Series and Parallel Circuits, Grade 9
Put three cells in parallel instead of in series and nothing happens to the brightness. That is the result, not a mistake.
It is also the practical most likely to be ruined before it starts, because many textbooks offer "two identical light bulbs or LEDs" and with LEDs this experiment produces no light at all, in every row of the table.
This page covers what actually happens to voltage, current and brightness in series and parallel circuits, the six practicals that show it, and the four things worth correcting before you teach it.
Use bulbs, not LEDs
| Activity | With 2,5 V torch bulbs | With LEDs |
|---|---|---|
| One cell, two lamps in series | Dim but visible | Nothing. Two red LEDs need about 3,6 V before they conduct |
| Three cells, two lamps in series | Bright | Lights, but with no resistor to limit the current the LEDs can burn out |
| Cells in parallel | Same brightness every time, which is the point | No light at all, in any row |
An LED is a diode, not a resistor. It conducts one way only, it needs a minimum voltage before it conducts at all, and it needs a resistor in series to survive. A caption in one popular Grade 9 textbook says the opposite, and that is where the LED substitution comes from.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 9 |
| Term | 3 |
| Strand | Energy and change |
| Topic | Topic 13, Series and parallel circuits |
| Status | No formally assessed task. Six practical activities |
| Marks | None prescribed. Our worksheet is 40 marks |
Cells in series: voltage adds up
Build a circuit with two lamps in series. Measure the voltage across the battery and note the brightness. Then add a second cell, then a third.
| Cells in series | Battery voltage | Brightness |
|---|---|---|
| 1 | 1,5 V | Dim |
| 2 | 3,0 V | Brighter |
| 3 | 4,5 V | Brightest |
Cells end to end add their voltages. Two 1,5 V cells make a 3 V battery.
Cells in parallel: nothing changes, and that is the lesson
Same circuit, but the cells side by side instead of end to end.
| Cells in parallel | Battery voltage | Brightness |
|---|---|---|
| 1 | 1,5 V | Normal |
| 2 | 1,5 V | Unchanged |
| 3 | 1,5 V | Unchanged |
Series adds voltage. Parallel adds capacity. Three cells in parallel do not make the lamp brighter, they make the battery last about three times as long.
A class that reports "nothing happened" has done it correctly. The follow-up question is the useful one: if the brightness is the same, what did we gain?
The three rules, and how to measure each one
| Rule | How to show it | What you get |
|---|---|---|
| Current is the same everywhere in a series circuit | Move the ammeter to three different points in the same loop | Three identical readings. Current is not used up |
| Voltages add up in series | Measure across each lamp, then across the battery | The separate voltages sum to the supply |
| Every parallel branch has the full voltage | Measure across each branch and across the battery | All the same |
| Branch currents add up to the main current | An ammeter in each branch and one in the main line | The main reading equals the sum of the branches |
"Current is used up as it goes round" is the most stubborn idea in this topic, and the three identical ammeter readings are the evidence against it. Make the class write their own numbers down before they answer.
Which circuit is brightest?
Same battery in all three. A has one lamp, B has two in series, C has two in parallel.
| Circuit | Brightness | Why |
|---|---|---|
| A, one lamp | Bright | The full battery voltage across it |
| B, two in series | Dimmest | The two lamps share the voltage, so each gets half |
| C, two in parallel | The same as A | Each lamp has the full battery voltage across it, exactly like the single lamp |
A and C are equally bright. Many marking memos say parallel is the brightest, and that is wrong. With a real cell, C is very slightly dimmer than A, because two lamps draw twice the current and the cell's own internal resistance drops a little more voltage. Never brighter.
Two things not to build
A circuit with nothing in it
A common textbook question asks what the ammeter would read if the resistors were removed. Answer it on paper. That circuit is a short: the cells and leads heat up, the cells can leak, and the ammeter can be damaged.
An ammeter straight across the battery
An ammeter has almost no resistance, so connecting it across a battery is a short circuit through the instrument. Ammeter in series, in the line. Voltmeter in parallel, across the component.
If it does not work
| Problem | Cause | Fix |
|---|---|---|
| Nothing lights | LEDs instead of bulbs, or a clip on insulation | Use 2,5 V bulbs, and check every clip is on bare metal |
| One lamp out, all lamps out | Series, and one filament has blown | Swap the bulb. This is also the answer to why house lights are wired in parallel |
| A bulb blows immediately | Three cells across a single 2,5 V bulb | Two cells maximum for one bulb |
| Cells in parallel change nothing | Correct | Nothing to fix. Ask what did change |
| The ammeter reads zero | It is connected across, not in the line | Ammeter in series |
| The needle goes backwards | Polarity | Positive terminal towards the battery positive |
| Voltages do not quite add up | Normal. Leads and cells lose a little | Say so. A learner who notices is doing well |
Safety
- This is 4,5 V and it cannot hurt anybody. Say it once and the class relaxes
- Do not short a cell to see what happens. It gets hot and it can leak
- Bulbs get hot enough to burn fingers after a few minutes
- Never connect an ammeter directly across a battery
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks. Part A is series circuits, Part B is parallel, both with results tables to fill in from the learner's own readings
- Marking memorandum, including the brightness question that most memos get wrong
Related practicals
- Resistors in series and parallel, Grade 10. The next version of this lesson, with resistors, an ammeter and a voltmeter instead of bulbs
- Measuring current and potential difference. Where each meter goes, and why
- Types of forces, Grade 9. The other Term 3 topic
- Internal resistance of a battery, Grade 12. Why two lamps in parallel are a fraction dimmer than one
What you need to run it
Bulbs, holders, battery holders and leads. About R75 a group.
A 2,5 V lamp is R6 and an MES holder is R5. An AA battery holder with leads is R6,50, and a pack of ten crocodile leads is R58. Six groups can be equipped for about R450, which is the cheapest full class practical in the Grade 9 range. Everything is in the electricity collection.
Buy spare bulbs. A Grade 9 class will put three cells across a 2,5 V bulb at least once, and that ends the bulb.
The meters are the real purchase, and they are used again every year from here to Grade 12. An ammeter is R295 and a dual range voltmeter is R350. One of each is enough if the groups take turns, and the activities are written so that they can.
We do not supply cells. Batteries leak in storage, and a school buys fresh AAs for less than we could ship them for.