AC current DC current

home/office outlets obtained from battery

periodically alternating “one direction” flow

amplitude varies with time amplitude is constant

long distance transmission no

impedance resistance only

frequency is 50 - 60 Hz frequency is zero

Part 2: Magnetism

useful definitions

magnet: an object/device that produces magnetic field and exerts attractive/repulsive

forces.

magnetic field: a region / space near a magnet or moving charges exerting magnetic

force.

electric currents + magnetic moments give rise to magnetic field. Magnetic field produces Magnetism, (attractive / repulsive force).

magnetic moment: property of a magnet that interacts with an applied magnetic field to

generate a mechanical moment. It’s a measure of the tendency to align with a magnetic

field.

magnetization: a vector field, it’s the process of making a substance temporarily or

permanently magnetic.

Magnetization delineates density of permanent or induced magnetic dipole moment

measured per unit volume.

magnetic flux = Φ = BA

spinning charge is associated with magnetic dipole moment

Now discussing………..magnetic field in sub-atomic level

spinning charge generates magnetic field

opposite spinsgenerate magnetic fields

in opposite directions

+ +

First discussing…………… permanent magnets

N

Sm

agne

tic fi

eld

lines

magnetic field lines

Lines are closed loops

properties of magnetic field lines

Direction from the north pole

Lines penetrate magnetic materials

Strength is proportional to the density

Lines bend at the surface of magnetic materials

“Ferromagnetism”, permanent magnetism is observed in magnetite, Fe3O4, a form of

natural iron.

S N

S N

N

SN

N

N

S

S

S

iron

pap

er c

lips

N

SN

N

N

S

S

S

iron

nai

ls

Now discussing…………..induced magnets

permanent magnet exhibits persistent alignment of unpaired electrons with / without

external magnetic field / electric current.

permanent magnet vs. induced magnet

N

S

I

I

Induced by electric current

Ferromagnetism observed in Magnetite

Paramagnetism temporarily and reversibly magnetized

Diamagnetism repelled by permanent magnet

Ferromagnetism susceptibility is large and positive

Paramagnetism susceptibility is small and positive

Diamagnetism susceptibility is small and negative

magnetic field

diamagnetic paramagnetic

Ferromagnetic Paramagnetic Diamagnetic

iron aluminium silver

cobalt platinum gold

nickel manganese copper

gadolinium magnesium mercury

dysprosium lithium petroleum

Part 3: Electromagnetism

useful definitions

S N

II

Solenoid

magnetic field inside the solenoid B = µ0N I

lI = electric current

l = length of the wire

N = number of turns

µ0 = magnetic constant

Solenoid: is a type of electromagnet with a purpose to generate controlled magnetic field.

Solenoids utilized to impede changes in electric current are called Inductors.

Electromagnetism

larger scale sub-atomic level

result of a magnetic field from

the movement of electrical charges

force between

electrically charged particles

usually concerns the use of electric current

to make electromagnets

one of the fundamental

interactions of matter

results in electrodynamics results in electromagnetic waves

electric force + magnetic force = electromagnetic force

Lorentz Force Law fusion of electric field and magnetic field

electric force magnetic force

Electrodynamics deals with interactions between electric, magnetic and mechanical forces.

Electrodynamics is phenomena associated with moving charges in varying electric and

magnetic fields.

electric currents have

Electromagnetic Induction: outlines

associated magnetic fields

magnetic fields can generate

electric currents

electric charges moving

in a magnetic field

create an electric field

electric charges moving

in an electric field

create a magnetic field

moving electric charges

create a magnetic field

a moving magnetic field

creates an electric field

magnetic moment = electric current X area

mag

netic

mom

ent

How electric field and magnetic field are related

current through wire in a magnetic field

F = B.I.l

voltage is induced

when

basic law of Electromagnetic Induction:

A phenomenon where magnetic field interacting with an electric circuit produces an

ElectroMotive Force (EMF).

Induction of EMF

by moving a conductor across a magnetic field

by varying magnetic flux within a magnetic field

a magnet moves near a stationary conductor

a conductor moves in a stationary magnetic field

Φ = BA cos θ

Now discussing……… Maxwell’s equations Maxwell’s equations: only 4 equations together can describe all.

classical electric phenomena & classical magnetic phenomena.

Maxwell’s equations are the foundation of classical electromagnetism.

First discussing……… Faraday’s laws Faraday’s first law:

• When a conductor is placed in a varying magnetic field, electromotive force is induced.

• If a closed circuit is placed in a varying magnetic field, then an induced current flows

through it.

Faraday’s second law:

The induced EMF in any closed circuit is the negative of the rate of change

of the magnetic flux enclosed by the circuit.

Hans Christian Ørsted, Ørsted’s Law

states that, a constant electric currentgenerates a magnetic fieldaround the conductor

Michael Faraday, Faraday’s Law

states that, a varying magnetic fieldinduces an electric current

Heinrich Lenz, Lenz’s Law

states that, this induced currentis in the opposite direction,a phenomenon called “self-inductance”

Faraday’s Law: • describes EMF generated by magnetic force

• describes EMF in coils of wire and moving wire

• describes EMF caused by magnet movement

• describes EMF caused by magnetic field change

Maxwell’s Equations contain Faraday’s Law

Faraday’s Law contains Lenz’s Law

Faraday’s Law vs. Lenz’s Law Lentz’s Law gave the direction of the electromotive force

ε = – Ndφdt

induced voltage(EMF)

number of loops

Lenz’s law

Lenz’s law

• The induced EMF generates a current

• This current has its own magnetic field

• That field opposes the change in the magnetic flux

• Loops and coils tend to keep the magnetic flux constant

• This law provides the polarity (direction) in Faraday’s law

Now discussing……… Ampere’s laws

∫ = line integral on a closed path or curve

∫ denotes integral in 1-dimension

∫ denotes line integral

∫∫ = surface integral on a closed surfaces

∫∫ denotes integral in 2-dimension

∫∫ denotes surface integrals

Faraday’s Law links a varying magnetic field with an induced electric field

Ampere’s Law links a varying electric field with an induced magnetic field

Ampere’s law: • Magnetic equivalent of Gauss’s Law

• Basic relation between electricity and magnetism

Ampere’s law

Gauss’s Law vs. Ampere’s Law

Gauss’s Law relates electric charge to electric field

Ampere’s Law relates electric current to magnetic field

Faraday’s Law leads to the definition of Inductance

Faraday’s Law fundamental operating principle of transformers,

generators and electrical motors

Gauss’s Law means The electric flux moving out of a closed surface

is proportional to the charge inside the volume

Gauss’s Law for Magnetism means Magnetic flux through a closed surface is zero,

there are no monopoles

Ampere-Maxwell Law means

Line integral of the magnetic field

around a closed loop is proportional

to the electric current plus

the rate of change of electric field

∫ =•l

encIdlB 0µ

∫∫ =•=s

EQdAE

0εφ

φE = electric flux through a closed surface S

E = electric field

A = surface area

Q = total electric charge

ε0 = electric constant

Gauss’s Law