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Background:

Assume that I have Bulb that consumes 10 watts of electrical energy (power). The operating values for full brightness are: 5 volts, and 2 amps. Since Power(p) = Voltage(V) x Current(I) = 5 x 2 = 10 watts

Question:

For a bulb to glow all it cares for is the flow of electrons at the required rate (amps) --> call it axiom. In the above circuit, assume that I keep the current constant at 2 amps by reducing both the voltage and resistance.

So, by my axiom, I expect the same brightness of the bulb, but the bulb glows at lower brightness. I know the lower brightness is because of lower power since voltage is reduced, but by my axiom: "For a bulb to glow all it cares for is the flow of electrons at the required rate (amps)", shouldn't the bulb remain bright?

Or, is my axiom wrong?

Edit: As many of you have mentined, I accept that usage of the word axiom is wrong in this context. I would rather use the word assumption, however, since many answers to this questions refers to the word axiom, I do not want to change it to assumption, as this would confuse future readers. Thanks.

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    \$\begingroup\$ no brightness is about heat dissipated hence energy. heat is all about power I^2 *R. you keep I constant but decrease R. your intuition is wrong. (that's why they sell bulbs regarding their wattage to relate it to brightness) \$\endgroup\$
    – user16307
    Jun 8 '16 at 17:30
  • \$\begingroup\$ @user16307 It doesn't matter. It won't work even in case of ideal 100% efficient bulbs. \$\endgroup\$
    – Eugene Sh.
    Jun 8 '16 at 17:33
  • \$\begingroup\$ @EugeneSh. do you agree that more photons are ejected with increasing thermal energy? \$\endgroup\$
    – user16307
    Jun 8 '16 at 17:37
  • \$\begingroup\$ The Light-bulb is non-linear device. Also how you want to change the resistance without changing the physical properties of the bulb? Also if the current is kept constant then the voltage across any device is I*R. \$\endgroup\$
    – G36
    Jun 8 '16 at 17:37
  • \$\begingroup\$ @user16307 I do not agree that thermal energy is necessary at all to emit photons. \$\endgroup\$
    – Eugene Sh.
    Jun 8 '16 at 17:39
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For a bulb to glow all it cares for is the flow of electrons at the required rate (amps).

Correct, if the filament gauge remains constant.

Assume that I keep the current constant at 2 amps by reducing both the voltage and resistance.

Reducing the voltage isn't a problem. Reducing the resistance is because you need to shorten the filament length.

If you could make a filament lamp with variable length filament you would get constant brightness per unit length. Total brightness would, of course, drop. Try it and see ...

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. 'Bulb' with variable filament length.

In Figure 1 we've created a lamp with variable filament length. You can adjust the filament resistance by switching out lamps as you step down the voltage.

"For a bulb to glow all it cares for is the flow of electrons at the required rate (amps)", shouldn't the bulb remain bright?

Yes, the filament temperature will remain the same, the light spectrum emitted will be the same and the light emitted per unit length will be the same. There will just be less of it overall.

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  • \$\begingroup\$ "the light emitted will be the same. There will just be less of it." i think it is better to say "the light emitted per unit filament will be the same" imao \$\endgroup\$
    – user16307
    Jun 8 '16 at 17:55
  • \$\begingroup\$ I meant to write "... and the light spectrum emitted will be the same." Thank you. \$\endgroup\$
    – Transistor
    Jun 8 '16 at 17:59
  • \$\begingroup\$ @transistor Thanks, you have answered the question comprehensively. The simulation really helped. \$\endgroup\$ Jun 8 '16 at 18:47
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Don't invent Axioms, as anything you can come up on this level is already invented before you. It can be simply answered in terms of energy conservation, but I'll give you the intuition. By reducing the resistance you have changed the physical properties of the bulb. Like, for instance, the length or the thickness of the filament wire in it. Thus reducing the working area and/or the number of photon generating atoms (roughly speaking).

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  • \$\begingroup\$ Thanks for your answer. Yes, usage of the word axiom is wrong in this context, I have amended my question. \$\endgroup\$ Jun 8 '16 at 18:49

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