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Electric heat
Electric heating is process of converting electric energy into heat. Common applications
include heating of buildings, cooking, and industrial processes.
Heat or heating energy is an internal energy that is either absorbed or radiated by an object
or is transfer to another object (heat transfer). Heat between objects can be transferred only of
their temperatures are not equal. The SI unit of energy is Joule and is denoted by the symbol J.
An electric heater is an electrical appliance that converts electrical energy into heat. The
heating element inside every electric heater is simply an electrical resistor, and works on the
principle of Joule heating: an electric current through a resistor converts electrical energy
into heat energy.
Joule’s first law, also known as the Joule effect or Joule heating, is a physical law
expressing the relationship between the heat generated by the current flowing through a
conductor. It is named for James Prescott Joule who studied the phenomenon in the 1840s. It is
expressed as:
where Q is the heat generated by a constant current I flowing through a conductor of
electrical resistance R, for a time t. The amount of generated heat is expressed in energy units:
1 J = 1 Ws = 0,239 cal.
Joule heating forms the basis for the myriad of practical
applications involving electric heating. However, in applications where
heating is an unwanted by-product of current use (e.g. load losses in
electrical transformers) the diversion of energy is often referred to as
resistive losses. Some electrical components have to be cooled so that
the excessive heat is dissipated and no damage is caused to the
component.
The heat obtained from electric energy is known for its safety and cleanliness as no fuel is
burned. Heat produced in this manner is mostly used in households and in industrial applications.
Types of devices utilizing electric
heat:
Electric heating devices  Electric cooling devices
Heat energy in technology:
1. Joule heating – is created when current flows through a resistor (or load).
2. Heat radiation – the carries is infrared radiation. Every object warmer than 0 K emits
infrared radiation.
3. Electric arc – source of heat with the highest temperature – is created by electric current
flow in the air between electrodes. The temperature is between 3000 and 6000 degrees
Celsius.
4. Dielectric loss – is created within dielectric insulation placed in an electric field. It is used
to warm and mold electrical insulators.
5. Eddy currents – are induced in metal objects placed in alternating magnetic current.
6. Hysteresis loss – is created in objects made from metallic magnetic materials placed in
alternating magnetic field created by alternating current (these loses are converted into
heat).
Transferring heat
a) Conduction
Heat is conducted between two solid objects when there is a heat differential between
them. Heat can be transferred directly (by touching) or indirectly through air or other
medium.
b) Convection
Convection is transfer of heat from solid object to a liquid or gas. The movement of the
liquid or gas can be natural or forced (e.g. using a pump or a fan).
c) Radiation
Transfer of heat between hotter and cooler objects by heat radiation (with wavelength
of λ = 0,75 to 10 µm). The source of radiation is therefore any object warmer than 0 K.
Economically, electric heat is very efficient, and can be compared to other sources of home
heating by calculating the cost per kilowatt hour multiplied by the efficiency of the heater, and
then multiplied by the number of kilowatts the heater uses.
Advantages of electric heating:
a) simple way of controlling the heat output,
b) automatic or programmable heat output,
c) finer gradient of heat output,
d) heat can be generated rapidly,
e) lower losses,
g) cleanliness,
h) smaller footprint for heating devices and source,
i) greater efficiency.
VOCABULARY
subsequently – následne
myriad of – nespočentné
expressing – vyjadrujúca
relationship – vzťah, pomer, súvislosť
phenomenon – jav, úkaz
radiation – sálanie
refrigerator – chladnička
fridge – chladnička
icebox – chladnička
involving – zahŕňajúce
conduction – vedenie
convection –prúdenie