SIR
JAMES ALFRED EWING – COINED THE WORD HYSTERESIS
He
was a Scottish physicist and engineer, best known for his work on the magnetic
properties of metals and, in particular, for his discovery of, and coinage of
the word, hysteresis.
In
1878, he served as a professor of mechanical engineering at the Tokyo Imperial
University, he was instrumental in founding Japanese seismology.
In
Tokyo, Ewing taught courses on mechanics and on heat engines to engineering
students, and on electricity and magnetism to students of physics.
He
carried out many research projects on magnetism and coined the word
'hysteresis'.
In
1883, Ewing returned to his native Dundee to work at the recently established
University College Dundee as its first Professor of Engineering.
In
1890, Ewing took up the post of Professor of Applied Mechanics at the
University of Cambridge, initially at Trinity College, though he later moved to
King's College.
In
1890, he observed that magnetisation lagged behind an applied alternating
current. He described the characteristic hysteresis curve and speculated that
individual molecules act as magnets, resisting changes in magnetising
potential.
Ewing
was a close friend of Sir Charles Algernon Parsons (Inventor of compound steam
turbine) and collaborated with him on the development of the steam turbine.
He
died on 7th January 1935. He lived for 79 years in this planet.
The
James Alfred Ewing Medal of the Institution of Civil Engineers has been awarded
for meritorious contributions to the science of engineering in the field of
research since 1938.
DIELECTRIC
–
A dielectric material (dielectric for short) is an electrical insulator that
can be polarized by an applied electric field. An important property of a
dielectric is its ability to support an electrostatic field while dissipating
minimal energy in the form of heat.
A
dielectric material is a substance that is a poor conductor of electricity, but
an efficient supporter of electrostatic fields.
If
the flow of current between opposite electric charge poles is kept to a minimum
while the electrostatic lines of flux are not impeded or interrupted, an
electrostatic field can store energy.
This
property is useful in capacitors, especially at radio frequencies.
Dielectric
materials are also used in the construction of radio-frequency transmission
lines and power cables.
In
practice, most dielectric materials are solid. [Examples – glass, plastics,
porcelain etc.]
Some
liquids and gases can serve as good dielectric materials.
Dry
air is an excellent dielectric, and is used in variable capacitors and some
types of transmission lines.
Distilled
water is a fair dielectric.
A
vacuum is an exceptionally efficient dielectric.
In
a perfect dielectric having infinite leakage reactance. If there is no flow of
leakage current, so will be no dielectric loss.
But in practice, it is not possible to get a perfect dielectric.
In
a cable, capacitances ‘C’ are formed between the conductors and the sheath,
which are separated by insulating material having leakage resistance R. The
equivalent circuit can be represented by a parallel combinations as shown in
figure.
DIELECTRIC
LOSS – When an insulating material is subjected to an
alternating electric field, the atoms get stressed and due to the inner atomic
friction heat is produced. This loss is known as dielectric loss
The
dielectric loss due to loss in leakage resistance is Pi = V^2/Rl
The
cosine of the angle θ is the power factor of dielectric, which provides a
useful measure of the quality of the dielectric.
The
power factor of impregnated paper is approximately 0.003. For a good dielectric
the value of θ is very close to 90 degree.
The
angle of δ is in radians and is known as the loss angle and which is very small
for good dielectric, therefore, δ = tan δ = sin δ = sin (90 – θ) = cos θ
δ
is the indicator of the state of the dielectric [healthy or unhealthy]
Higher
value of δ indicates dialectic is healthy.
Lower
value of δ indicates the dielectric is unhealthy.
Dielectric
loss P = V^2ωCδ, = V^2 2∏f [ϵo ϵr A/d]δ , i.e. P ∞ V^2 and P ∞ f
The
product of ϵr
x δ is known as the loss factor. [The relative permittivity is an indication of
the retained energy due to molecular deformation whereas δ is an indication of
the amount of leakage current which will flow through the insulator and so
produce a heat loss.
DIELECTRIC
HEATING – When a ferro-magnetic material is subjected to
alternating magnetic field, it gets heated up due to eddy currents induced in
the material and the hysteresis loss in the material.
LOSS
FACTOR – The product of ϵr δ is known as
dielectric loss factor.
ϵr
– relative permittivity and it is an indication of retained energy due to
molecular deformation.
δ is an indication of amount of leakage current
which will flow through the insulator and so produce heat.
RANGE
OF OPERATING VOLTAGE AND FREQUENCY
The
operating voltage range of dielectric heating is 600 – 3000V
The
frequency range – 10 – 40 MHZ.
Advantages
1.
No flame appears since heat is generated within the dielectric.
2.
By increasing the frequency the heating becomes faster.
3.
Heating can be stopped immediately as and when desired.
4.
Uniform heating can be obtained.
5.
Plastics and wooden products etc., can be heated safely.
Disadvantage
–
Costly method
Applications
– 1. Chocolates and popcorn preparation 2. Rubber vulcanizing
3.
Gluing of laminated glass 4. Sterilization of surgical instruments 5. Removal
of moisture from oil emulsions and 6. Sterilization of cereals.
