A note on Electric Heating…

Heating coil in an oven.

Heating coil in an oven.

Somebody asked me last day about the relations between electrical resistance and heating and light emission. So I thought i would rather give a short note on resistive heating as a reply. Here is a ‘not so short’ version of that reply:

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 = \frac{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. The amount of heat released is given by
Q \propto I^2 \cdot R
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.

Incandescent light bulb.

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