Brightness of a Bulb: Power vs. Current Explained

[quantum123]

What does the brightness of a bulb depend on: power or current?

 

[Bloodthunder]

Increasing the current increases the brightness. However, P = IV, so you could increase either one.

 

[russ_watters]

Incandescent bulbs glow because they are hot...

 

[KingNothing]

Maybe you should do an experiment: hook up a bulb to a constant voltage source, and vary the current while holding power constant.

 

[Bloodthunder]

Maybe you should do an experiment: hook up a bulb to a constant voltage source, and vary the current while holding power constant.

Can't do that. P = IV, so if V is constant, whatever changes you make to I will change P as well.

 

[KingNothing]

Can't do that. P = IV, so if V is constant, whatever changes you make to I will change P as well.

Yes...that was my point :). To make the original poster see the silliness of his or her question.

 

[Bloodthunder]

Yes...that was my point :). To make the original poster see the silliness of his or her question.

Oh... ^_^

 

[Physicist1231]

I'm not that great with electronics but doesn't have something to do with how much resistance the filiment is able to take? The more current you put into a filiment the brigher but at the same time if you put the same current into a more resistant filiment it too would be brighter?

Of course it may not last as long but still. Just curious about that one. Any ideas?

 

[Drakkith]

I'm not that great with electronics but doesn't have something to do with how much resistance the filiment is able to take? The more current you put into a filiment the brigher but at the same time if you put the same current into a more resistant filiment it too would be brighter?

Of course it may not last as long but still. Just curious about that one. Any ideas?

I think that is correct, but in the terms of the OP's original question it was assumed that the bulb had the same filament throughout.

 

[ASD16]

the brightness depends on power...so whther u increase the current or the voltage the brightness will increase...

 

[quantum123]

Imagine you put up a voltage of 1,000,000 volts, but only allow one electron to pass through per second. Will the lamp be bright?

 

[pallidin]

Imagine you put up a voltage of 1,000,000 volts, but only allow one electron to pass through per second. Will the lamp be bright?

This is not a yes or no answer.
Some light emitting devices rely on voltage as the primary determinant; others rely on current flow.
Thus, arguing this can become pointless unless you reference a specific light-emitting device.

 

[Dr Lots-o'watts]

Imagine you put up a voltage of 1,000,000 volts, but only allow one electron to pass through per second. Will the lamp be bright?

Uhh... But I=V/R... It's the voltage that "allows" the current.

 

[AtomicJoe]

What does the brightness of a bulb depend on: power or current?

Yes it depends on current and power or both.

The resistance of a bulb is fixed.

Power = I^2 R

The brightness depends on the square of the current and is directionally proportional to the power.

Double the power and you have 2 times the brightness.

Double the current and you have 4 times the brightness!

 

[SpectraCat]

[STRIKE]It's easy enough to check .. get a power supply that can give constant current (like a potentiostat), and monitor the brightness as a function of voltage. As pointed out, for an incandescent bulb, it is the temperature that determines the brightness, so I expect you will find that it is the power, rather than the current, which is important. Once you get the filament hot enough that it starts emitting in the visible spectrum, I think you will find it gets brighter as you continue to increase the voltage at constant current. OTOH, if you have an quantum-based emitter, like an LED, which has a threshold voltage for emission, then I would expect the current to be more important, provided that you are above the threshold voltage for emission.[/STRIKE]

EDIT: Wow .. I guess I need to go to bed .. please disregard the above. I was treating resistance as if it were a free parameter, and of course it is not .. it is fixed for a given electric load (i.e. a bulb).

 

[AtomicJoe]

Imagine you put up a voltage of 1,000,000 volts, but only allow one electron to pass through per second. Will the lamp be bright?

No.

Power is I^2 R. The resistance is fixed, for one electron the current is tiny.

Any experiment to achieve this would be to put the voltage across a brick in series with the bulb, or alternatively leave the bulb switched off!

 

[AtomicJoe]

Interesting fact of the day, if you take a 50W a 100W and a 150W bulb and connect them in series (not parallel as normal) the 50W bulb will be brightest!

 

[AtomicJoe]

Yes a good experiment would be to put the voltage across a brick and see how bright it gets. Answer not very!

 

[SpectraCat]

No.

Power is I^2 R. The resistance is fixed, for one electron the current is tiny.

Any experiment to achieve this would be to put the voltage across a brick in series with the bulb, or alternatively leave the bulb switched off!

Hmmm .. you are right in practice of course, but in principle V=IR, so P=IV, which implies that even for a current of 10^-18 Amps (which is about 1 electron per second), a 60 W bulb would glow if you ran the current at a potential difference of 60x10^18 Volts. The issue is that such high voltages are not achievable in practice. However, in principle it shouldn't matter if he runs the bulb at 1 mA and 60 kV, or 1 A and 60 V. Both amount to 60 W, and so should produce the same brightness.

EDIT: This is wrong too .. for the same reason that my earlier post is wrong. Resistance is constant, as AtomicJoe says below, so power and current are unavoidably coupled for this kind of question.

 

[AtomicJoe]

Hmmm .. you are right in practice of course, but in principle V=IR, so P=IV, which implies that even for a current of 10^-18 Amps (which is about 1 electron per second), a 60 W bulb would glow if you ran the current at a potential difference of 60x10^18 Volts. The issue is that such high voltages are not achievable in practice. However, in principle it shouldn't matter if he runs the bulb at 1 mA and 60 kV, or 1 A and 60 V. Both amount to 60 W, and so should produce the same brightness.

The only thing which defines a bulb is it's resistance, we only rate them in watts because we use a standard household voltage.


If you put 60x10^18 Volts across a 60W bulb it will either blow or become a new star.

 

[quantum123]

Pleae note that resistance is not constant, as the tungsten filament is not Ohmic.

 

[ASD16]

absolutely correct...that thought experiment is rather...i would say...ridiculous...

 

[KingNothing]

Pleae note that resistance is not constant, as the tungsten filament is not Ohmic.

Actually it is ohmic at any given point in time. I disagree with calling something "non-ohmic" because it's resistance depends on temperature. In practice almost every device's characteristics change to some degree as temperature changes.

 

[quantum123]

Ohmic means obey ohms law. The characteristic curve is a straight line through origin. the filament does not obey this.