Simple Circuits: Paths, Switches & Energy — Print · Grade 6 Science

Simple Circuits: Paths, Switches & Energy

Learning goals

I can explain why the current is the same before and after a bulb in a series circuit.
I can predict the brightness of bulbs wired in series and in parallel.
I can explain what voltage measures and why mains voltage is dangerous.

Readings for this unit

A Reading Lamp Is More Than a Bright Object

Before this unit

Read the chapter opening and the named observation or diagram. Clarify one word or unit that blocks understanding. Recall the relevant earlier measurement, matter, or energy idea.

Study the unit across three 35–45-minute class sessions. Use the worked examples after an attempt and the supplied records when actual equipment is not available.

A circuit has to be a complete loop. Break it anywhere, with a switch or a loose wire, and nothing flows anywhere in it, not even in the part still connected to the battery. That alone tells you something important: current is not something the battery squirts out and sends off to be consumed.

Measure the current just before a bulb and just after it, with two ammeters in the same loop, and both read the same, perhaps 0.30 A. The bulb is glowing brightly and the charge arriving at it leaves again in exactly the same quantity. Charge is not used up by the bulb.

What the bulb does use is energy. The charge carries energy from the battery, delivers it in the filament where it becomes light and heat, and continues round the loop carrying less. A battery is better thought of as a pump for energy than a reservoir of current.

Build your understanding

Where this fits

The last lesson was about charge sitting still. This one is about charge moving, which is all a current is. The single idea that makes everything else in the lesson predictable is in the second section, and most confusion about circuits at this level traces back to getting it the wrong way round.

Use three flexible 40-45-minute sessions: complete loops and what current is; why charge is conserved while energy is not; series against parallel, and what voltage measures, then the practice set, revision and the exit check. Workshop, review, quiz and exams have separate time.

Part 1 · The loop has to be closed

An electric current is the movement of charged particles, and in a metal wire those particles are electrons, the same ones that moved during rubbing in the last lesson. The difference is that here they keep moving, pushed round the circuit by the battery. For that to happen there must be an unbroken conducting path from one terminal of the battery, through the components, and back to the other terminal. Open a switch at any point and the whole current stops instantly everywhere, including in the wires nowhere near the switch.

That instantaneous stop is worth a moment's thought, because the electrons themselves move astonishingly slowly, typically a fraction of a millimetre per second. The lamp lights immediately because the push travels through the wire at close to the speed of light while the electrons shuffle along at walking pace or slower. A useful comparison is a long pipe already full of water: push water in at one end and water comes out of the far end at once, even though no individual drop has travelled the length of the pipe. The water comparison is worth pushing a little further because it also shows the model's limit. A pipe can leak and a circuit cannot, since charge has nowhere to go; a pipe can be left open at one end and still flow, while an open circuit stops dead. Use the comparison for the speed puzzle and drop it before it starts producing wrong predictions, which is the habit chapter 1 asked for with every model.

The idealized battery path and B branch form a complete loop while A’s switch gap interrupts A alone. Charge is not used up at a lamp. Component brightness, source lifetime, and construction ratings are not specified; this is a paper path model, not a build instruction.

Pause and explain

Why does a lamp light the instant a switch is closed when electrons move only a fraction of a millimetre per second?

Check your reasoning after answering

Because the wire is already full of electrons and the push travels through it at nearly the speed of light. Closing the switch starts every electron in the loop moving almost at once, including those already inside the filament, so energy is delivered immediately even though no individual electron has travelled from the battery to the lamp.

Part 2 · Charge is conserved, energy is not

Put two ammeters in a simple series loop, one each side of the bulb, and they read the same value, say 0.30 A. Charge is not used up in the bulb, and the reason is conservation: the electrons cannot pile up inside the filament or vanish from it, so however many arrive each second, the same number must leave. This is the single most common misconception at this level, and it matters because almost every wrong prediction about circuits comes from imagining current being consumed as it goes round.

What is consumed is energy. Each unit of charge collects energy at the battery and gives it up in the filament, where it becomes light and heat, and then continues round with less. Put a voltmeter across the battery and across the bulb and you will see this directly: the battery might read 1.5 V and the bulb 1.5 V, because the energy supplied per unit of charge equals the energy delivered per unit of charge. A battery runs flat not because it has run out of charge but because the chemical reaction inside it that does the pushing has finished. There is a neat way to see the energy being delivered rather than the charge being consumed. Put an identical bulb on each side of the battery in a simple loop and both glow equally brightly, which would be impossible if the first one consumed part of the current before it reached the second. Students almost always predict the first bulb will be brighter, and watching that prediction fail does more than any explanation.

Pause and explain

Two ammeters either side of a glowing bulb read the same current. What is being used up, if not the current?

Check your reasoning after answering

Energy. The number of electrons arriving each second must equal the number leaving, since they cannot accumulate or disappear inside the filament, so the current is unchanged. What each unit of charge loses as it passes through is energy, which appears as the light and heat the filament gives out.

