Why must the material of heating element of an electric heater habe high resistivity ? I mean, more resistivity, more resistance and hence less current and current is directly proportional to heating effect. And ive read that it is because if current is increased then resistance will decrease but heating effect is proportional to square of current and single term of resistance. So if high resistivity then very less current right? In that case heating effect will be less too. Decrease in current should decrease heat more than decrease in resistance does. So why are high resistivity materials used then?
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$\begingroup$ My best guess involves materials/safety concerns. Most heating elements glow orange rather than white hot. Current must be somewhat limited to keep the heating element in a narrow temperature range (where it glows orange). Another consideration - the heating element usually exists in an "open air" atmosphere. If the temperature gets too high, the heating element will oxidize (e.g., burn up). $\endgroup$– David WhiteCommented Jul 3, 2018 at 18:05
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$\begingroup$ Heat loss in a conductor goes with the square of the current, but is directly proportional to resistance. If there is no resistance, then no heat loss. Note the word 'loss'. Currents induced in a superconductor have no potential across the superconductor, no loss. The energy is trapped within a field. When conductor atoms start jiggling they give up energy as radiated heat. More resistance, more impedance, more jiggling. $\endgroup$– docscienceCommented Jul 9, 2018 at 4:15
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3 Answers
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Whatever current is flowing in the heating element will also have to flow in the wires leading to the heating element, including the wires in a transformer, which could be very long.
So, if we want to minimize heat losses in the wires, the resistance of the heating element has to be substantially higher than the resistance of the wires and therefore, for a reasonable geometry of the heating element, its material has to have high resistivity.
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There is no 'high resistivity' requirement for generating heat by electrical current. A soldering gun heating element may be a fraction of an ohm, and dissipate 50 watts, or a lamp filament may be 290 ohms, and dissipate 50 watts.
There is, however, a practical matter of delivering the current to the heater: thin wire and 120V delivers the current to that lamp filament, while a massive copper tube (the secondary winding) of the soldering gun transformer delivers much higher current.
For efficiency (and minimization of expensive metal), a complete circuit with a heater must put more energy into the heater element than into the rest of the circuit wiring.
A heater must also draw SOME current, or it is 'turned off'. The minimum (zero resistance) and maximum (very high resistance) values are both bad heater values. One blows fuses, the other leaves you cold.
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The premise is incorrect; it's not true that one should maximize the resistivity of the heater.
To maximize power transfer (which is what the temperature increase generally scales with), one should match the load impedance to the source impedance. This is the happy medium between a low-resistance load (which doesn't generate much Joule heating) and a high-resistance load (which reduces the current substantially for a given voltage, as you note).
To increase efficiency, one should also minimize the source impedance that one is matching the load impedance to.
answered Jul 8, 2018 at 23:48