Bulb
Comprehensive notes, formulas, and practice questions for Bulb.
Bulb
Electric Bulb & Circuit
What you'll learn
- A cell (battery) provides energy to push electric current through a circuit.
- How a bulb glows when current passes through its filament (or LED in modern devices).
- To build a complete circuit — unbroken path from cell → bulb → back to cell.
- Symbols for cell, bulb, and connecting wires in circuit diagrams (intro).
Key concepts
Level 1 — Complete vs broken circuit
Verbal: Current flows only in a closed loop. If the path breaks, the bulb does not glow.
Visual (complete circuit): (+) Cell (−) → wire → Bulb → wire → back to cell. Closed loop → bulb glows.
Open circuit: Loose wire, burnt bulb, or missing connection → no glow.
Cell terminals: Positive (+) and negative (−) — connect so current can circulate (Class 6 uses one cell and one bulb).
| Condition | Bulb |
|---|---|
| Closed path | Glows |
| Open anywhere | Off |
| Cell exhausted | Dim or off |
Level 2 — Filament, brightness, and safety
Incandescent bulb: Thin filament heats up and emits light when current passes.
LED bulb (modern): Light-emitting diode — lower heat, used in torches and study lamps.
Series (two bulbs, one path): Both glow dimmer than one bulb alone — same current shared.
Safety: Never experiment with mains electricity (230 V at home). Class 6 uses 1.5 V or 9 V cells only.
Circuit diagram symbols: Cell (long/short line), bulb (circle with cross), wire (straight line).
Worked example
One bulb glows; adding a second bulb in series makes both dim. Why?
Step 1 — Same cell voltage now shared across two bulbs
Step 2 — Each filament gets less energy per second
Step 3 — Both glow, but dimmer than single-bulb circuit
Step 4 — If one bulb removed (open), series path breaks → both off
Common mistakes
| Mistake | Why it happens | Fix |
|---|---|---|
| Bulb glows without return wire | Thinking one wire enough | Need complete loop to cell |
| Connecting only one terminal | Partial contact | Both bulb terminals must be in circuit |
| Using household mains for school activity | Confusion with "electricity" | Use low-voltage cells only |
| Reversing cell stops all current (single cell) | Misconception | Bulb still glows; polarity matters for LEDs later |
Quick check
- What is a complete (closed) circuit?
- Why does an electric bulb glow?
- Draw a simple circuit with one cell and one bulb using symbols.
- What happens if a wire is loose in the circuit?
Open the Practice tab for graded questions on Electric Bulb & Circuit.
Key Takeaways (TL;DR)
- What you'll learn
- Key concepts
- Worked example
- Common mistakes
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