What’s cooking?

An incandescent light bulb

Image via Wikipedia

Ever wondered how an electric incandescent bulb, an electric room heater or an electric stove worked?  Ever wondered how they produce so much heat and light?  Today, we will explore the working of electric bulbs and heaters.  I will given an overview of how electric power is converted to heat and light.  As a bonus, here is a quick poll to give you a teaser.

Solution

Counter intuitively, an electric bulb operates at a much higher temperature than that of an electric stove.

What is electricity?

Electricity is, roughly speaking, the movement of electric charge.  It is quantified through a physical quantity called electric current.  Current at a point on a wire is defined as the net electric charge that flows across that point per unit time.  Current is usually measured in terms of a unit called ampere.  This is a fancy name for a certain number of electric charges flowing past a point on a wire per second.  In common conductive wires (such as copper wires), the charge that moves is the electron.  Now, as we know, all common materials on earth are made of atoms and molecules.  Atoms consist of a core, a nucleus, consisting of protons and neutrons, and a cloud of electrons that surround the nucleus.  When electrons flow in a wire, they come detached from their nucleus and move around the wire freely (like a gas).  They don’t glide gently through the wire though and follow jagged paths.  Electrons in a wire travel extremely rapidly – approaching a fraction of the speed of light.  However, without any external influence, the electrons move in independent random directions, and on average, don’t cause a current.  If externally compelled to, the electrons then drift preferentially towards one side at a gentle speed.  We can set this up by using an external power source such as a battery or a power generator.

Viva La Resistance!

It helps to think of electrons drifting through wire as similar to wind.  Air molecules move really rapidly.  They are constantly in motion in completely random directions, making air not move in general.  Nevertheless, it is possible to gently make them, on average, move in one direction more than the other.  This is known as a wind when it occurs in air and a current when it occurs with electrons in a wire.  When a wind encounters a barrier, the air deflects around the barrier, possibly slowing down due to friction.  Similarly, when electrons are drifting in one direction and picking up speed, they slam into atoms and other electrons and bounce.  This “bounce” causes energy to be released and the electron slow down during the process.  There is no good way to explain why this occurs – a meaningful explanation would require an understanding of quantum mechanics.  The amount of “bounce” that a wire induces in known as the resistance of a wire.  Resistance is a function of the type of material used to make the wire, the geometry of the wire (how thick or long it is) and the temperature of the wire.  Thick, short wires offer lesser resistance.  Thin, long wires offer greater resistance.

Radiation

A side effect of all this rattling and the ensuing subatomic commotion is a release of electromagnetic radiation.  What is radiation?  It is a word that is heard in the media a lot, sometimes as an evil villain.  Electromagnetic (EM) radiation is a form of energy that is associated with vibrating electric and magnetic fields.  The faster the fields are changing, the stronger the radiation.  The slower they change, the weaker the radiation.  EM Radiation comes in all forms.  The weakest forms are radio waves.  Microwaves are a bit stronger and pack more punch.  The next step up is infra-red radiation, which we humans feel as heat.  Then comes visible light, with red being the weakest (least energetic) and blue being the strongest.  Higher radiation comes in various flavors such as UV rays, X-rays and Gamma rays in order of increasing energy.

Hot Hot Hot

When our rattling electron releases EM radiation, it is one of the above types of radiation.  The exact form of energy output depends on the temperature of the material and is mostly as infra-red radiation which we perceive as heat.  The hotter the material, the more high energy radiation it is giving off.  Hence, an electric stove, which operates at 300-800°C, emits most of its energy in the form of infra-red radiation a.k.a. heat and maybe a little in the form of light when it is glowing red-hot.  An incandescent light bulb operates at about 2500-3000°C.  It still gives off most of its energy in the form of heat, but a larger portion of it is emitted as light.  We can see from this that, counter intuitively, a simple light bulb operates at a far greater temperature than a stove.  We can rationalize this by realizing that temperature is not a measure of the energy output of a device.  A stove or heater consumes many kilowatts of power whereas a light bulb consumes perhaps 100W.  Hence, a stove, while consuming (and delivering) over ten times as much power, operates at less than a third of the temperature of a light bulb.

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