Current in a complete series circuit
Trace a complete circuit, predict current at different positions and evaluate ammeter readings.
Before you begin
- Energy transfer in a lamp
- Reading decimals and calculating a mean
Trace one complete path
A simple cell-and-lamp circuit needs a continuous conducting path from one terminal of the cell, through the lamp, back to the other terminal. Components in series share one unbranched path. A closed switch completes that path; an open switch breaks it. For this simple direct-current circuit, after brief changes have settled, opening the switch stops current throughout the loop, not just beyond the switch. A circuit diagram shows electrical connections, not the physical spacing of the equipment.
Distinguish charge flow from energy transfer
Electric current is the rate at which electric charge passes a point. It is measured in amperes, symbol A. In a steady series circuit, the same current flows at every position: charge does not continuously build up in one component. A lamp transfers energy to its surroundings, but does not use up the current or destroy the charge. The cell supplies energy to the circuit; charge is already present in the conducting materials. Conventional current is drawn from the positive terminal through the external circuit towards the negative terminal. In metal wires, electrons move in the opposite direction.
Measure current in series
An ammeter measures the current passing through it. Insert it into the path in series with the lamp, so the same current passes through both. Switch the supply off before moving the meter and use the appropriate meter connections, polarity and range under teacher supervision. Do not connect an ammeter directly across the cell: it provides a very low-resistance path and can cause a large current. Use only supervised, suitable low-voltage school equipment; never build or test a mains circuit.
Compare readings fairly
To compare positions, keep the cell or supply, lamp, meter range and other circuit conditions the same. Allow readings to settle and consider temperature or supply changes when taking repeats. A small difference between displayed readings is not enough to show that a lamp uses up current. Compare the pattern across repeats and investigate measurement variation. A display step of 0.01 A describes resolution, not a guarantee that every reading is within 0.01 A of the true value.
Worked example
A steady, unbranched cell-and-lamp circuit carries 0.30 A just before the lamp. Predict the current just after the lamp. Explain how the lamp can light without using up current.
Show the worked solution
- There is only one path, so charge cannot continuously accumulate at the lamp.
- The same rate of charge flow passes into and out of the lamp: 0.30 A.
- The lamp transfers energy to light and heating. Energy transfer is different from consuming charge or current.
0.30 A after the lamp. The lamp transfers energy; it does not use up current.
Try it yourself
In a steady series circuit, the current before a lamp is 0.18 A. Which prediction and explanation are consistent?
Common mistakes
- Current is measured in amperes; energy is measured in joules.
- The lamp transfers energy, not a portion of the current.
- An ammeter goes in series, never directly across the cell.
- Different readings need evaluation; one small difference does not overturn the steady-series model.
- Written planning does not demonstrate practical competence.
Trace, measure and explain
Explore the series-current lesson and question mind map.
Plan the comparison
Trace the loop from the positive terminal through P, the lamp, Q, the closed switch and R to the negative terminal. Predict the current at all three positions. Then explain how to compare them using one ammeter.
Compare my plan
- Predict the same current at P, Q and R once the circuit is steady. There are no branches where current could split.
- Under teacher supervision, switch the supply off before moving the ammeter into the path at Q or R. Reconnect the original gap with a wire. Do not add a branch or connect the meter directly across the cell.
- Keep the same supply, lamp, meter and range. Check connections and allow readings to settle; lamp temperature and a weakening cell can affect comparisons made at different times.
- Repeat readings at each position. Vary the order of positions in later rounds to help identify drift over time. Repetition does not remove a systematic meter error.
Does the lamp use up current?
These original constructed readings illustrate a comparison; they are not laboratory results. One ammeter is moved between positions in separate trials while the circuit conditions are kept as similar as possible. The display changes in steps of 0.01 A. No complete meter-accuracy specification is supplied.
| Position | Reading 1 | Reading 2 | Reading 3 |
|---|---|---|---|
| P | 0.24 | 0.25 | 0.24 |
| Q | 0.24 | 0.24 | 0.25 |
| R | 0.25 | 0.24 | 0.24 |
A student compares only the first readings at P and R and says, “Current gets bigger as it returns to the cell.” Calculate the mean and range at each position, evaluate that claim, and explain what these readings can and cannot establish. Write your answer before opening the guidance.
Compare my data reasoning
- Each row totals 0.73 A across three readings, so each mean is 0.73 ÷ 3 = 0.2433… A, or 0.24 A rounded to two decimal places. Adding repeated readings here is a calculation step, not adding currents in branches.
- Each range is 0.25 − 0.24 = 0.01 A. The first-reading difference is not a consistent positional pattern across repeats.
- The data are consistent with the same current around this steady series circuit. They do not support a claim that the lamp uses up current or that current grows on its way back.
- These readings do not prove exact equality or perfect instrument accuracy. Display resolution is not an accuracy guarantee. Supply drift, lamp temperature, connections or meter limitations need evaluation; more repeats alone do not fix every cause.
Transfer to a changed circuit
A second lamp is added in series. After the circuit settles, an ammeter at P reads 0.16 A. A student predicts 0.08 A at Q because there are now two lamps. Predict the currents at Q and R, explain the flaw and state whether the old 0.24 A reading alone could have predicted the new current.
Compare my transfer reasoning
Q and R each carry 0.16 A. Two series lamps share one path; they do not split the current into two branches. Adding a lamp changes the circuit, but the steady current is still the same at all positions within that new circuit. The old reading alone does not determine the new value: information about the supply and the components’ behaviour is needed. Energy can be transferred in both lamps without charge being consumed.
The guidance gives reasoning points for self-checking, not official marks or a saved score. It does not demonstrate supervised practical competence.
Connect current to energy transfer and power · Revisit controlled comparisons · Later extension: parallel circuits and resistance
Try fresh questions
Open the six-question series-current practice. These use new connections, comparisons and data. No practice score is saved.
Scope and review status
Grade 8 is StudyForA’s suggested placement, not an official grade requirement. England KS3 science and AQA KS3 science, section 3.2.2 include current as flow of charge and series circuits. This lesson covers one introductory part of that scope; parallel branches, potential difference and resistance remain further study. Specialist review has not been completed.
What can you explain now?
A cell, two lamps and a switch form one series loop. The current is initially 0.12 A. The switch is opened between the lamps. After brief changes settle, what is the current on each side of the switch, and does its position matter?
Compare with the explanation
Zero on both sides. Opening the switch anywhere in this single loop breaks the complete path.
Charge does not keep flowing as far as the break in a steady open circuit. There is no complete conducting loop, so neither lamp carries a steady current. This answer is for the simple direct-current circuit described, after transients.
After trying it yourself, choose your next review. This is your self-assessment.
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