Part 3 · Series and parallel

In a series circuit the components sit one after another on a single loop, so the same current passes through every one of them and the battery's voltage is shared between them. Add a second identical bulb in series and each now receives about half the energy per unit of charge, so both glow dimly, and the current through the whole loop falls. Remove or break one bulb and the loop is open, so the other goes out as well, which is why old Christmas lights failed completely when a single bulb blew.

In a parallel circuit each component sits on its own branch across the battery, so each receives the full battery voltage and each draws its own current. Two bulbs in parallel both glow at full brightness, and breaking one leaves the other lit because its branch is still a complete loop. The total current the battery supplies is the sum of the branch currents, so a parallel arrangement drains a battery faster. House wiring is parallel for exactly these reasons: every socket gets the full 230 V, and switching off a lamp does not darken the rest of the house. There is a cost to the parallel arrangement that household wiring has to manage. Because every appliance adds its own branch current, the total drawn through the main cable is the sum of everything switched on at once, and that cable has a limit. A fuse or circuit breaker is there to cut the supply when the total climbs too high, which is why running several high-power appliances on one extension lead can trip it.

Pause and explain

Why is household wiring arranged in parallel rather than in series?

Check your reasoning after answering

So that every appliance receives the full supply voltage and works independently. In parallel each device sits on its own branch across the supply, gets the whole 230 V, and can be switched off without breaking the path to anything else. In series the voltage would be shared, everything would run weakly, and one failure would stop the lot.

Part 4 · Volts, amps and resistance

Current, measured in amperes, counts how much charge passes a point each second, and an ammeter is placed in the loop so that the current flows through it. Voltage, measured in volts, is the energy given to each unit of charge, and a voltmeter is placed across a component to compare the energy on either side of it. Resistance, measured in ohms, is how strongly a component opposes the flow, and a thin filament has far more of it than the copper wire leading to it, which is why the filament glows and the wire does not.

Putting real numbers on these makes the ideas concrete. A single AA cell supplies 1.5 V and a small torch bulb might draw 0.30 A from it. Mains electricity in much of the world is 230 V, more than a hundred and fifty times the cell, which is why a battery on your tongue is a mild tingle and a mains socket is lethal. A domestic kettle draws about 10 A at that voltage, delivering energy at roughly 2300 W, which is why kettles have thick cables and torches do not. These three quantities are linked by a single relationship you will meet formally next year: the current through a component equals the voltage across it divided by its resistance. You can already see it working in this lesson. Adding a second bulb in series roughly doubles the resistance and roughly halves the current, while putting the same bulb in parallel leaves the voltage unchanged and so leaves that branch's current unchanged too.

Pause and explain

A thin filament and the thick copper wire feeding it carry the same current, yet only the filament glows. Why?

Check your reasoning after answering

Because the filament has far greater resistance. Resistance measures how strongly a component opposes the flow, and energy is given up where that opposition is greatest, so almost all of the energy per unit of charge is delivered in the filament rather than in the wire. The current is identical in both, since they are in the same loop.

Apply the whole unit · Predict the circuits

A 1.5 V cell is connected to one bulb, and the ammeter reads 0.30 A. A second identical bulb is then added, first in series and then, in a separate circuit, in parallel.

Your task

Predict the brightness and current in each arrangement, and explain what happens if one bulb is removed.

What your response should show

Predicts dim bulbs and reduced current in series, with the shared voltage as the reason
Predicts full brightness and doubled total current in parallel
Gets both removal cases right and explains them by whether a loop remains closed
States that current is unchanged around a loop rather than consumed

Response guidance — after your attempt

Describe the brightness of each bulb in the series circuit and what happens to the current.
Describe the brightness and the total current in the parallel circuit.
Say what happens to the remaining bulb in each arrangement when one bulb is removed.

In series the two bulbs sit on one loop, so the 1.5 V is shared and each bulb gets about 0.75 V. Both glow dimly and the current through the whole loop falls below 0.30 A, because the total resistance has roughly doubled. In parallel each bulb sits on its own branch and receives the full 1.5 V, so both glow at normal brightness and each draws about 0.30 A, making the total current from the cell about 0.60 A. The cell therefore runs flat about twice as fast. Removing a bulb from the series circuit breaks the only loop, so the remaining bulb goes out. Removing one from the parallel circuit leaves the other branch complete, so the remaining bulb keeps glowing at full brightness and the total current drops back to 0.30 A. In both circuits the current is the same at every point within any single loop, because charge is not used up anywhere.

Scientific references for this unit

Parent / teacher teaching notes

Begin with the literal observation, quantity, or diagram relationship. Ask the learner to explain why the evidence supports the claim, check a relevant unit or boundary, then discuss an alternative. The supplied numbers are illustrative, not real observations. A parent can read aloud and allow a spoken rehearsal before writing. Judge science and evidence rather than political, faith, or family agreement. Use the complete data option whenever an actual procedure is unavailable or unsuitable.

