# Brightness of bulbs depending on their watt level

• momowoo
In summary, two 110-V light bulbs, one "25W" and the other "100W", are connected in series to a 110 V source. The current is the same through both bulbs, eliminating some choices. The 25W bulb has a higher resistance and therefore less current and power when connected in a circuit. This eliminates the option of equal voltage drops. The 100W bulb consumes 100W of electric power only if the potential difference across it is 110 V. When connected in series, the potential difference across each bulb is not 110 V. By analyzing the circuit, we can see that the "100W" bulb actually only dissipates 4W. Therefore, the correct answer is b: the
momowoo

## Homework Statement

Two 110-V light bulbs, one "25W" and the other "100W" are connected in series to a 110 V source. Then:
a. the current in the 100W bulb is greater than in the 25W bulb
b. the 25W bulb will be brighter
c. the current in the 100W bulb is greater than that in the 25W bulb
d. the voltage drop across both bulbs will be the same
e. the 100W bulb will be brighter

P=IV=V^2/R=I^2*R

## The Attempt at a Solution

I thought that the power would tell me which bulb is brighter, so I instantly saw 100W and assumed that bulb would be the brighter one. But the answer is b, which I totally don't understand. I'm guessing it might have to do with resistance, but every time I try to reason with the relevant equations, nothing makes sense to me.

Yeah, it's not a great question, but I think the reasoning is this.

As you know, the current is the same through both, since they are in series, which eliminates a couple of the choices.

The Lower power bulb probably has a higher resistance (hence less current and power when correctly connected in a circuit), so equal voltage drops is out as an answer.

So you are left with which would be brighter when connected in series, which to me is not an obvious thing. Again, though, one has a higher resistance than the other, and the current is the same through both, so which would have the higher power dissipation (so put out the brighter dim light)?

momowoo said:
I thought that the power would tell me which bulb is brighter, so I instantly saw 100W and assumed that bulb would be the brighter one. But the answer is b, which I totally don't understand.
A 100W bulb consumes 100W of electric power only if the potential difference across it is 110 V. When the two bulbs are connected in series across a 110 V source, does each bulb have a potential difference of 110 V?

I'm guessing it might have to do with resistance
Yes, good.

but every time I try to reason with the relevant equations, nothing makes sense to me.
If a bulb is rated at 100W when it has a potential difference of 110 V, can you find the resistance of the bulb?

Resistance would be 110 ohms.

momowoo said:
Resistance would be 110 ohms.
I suggest that you do a full analysis of this circuit, developing all voltages, currents, and power dissipation. The results are quite interesting.

I'm unsure how to go about doing that. Do I use the bulb wattage as the power, or should I not because that isn't the true power they dissipate?

momowoo said:
I'm unsure how to go about doing that. Do I use the bulb wattage as the power, or should I not because that isn't the true power they dissipate?
Analyze each bulb at full voltage to get the resistance and then do a circuit with those two resistances in series. Give us a picture when you've filled in all the values.

momowoo said:
I don't read sideways stuff. Makes my neck hurt.

momowoo said:
Excellent. Interesting to see the "100W" bulb only dissipating 4W, yes?

Yes, very interesting haha. Thank you!

## 1. How does watt level affect the brightness of a bulb?

The watt level of a bulb directly affects its brightness. The higher the watt level, the brighter the bulb will be. This is because watt level measures the amount of energy a bulb uses, and more energy means more light produced.

## 2. Can a lower watt bulb be just as bright as a higher watt bulb?

Yes, it is possible for a lower watt bulb to be just as bright as a higher watt bulb. This is because advances in technology have allowed for bulbs with lower watt levels to produce the same amount of light as bulbs with higher watt levels. This is known as energy efficiency.

## 3. Is watt level the only factor that affects the brightness of a bulb?

No, watt level is not the only factor that affects the brightness of a bulb. The type of bulb, its design, and the lumens (measure of light output) also play a role in determining the brightness of a bulb. For example, a LED bulb may have a lower watt level but produce the same amount of light as a traditional incandescent bulb due to its design and use of technology.

## 4. Do all bulbs with the same watt level produce the same amount of light?

No, not all bulbs with the same watt level produce the same amount of light. As mentioned before, the type of bulb and its design can also affect the brightness. Additionally, the quality and age of the bulb can also impact its brightness. A newer, high-quality bulb may produce more light than an older, lower-quality bulb with the same watt level.

## 5. Is it necessary to use higher watt bulbs to have a brighter room?

No, it is not necessary to use higher watt bulbs to have a brighter room. Instead of focusing on watt level, it is important to look at the lumens when choosing a bulb. Lumens measure the amount of light produced, so a bulb with a higher lumen count will be brighter regardless of its watt level. Additionally, factors such as the placement and number of bulbs in a room can also impact its overall brightness.

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