When learning basic electronics, one of the most mind-twisting but fascinating questions is:
“If a 60W and 100W bulb are connected in series, which one will glow brighter?”
Most people assume the 100W bulb will glow brighter—after all, it’s rated for more power, right?
But in a series circuit, the rules change. Let’s break it down and understand why the 60W bulb actually glows more.
🔍 Understanding the Basics
Each bulb has a power rating given for a specific voltage (typically 220V or 110V). The wattage rating tells us how much power the bulb consumes when connected to its rated voltage.
For example:
- 100W bulb @ 220V
Resistance R=V2P=2202100=484 ΩR = \frac{V^2}{P} = \frac{220^2}{100} = 484 \, \OmegaR=PV2=1002202=484Ω - 60W bulb @ 220V
Resistance R=220260≈806.7 ΩR = \frac{220^2}{60} \approx 806.7 \, \OmegaR=602202≈806.7Ω
So, the 60W bulb has higher resistance than the 100W bulb.
🔌 What Happens in a Series Connection?
In a series circuit:
- Current is the same through all components.
- Voltage is divided based on resistance.
Since the 60W bulb has higher resistance, it gets a higher voltage drop across it.
🔥 Power in Series
Power consumed by a component = P=I2×RP = I^2 \times RP=I2×R
And since current (I) is the same in both bulbs, the bulb with higher resistance consumes more power in this case.
Thus, 60W bulb (with higher resistance) consumes more power and glows brighter in series!
💥 Common Misconception
People often think “100W is more powerful, so it should glow more.” But remember:
- Wattage rating is for normal operation at full voltage.
- In a series circuit, both bulbs share voltage unequally.
- Brightness depends on actual power consumed, not rating.
🧠 Final Answer:
✅ The 60W bulb glows brighter than the 100W bulb when connected in series.
🔧 Real-World Tip
Never connect bulbs of different wattages in series for regular use. It’s inefficient and may reduce the life of both bulbs due to unequal voltage stress.