Question
In an experiment to test whether a liquid conducts electricity, why is an LED preferred over a regular electric bulb as an indicator? Which shows a weaker current better?
Solution — Step by Step
Both an LED (Light Emitting Diode) and a filament bulb glow when current passes through them. The question is: which one glows at lower current levels?
This matters in electrolysis experiments where some liquids (like weak electrolytes) allow only a small current to flow.
A filament bulb produces light by heating a tungsten filament to very high temperatures until it glows (incandescence). For this to happen, the filament needs significant current — typically several hundred milliamperes. If the current is low, the filament barely heats up and gives no visible light.
This makes a filament bulb unsuitable for detecting weak currents.
An LED produces light through electroluminescence — electrons recombine with holes in the semiconductor, releasing energy directly as photons of light. No heating is required. An LED can glow visibly with currents as low as a few milliamperes (mA).
This is why LEDs are so efficient — almost no energy is wasted as heat.
| Feature | Filament Bulb | LED |
|---|---|---|
| Mechanism | Heating (incandescence) | Electroluminescence |
| Minimum current to glow | High (~100–500 mA) | Very low (~5–20 mA) |
| Detects weak currents? | No | Yes |
| Energy efficiency | Low | High |
| Suitable for electrolysis test | Only for strong conductors | For both weak and strong conductors |
When we test lemon juice or dilute salt water, the current through the circuit is small. A filament bulb will not glow — the student might mistakenly conclude the liquid does not conduct. An LED, by contrast, will glow even with this small current, correctly indicating conductivity.
Answer: An LED is better than a filament bulb for detecting current in conductivity experiments because it glows at much lower current values.
Why This Works
The key difference is the mechanism of light production. The bulb converts electrical energy → heat → light (two-step, inefficient). The LED converts electrical energy → light directly (one-step, efficient). Because the LED doesn’t need to heat anything, it responds to much smaller currents.
This same principle is why LEDs replaced incandescent bulbs in everyday lighting — they do more with less current.
Alternative Method
You can also connect a sensitive galvanometer (current meter) instead of an LED or bulb. A galvanometer shows a deflection even for microampere currents, but it is more expensive and fragile. For classroom demonstrations, an LED is the practical choice.
In CBSE Class 8 experiments, always use an LED in conductivity testers. When asked “which glows brighter” — a bulb in a circuit with strong electrolyte (like NaCl) vs weak electrolyte (like distilled water) — the bulb only glows in the NaCl circuit. An LED would glow in both, but more brightly for NaCl.
Common Mistake
Students sometimes write that an LED “doesn’t need electricity” or “produces less electricity.” This is wrong. An LED needs the same voltage source — it just consumes far less current. Also, don’t confuse current with voltage; a liquid’s conductivity is shown by how much current flows, not by the voltage applied.