Resistors in Series and Parallel: Potential Difference and Current
Wire two bulbs in series and they are dim. Wire three in parallel and they are bright. Same battery.
Everything in this topic follows from why that happens. In these circuits the bulbs are the resistors, and you can measure the whole of it with an ammeter, a voltmeter and about eighty rand of bulbs.
Before you connect anything: where the meters go
An ammeter wired in parallel across a battery is a short circuit through a delicate instrument. It takes about two seconds and it is the most expensive mistake in Grade 10 physics.
| Meter | Goes | Why |
|---|---|---|
| Ammeter | IN SERIES, in the path of the current | It measures what flows through, so the current must pass through it. It has very low resistance so it does not change what it is measuring |
| Voltmeter | IN PARALLEL, across the component | It measures the difference between two points, so it must touch both. It has very high resistance so almost no current is diverted through it |
And polarity matters. The ammeter's positive terminal faces the battery's positive terminal. Connected backwards, the needle drives against its stop.
Mark the meter positions on a circuit diagram on paper before anybody touches the apparatus. The syllabus sets this as an exercise in its own right, immediately before the practical, and it is there for exactly this reason.
The multi-scale ammeter trap
Some ammeters have more than one positive terminal, one for each scale, for example 0 to 5 A and 0 to 500 mA. The number beside the terminal is the largest value that scale can read.
Both scales share one needle. The same needle position is 350 mA on one terminal and 3,5 A on the other, and learners read the wrong row constantly.
Always start on the largest scale. If the reading is small enough for the smaller scale, move the lead down. Doing it the other way round drives the needle off the end.
Series: one path
All the current goes through every component, one after the other.
| In a series circuit | |
|---|---|
| Current | The same at every point. There is nowhere else for it to go |
| Voltage | Divides. V = V1 + V2 + V3 |
| Resistance | Adds. Rs = R1 + R2 + R3 |
Voltage divides in proportion to resistance, so the bigger the resistance the bigger its share. That is why a series circuit is also called a voltage divider.
Parallel: several paths
| In a parallel circuit | |
|---|---|
| Voltage | The same across every branch, and equal to the battery |
| Current | Divides between the branches. The branch currents add to the total |
| Resistance | Goes down as you add branches |
The voltage result has a clean reason: the potential difference between two points is the same whatever path you take between them. Every parallel branch sits between the same two points, so every branch has the same voltage across it.
Why more branches means less resistance
This one feels backwards. More components ought to mean more resistance.
More branches means more routes for the current to take, so more current flows in total for the same voltage, which means the circuit's overall resistance has fallen.
Think of lanes on a road, not obstacles in a pipe.
Where this fits in the curriculum
| Subject | Physical Sciences |
|---|---|
| Grade | 10 |
| Term | 3 |
| Topic | Electricity and magnetism |
| Status | CAPS prescribed experiment |
| Marks | 40 on our worksheet. None is prescribed |
The practical
Apparatus
| Item | Qty |
|---|---|
| Ammeter, DC | 1 |
| Voltmeter, dual range | 1 |
| Bulbs and bulb holders | 3 each |
| Battery holder and a 1,5 V cell | 1 |
| Connecting leads | 8 |
| Knife switch | 1 |
Get six bulbs, not three. They are the one thing that fails, and they fail in the way most likely to waste a lesson. At R6 each, spares are the cheapest insurance in the box.
Part A: series
- Two bulbs in series with the battery. Draw the circuit diagram
- Mark on the diagram where the ammeter and voltmeter go to measure through and across one bulb, and both bulbs
- Connect and read
- Compare the current through one bulb with the current through both
- Compare the voltage across one bulb with the voltage across both
Part B: parallel
- Three bulbs in parallel with the battery. Draw the circuit diagram
- Mark where the meters go for one bulb and for the battery
- Connect and read
- Compare the voltage across one bulb with the voltage across the battery
- Compare the current through one bulb with the current through the battery
What you should see
With a 1,5 V cell and 2,5 V bulbs. Your numbers will differ. The relationships will not.
Series, two bulbs
| Measurement | Roughly | The point |
|---|---|---|
| Current through bulb 1 | 0,10 A | |
| Current through bulb 2 | 0,10 A | Identical. Current is not used up |
| Voltage across bulb 1 | 0,7 V | |
| Voltage across bulb 2 | 0,8 V | |
| Voltage across the battery | 1,5 V | The two bulb voltages add to it |
Parallel, three bulbs
| Measurement | Roughly | The point |
|---|---|---|
| Voltage across one bulb | 1,5 V | |
| Voltage across the battery | 1,5 V | The same. Every branch gets the full voltage |
| Current through one bulb | 0,12 A | |
| Current through the battery | 0,36 A | Three times. The branch currents add |
The bit they notice before they measure it
The parallel bulbs are visibly brighter than the series bulbs, on the same battery.
