Question about how light bulb brightness varies when an AC supply is used

• ellieee
In summary, the conversation revolved around the observation of a flashing light bulb when using an AC supply. The experts discussed the physics behind this phenomenon and explained that the flashing is due to the energy produced in the bulb, which has double the frequency of the supplied voltage or current. They also mentioned that the thermal time constant of the filament and the flicker fusion rate of the eye play a role in the perception of the flashing. Additionally, there was a discussion about the difference between an incandescent light bulb and an LED, with the experts noting that the amount of light produced is proportional to the current in an LED, while it is not affected by the direction of the current in a light bulb.
ellieee
Homework Statement
in pic
just wanted to check if my answer is acceptable
Relevant Equations
nil

ellieee said:
Homework Statement:: State and explain your observations clearly.
just wanted to check if my answer is acceptable
Relevant Equations:: nil
Did you actually observe that the bulb was flashing?
The 2 coils of wire are really just a bad transformer, and if you input 50 or 60 Hz AC, you will get 50 or 60 Hz AC out. An incandescent lamp will actually flash at twice the frequency, so 100 or 120 Hz. (if the ac voltage is proportional to sin( at), the energy produced in the lamp is sin2(at). and this has double the frequency).
I don't think you can see that with the naked eye. You can probably with a phone camera, if you look at individual pictures.

Delta2
willem2 said:
the energy produced in the lamp is sin2(at). and this has double the frequency).
Also there is a thermal time constant for any filament that is longer usually much longer than the 10 ms time of the AC. The light will not "blink". It will flicker slightly at the 100 (or 120) Hz.

Your thread title says "how light bulb brightness varies when an AC supply is used" and yet I don't see anything in the question that suggests there will be any visible variation other than on and off- Is that your interpretation of the question or was it explicitly stated in the first part of the question ?

The answer required here will depend on the level of the course being followed, but this could easily be taken from an A-level paper. In that case, the answer would include the following points:

When the switch is closed an alternating current flows in the solenoid
This produces an alternating (or changing) magnetic field
Therefore there is a changing flux linkage in the coil
Therefore an alternating emf is induced in the coil
Since there is a closed circuit, the induced emf produces a current and the bulb lights up.

then of course, when the ac supply is switched off, all of this stops.

I wanted to point out that the light will appear "on" and not "blinking". In fact there are two reasons: the thermal issues I mentioned and the flicker fusion rate of your eye.
rsk said:
Therefore there is a changing flux linkage in the coil
I believe this is incorrect use of the term "flux linkage". It describes the flux geometry. There will be a changing flux thru the secondary coil. The flux linkage changes if you move the coil. Not a huge deal

rsk
rsk
hutchphd said:
I wanted to point out that the light will appear "on" and not "blinking". In fact there are two reasons: the thermal issues I mentioned and the flicker fusion rate of your eye.

I believe this is incorrect use of the term "flux linkage". It describes the flux geometry. There will be a changing flux thru the secondary coil. The flux linkage changes if you move the coil. Not a huge deal
Noted, thanks :)

hutchphd
willem2 said:
An incandescent lamp will actually flash at twice the frequency, so 100 or 120 Hz. (if the ac voltage is proportional to sin( at), the energy produced in the lamp is sin2(at). and this has double the frequency).
Why are you saying that the bulb will flash at the frequency of the supplied energy? (which is indeed double the frequency of the supplied voltage or current). The way I know it the bulb blinks at the frequency of the supplied current, however it will not blink due to thermal considerations as @hutchphd notes.

kuruman said:
Watch this demo.
That's fab - so simple but so effective.

Delta2 said:
Why are you saying that the bulb will flash at the frequency of the supplied energy? (which is indeed double the frequency of the supplied voltage or current). The way I know it the bulb blinks at the frequency of the supplied current, however it will not blink due to thermal considerations as @hutchphd notes.
Because the energy is what is heating up the bulb making it glow. Also the direction of the current doesn't matter for the amount of light produced.
If you used a single LED with a resistor, The amount of light would be proportional to the current, and you would only get half the frequency of incandescent light bulbs.

willem2 said:
Because the energy is what is heating up the bulb making it glow. Also the direction of the current doesn't matter for the amount of light produced.
If you used a single LED with a resistor, The amount of light would be proportional to the current, and you would only get half the frequency of incandescent light bulbs.
What's the difference between a LED and a bulb?

willem2 said:
Because the energy is what is heating up the bulb making it glow. Also the direction of the current doesn't matter for the amount of light produced.
If you used a single LED with a resistor, The amount of light would be proportional to the current, and you would only get half the frequency of incandescent light bulbs.
But the incandescent bulb light output is mostly DC with a small (<10% typically) 120 Hz ripple on top because of the thermal response time. An LED response at this time scale is instant (I have driven them at 70 MHz successfully) and they will be off for half the cycle.

hutchphd said:
But the incandescent bulb light output is mostly DC with a small (<10% typically) 120 Hz ripple on top because of the thermal response time.

​

(This is 50 cycle, 100Hz)

hutchphd

1. How does the brightness of a light bulb change when using an AC supply?

The brightness of a light bulb varies when using an AC supply due to the fluctuation of the alternating current. As the current changes direction, the filament in the bulb heats up and cools down, causing the brightness to fluctuate.

2. Is there a difference in brightness between using an AC supply and a DC supply?

Yes, there is a difference in brightness between using an AC supply and a DC supply. This is because DC current flows in one direction, providing a steady flow of energy to the filament, resulting in a consistent brightness. AC current, on the other hand, changes direction, causing the filament to heat up and cool down, resulting in a fluctuation of brightness.

3. Does the voltage of the AC supply affect the brightness of the light bulb?

Yes, the voltage of the AC supply does affect the brightness of the light bulb. The higher the voltage, the brighter the bulb will be. This is because a higher voltage results in a higher current flow, providing more energy to the filament, causing it to glow brighter.

4. Why does the brightness of a light bulb decrease over time when using an AC supply?

The brightness of a light bulb decreases over time when using an AC supply due to the gradual weakening of the filament. As the filament heats up and cools down repeatedly, it weakens and eventually breaks, resulting in a decrease in brightness.

5. Can using a dimmer switch affect the brightness of a light bulb when using an AC supply?

Yes, using a dimmer switch can affect the brightness of a light bulb when using an AC supply. Dimmer switches work by reducing the voltage and current flow to the bulb, resulting in a decrease in brightness. This is why the bulb may appear dimmer when a dimmer switch is used with an AC supply compared to a regular on/off switch.

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