# Thread: Lightbulbs in series relative brightness

1. ## Lightbulbs in series relative brightness

Hi electronic experts,
My daughter is doing high school physics and in an electronics test got a question about relative brightess of light bulbs in series.

She answered that all bulbs are of equal brightness, which I reckon is correct. Her answer was marked wrong. We have questioned the teacher, who responed "In series circuit the current that goes through each light bulb is the same. However, since light bulbs are resistors, as electrons pass through each light bulb some energy is lost to light (and heat) and as such, bulb 5 will be the brightest and bulb 1 the dimmest."

I thought than in a series circuit, current was the same all the way and all resistors evenly divided the voltage. This would mean all bulbs recieved the same current and voltage, therefore have the same brightness. Am I correct?

If anyone has links, I would really appreciate it.

Cheers
Chook

2. ## Re: Lightbulbs in series relative brightness

Originally Posted by ChookChoker
Hi electronic experts,
My daughter is doing high school physics and in an electronics test got a question about relative brightess of light bulbs in series.

She answered that all bulbs are of equal brightness, which I reckon is correct. Her answer was marked wrong. We have questioned the teacher, who responed "In series circuit the current that goes through each light bulb is the same. However, since light bulbs are resistors, as electrons pass through each light bulb some energy is lost to light (and heat) and as such, bulb 5 will be the brightest and bulb 1 the dimmest."

I thought than in a series circuit, current was the same all the way and all resistors evenly divided the voltage. This would mean all bulbs recieved the same current and voltage, therefore have the same brightness. Am I correct?

If anyone has links, I would really appreciate it.

Cheers
Chook
You are right and the teacher is wrong. But handling this correctly is important and not easy. In a series circuit, each device gets the same current. If all devices are identical, they have the same voltage drop across each - a 12v source would give 1v to each of 12 bulbs in series. Since power is Volts times Amps, then each bulb will dissipate the same amount of power and emit the same amount of light.

Any resource on series circuits will back you up. Here's one with pictures specific to light bulbs

3. ## Re: Lightbulbs in series relative brightness

Thanks Apple.

4. ## Re: Lightbulbs in series relative brightness

If you have some christmas lights, you could show her on the cheap that you are right. The rated voltage for individual christmas bulbs can be found by dividing 120v (house AC voltage) by the number of bulbs. If the bulbs aren't identical then no conclusion can be made. But assuming identical bulbs, they'll get the same voltage and current.

Here are more resources.

This is close to what you want. "The supply voltage is divided equally between the lamps" does not come right out and say that "These identical lamps are of identical brightness" but you can reason it out. Also, the electrons are not "used up." Electrons are not created or destroyed in home circuitry. Any electron you put in a circuit on one side comes out the other. Any electron going through one bulb goes through the next one in series. That is, the same number of electrons flow out of the (-) terminal of the battery, through the wire, through each bulb, and to the (+) terminal of the battery. "Current direction" is opposite of electron direction because some famous guy had to pick, and he picked wrong way back then.

5. ## Re: Lightbulbs in series relative brightness

Originally Posted by ChookChoker
...the teacher, who responed "In series circuit the current that goes through each light bulb is the same.
Correct.

Originally Posted by ChookChoker
However, since light bulbs are resistors, as electrons pass through each light bulb some energy is lost to light (and heat) and as such, bulb 5 will be the brightest and bulb 1 the dimmest."
No wonder I can never water my garden.

Originally Posted by ChookChoker
I thought than in a series circuit, current was the same all the way
Yes

Originally Posted by ChookChoker
and all resistors evenly divided the voltage. This would mean all bulbs recieved the same current and voltage, therefore have the same brightness.
This is true, if and only if the resistances are all equal.

You may, for example, string a mixture of 120V worth of 3V, 6V, 12V and 24V bulbs, all rated at the same operating current accross your 120V supply, and all bulbs will run at their design voltages, but the brightnesses will be very different.

Or in the Christmas tree light example, a string of identical bulbs will all light up with equal brightness, whether fed with DC or AC.

If anyone has links, I would really appreciate it.

Cheers
Chook[/QUOTE]

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