The key idea

Charge travels round a complete loop and is not used up anywhere in it; what gets used up is energy. Getting that distinction right is what makes series and parallel circuits predictable instead of mysterious.

Worked examples

Example 1 · Compare two circuit records

Recommend a design for independent lamp control and explain a limitation.

Trace the available current paths.
Use the branch and shared-switch records.
Separate logic from actual brightness and safety performance.

Answer: The parallel branch design provides the modeled independent control: opening A does not interrupt B’s complete path. The common switch can still stop both. The records do not establish equal brightness, battery lifetime, or a safe real construction; those require a rated design and further evidence.

Example 2 · Transfer to a different case

A lamp fails although the drawn loop is complete. Give two plausible checks without using wall electricity.

Identify the distinction and conditions in this new case.
Use the relevant mechanism or quantity; preserve units and the chosen boundary.
Explain why the answer fits and what further evidence would be needed.

Answer: Inspect an approved kit’s contacts and source condition under adult guidance, or evaluate those possibilities in the supplied record. A complete drawing does not establish that every actual component functions.

Example 3 · Separate charge movement from energy transfer · a focused example

Is charge used up at the lamp?

State the literal condition or observation in the example.
Apply the distinction explained in this teaching part.
Check whether the claim stays within the given evidence.

Answer: No. The lamp receives energy and transforms its distribution; charge continues through the circuit under the model.

Avoid these mistakes

Misleading claim: The lamp consumes the charge, leaving none in the return path.
Misleading claim: A picture of a closed circuit proves a real device is safe and functioning.
Omitting units, measurement timing, or a model boundary makes the evidence hard to interpret.
Treating illustrative classroom data as real field measurements or copying a model is not an independent explanation.

Check your understanding

Why does closing the switch matter?

Discuss the answer

It connects the conducting path so current can be sustained in the stated battery–lamp model.

Is charge used up at the lamp?

Discuss the answer

No. The lamp receives energy and transforms its distribution; charge continues through the circuit under the model.

Does every open switch necessarily turn off both lamps?

Discuss the answer

No. Its location matters: a branch switch and a switch in the shared path affect different routes.

What must be checked beyond a diagram?

Discuss the answer

Actual connections, component ratings, source condition, and the intended switch positions. Paper evidence establishes the modeled logic, not a tested household appliance.

Sustained scientific explanation and revision

These activities are scheduled in the parent plan. Save a first response, a correction, and the reason for the change in the science notebook.

Activity 1 · Simple Circuits: Paths, Switches & Energy · Model and explain

Assigned reading and preparation

Read A Reading Lamp Is More Than a Bright Object and the linked unit teaching; use its complete supplied case and diagrams.

Your task

For Simple Circuits: Paths, Switches & Energy, study Parts 3–4 and attempt the worked case: Recommend a design for independent lamp control and explain a limitation. Explain the steps before opening the model.

Success criteria

An accurate scientific claim answering the actual question.
Relevant evidence or a labeled model with quantities and units when applicable.
Reasoning that connects evidence with the mechanism rather than only repeats it.
A specific method, boundary, scale, or alternative-explanation limit.

Hint

Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.

Response guidance — discuss after your attempt

The parallel branch design provides the modeled independent control: opening A does not interrupt B’s complete path. The common switch can still stop both. The records do not establish equal brightness, battery lifetime, or a safe real construction; those require a rated design and further evidence.

Activity 2 · Simple Circuits: Paths, Switches & Energy · Practice, revise and exit

Assigned reading and preparation

Read A Reading Lamp Is More Than a Bright Object and the linked unit teaching; use its complete supplied case and diagrams.

Your task

For Simple Circuits: Paths, Switches & Energy, complete practice questions 1–6 and both choice checks, then answer independently: A lamp fails although the drawn loop is complete. Give two plausible checks without using wall electricity. Save a correction explaining one changed claim or method.

Success criteria

An accurate scientific claim answering the actual question.
Relevant evidence or a labeled model with quantities and units when applicable.
Reasoning that connects evidence with the mechanism rather than only repeats it.
A specific method, boundary, scale, or alternative-explanation limit.

Hint

Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.

Response guidance — discuss after your attempt

Inspect an approved kit’s contacts and source condition under adult guidance, or evaluate those possibilities in the supplied record. A complete drawing does not establish that every actual component functions.

Words to know

circuit: A connected path supporting charge flow and energy transfer under stated conditions.
parallel branch: A separate conducting route between shared connection points in a circuit.
evidence: A relevant observation, measurement, or record used to evaluate a claim.
system boundary: The chosen limit specifying what is included and which exchanges cross it.
uncertainty: A stated limit in measurement or knowledge, distinct from a careless mistake.

In the real world

Exit check

A lamp fails although the drawn loop is complete. Give two plausible checks without using wall electricity.

Check your explanation after answering

Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.