Last day I was going through a class 6 school science textbook. I
was reading the chapter on 'electricity' where there was a discussion on the
kinds of materials used in different electrical apparatus like iron box,
heating coils, water heaters, bulb filaments, etc. It was mentioned in the
section, certain materials are used as heating elements since they have high
electrical resistance. I just wondered if a class 6 student will be able
to understand the relation between electrical resistance and heating. So i
sent a short note on resistive heating to the teachers' manual as an explanation.
Here is a 'not so short' version of that:
What is Resistance?
We know that (electric) current is the flow of electric
charges in a circuit. In electric circuits, this charge is carried by the
moving electrons through a wire driven by the potential difference between the
ends of the wire. Often, an electron moving through an electric circuit encounters
hindrance to its flow due to its many collisions with the atoms in the
conducting wire. This is called resistance. There are numerous factors which
causes this resistance.
First is the length of the wire. The longer the wire, more
are the chances of collisions and hence, more resistance.
Second, the cross-sectional area of the wire; thinner wires
offer more resistance to the flow (just like a thinner tube providing more
resistance to the flow of water through it compared to a wider tube).
Third, the material used. Some materials offer less
resistance to the flow of charge. Meaning, they are better conductors. Examples
include copper and aluminium used in household circuits
Ohm's Law
Ohm's law states that the current through a conductor is directly
proportional to the potential difference between the two ends. The law can be
written in terms of resistance as,
I = V/R
where I is the current through the conductor and V is the
potential difference measured across the conductor, R being the resistance of
the conductor. The equation says that greater the voltage supplied to the
circuit, greater will be the current flowing through and larger the resistance,
lesser will be the current flowing through.
Joule Heating
Now, let us come to the heating part. We just saw that
materials or elements with resistance (or simply resistors) oppose the flow of
electric current. So, the electrical energy needs to push the current through
the material which results in the current converting the electrical energy to
heat. This is called Joule heating or ohmic heating or resistive heating.
We know that heat is the kinetic energy of the particles as
they vibrate and collide with other particles. With an increase in voltage, for
a material with high resistance, there will be an increased collision of the
flowing electrons with the atoms in the material. This results in the heating
of the resistor.
This dissipation of electrical energy is often undesired, like
in the case of transmission losses in power lines. On the other hand, Joule
heating is sometimes useful: examples include electric stoves and other electric
heaters. The electric heaters are also called resistive heaters.
Another example of resistive heating is incandescence.
Incandescence is the emission of light from a hot body (in general, emission of
electromagnetic radiation from a hot body). Incandescent lamps and the electric
heaters rely on Joule heating: the filament is heated to such a high
temperature that it glows 'white hot'. Higher the temperature the material
attains, the brighter it will be due to the increased emission of the EM light
which in turn is due to the increased vibration and collision of charges.
Incandescent light bulb.
