What it shows
In a series circuit the same current flows through every bulb and the voltage is shared, so each bulb glows dimly and all go out if one is removed. In parallel each bulb gets the full cell voltage, glows brightly, and the others stay lit when one is removed.
The science
Resistances in series add up (R = R1 + R2 + R3), so three bulbs in series draw a third of the current one bulb would, and each receives one third of the cell voltage. In parallel the reciprocals add (1/R = 1/R1 + 1/R2 + 1/R3), the total resistance is lower than any single bulb, each bulb has the full 3 V across it, and the cell supplies three times the current of one bulb. House wiring is parallel for exactly this reason.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Six 2.5 V torch bulbs with holders (about Rs 10 each)
- Two AA cells in a holder (3 V)
- A cardboard or plywood board about 30 cm by 40 cm
- Insulated single-strand wire, 3 m
- Three slide switches or paper-clip switches
- A multimeter to read current and voltage
- Drawing pins and glue
Safety
For this project: Cells only, never mains. Bulbs and wires get slightly warm; switch off when not demonstrating. An adult must help with the cutter when stripping wire.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Draw the two circuits on the board in pencil: three bulbs in a single loop on the left, three bulbs on parallel rungs like a ladder on the right.
Fix the bulb holders with glue or drawing pins over the drawing, and fix the cell holder at the bottom centre.
Wire the series circuit first, with a switch in the loop, and test it: all three bulbs should glow dimly together.
Wire the parallel circuit with its own switch and test it: all three should glow brightly.
Unscrew one bulb in each circuit while switched on and note what happens to the others; this is the key demonstration.
Use the multimeter on DC volts across each bulb and on DC milliamps in the main line, and record a table: circuit, voltage across bulb, current from cells.
Add a third small section with two bulbs in parallel and then one in series with that pair to show a mixed circuit.
Label every part on the board and write the two resistance formulas beside each circuit.
What you should see
Series bulbs glow dim and share about 1 V each; parallel bulbs glow bright at about 3 V each and the cells supply roughly three times the current. Removing a series bulb switches off the whole loop; removing a parallel bulb leaves the others lit.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why are the lights in a house wired in parallel?
So each gets the full supply voltage and one failing does not switch off the others.
2Which circuit drains the cells faster?
The parallel circuit, because its total resistance is lower and it draws more current.
3What is the total resistance of three equal bulbs R in parallel?
R/3.
4What stays the same for every component in a series circuit?
The current.
Take it further
Replace one bulb with an LED and a 220 ohm resistor and explain why the LED needs the resistor.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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