In series each bulb gets a share of 1,5 V. In parallel each gets the whole 1,5 V. Ask the class to predict which will be brighter before they build either one, then let the voltmeter explain it.
Current is not used up, and here is the proof
The most persistent misconception in this topic is that current gets "used up" as it passes through a bulb. It sounds reasonable: the bulb is doing something, so something must be being consumed.
Your own series readings disprove it. The current through bulb 1 and the current through bulb 2 are the same number.
What gets used up is energy, not charge. Every electron that goes into the bulb comes out the other side; it just leaves some energy behind as light and heat. Measure the current at four different points around the series circuit and all four readings match.
If it does not work
| What you see | What caused it |
|---|---|
| The whole series circuit is dead | One dud bulb. In series, one failure stops everything. Swap bulbs one at a time |
| One parallel bulb out, the others fine | Correct behaviour, and it is the point. It is why house lighting is parallel |
| The ammeter needle slams backwards | Polarity reversed. Disconnect at once |
| The ammeter reads nothing on a live circuit | Connected in parallel instead of series, or the scale is too large for a small current |
| The voltmeter reads battery voltage whatever you do | It is across the battery, not across the component |
| Readings drift down all lesson | Tired cell. Open the switch between readings |
| Bulbs blow when rewired from series to parallel | Two 2,5 V bulbs sharing 1,5 V were fine; each getting the full 1,5 V is a different matter |
| The numbers do not add up | Check which ammeter terminal the lead is in. The scale trap |
| Everything is very dim | Flat cell, or a poor crocodile clip contact |
The failure worth keeping
Unscrew one bulb from the parallel circuit and the others stay lit. Unscrew one from the series circuit and everything goes dark.
That is not a fault, it is the answer to "why does this matter". House lighting is wired in parallel so one blown bulb does not darken the house. Old Christmas light strings were wired in series, which is why a single dead bulb killed the whole string.
How the 40 marks are made up
| Section | Marks |
|---|---|
| Meter placement, on paper | 6 |
| Series circuit diagram and readings | 8 |
| Parallel circuit diagram and readings | 8 |
| Current: same in series, divides in parallel | 6 |
| Voltage: divides in series, same in parallel | 6 |
| Why parallel is brighter | 3 |
| Conclusion | 3 |
No mark allocation is prescribed. The worksheet and this split are ours.
Meter placement carries 6 on paper alone, because knowing where an ammeter goes is examined every year and it is also what stops the instrument being destroyed.
The mark most often dropped is saying current is used up. The evidence against it is in the learner's own results table, and the answer has to quote it.
If you have time
Add a third bulb to the series circuit. Everything dims further and the voltage divides three ways. Then add a third parallel branch: brightness is unchanged and the battery current rises again.
Measure the current at four points around the series circuit. All four readings match. It is the most direct possible answer to the misconception.
Ask why house wiring is parallel. One failure must not darken the house, and every appliance needs the full supply voltage. Both follow from what they have just measured.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks, with the meter placement exercise, both circuit diagrams, both results tables and the current-is-not-used-up question
- Marking memorandum, with a sample set, guidance on marking relationships rather than numbers, and the six places learners most often drop marks
Related practicals
- Ohm's Law, Grade 11. Where V = IR gets measured properly
- Internal resistance of a battery, Grade 12. The same apparatus, three years on
- Magnetism, Grade 10. Same term
Buy this experiment
We are putting together a Series and Parallel Circuits Kit for Grade 10 with an ammeter, a dual-range voltmeter, bulbs and holders, battery holders, connecting leads and a switch in one box, plus a printed teacher guide and the marking memo. Coming shortly.
The two meters are what the practical stands on. A DC ammeter and a dual-range voltmeter reading 0 to 1 V and 0 to 5 V. The dual range matters, because one bulb in a series pair sits well under a volt while the battery is at 1,5 V, and one instrument should cover both. They are in the electricity range.
Buy bulbs in tens. They are R6 and they are the only thing in this practical that fails.
Cells are not included and should not be. They have a shelf life and they leak in storage.