Why a Bulb Only Glows When the Path Is Complete
Learn what each part of a circuit does, why a switch works wherever you put it, how to draw proper circuit symbols, and how to test what conducts electricity.
What are the parts of a simple electric circuit?
A simple circuit needs a source of electricity, something to use it, and an unbroken path of wires joining them. Break the path anywhere and everything stops — which is the single idea that explains switches, testers and faults alike.
This page covers everything in the CBSE Class 7 Science chapter on electricity and circuits: what each component does, why a circuit must be closed, how to draw circuit diagrams, and how to tell conductors from insulators.
This page covers everything in the CBSE Class 7 Science chapter on electricity and circuits: what each component does, why a circuit must be closed, how to draw circuit diagrams, and how to tell conductors from insulators.
What does each circuit component do?
An electric cell stores chemical energy and supplies electricity. It has two terminals: a positive terminal (the metal cap) and a negative terminal (the flat metal disc).
A battery is two or more cells joined together, the positive terminal of one touching the negative terminal of the next. A torch using two cells contains a battery.
An incandescent lamp glows because current heats a thin filament until it gives out light — so it becomes hot and wastes much of the electricity as heat. An LED gives light directly without a hot filament, so it stays cool and uses far less electricity for the same brightness. An LED also has a longer and a shorter leg, and must be connected with the longer leg towards the positive terminal, whereas an incandescent lamp works either way round.
Connecting wires carry the current, and a switch opens or closes the path.
A cell that no longer lights a lamp has not "run out of electricity" — its chemicals have been used up, which is why it must be replaced or recharged.
A battery is two or more cells joined together, the positive terminal of one touching the negative terminal of the next. A torch using two cells contains a battery.
An incandescent lamp glows because current heats a thin filament until it gives out light — so it becomes hot and wastes much of the electricity as heat. An LED gives light directly without a hot filament, so it stays cool and uses far less electricity for the same brightness. An LED also has a longer and a shorter leg, and must be connected with the longer leg towards the positive terminal, whereas an incandescent lamp works either way round.
Connecting wires carry the current, and a switch opens or closes the path.
A cell that no longer lights a lamp has not "run out of electricity" — its chemicals have been used up, which is why it must be replaced or recharged.
Why does a lamp glow only when the circuit is closed?
Current can only flow around a complete, unbroken loop from one terminal of the cell, through the components, and back to the other terminal. Such a loop is a closed circuit, and the lamp glows. If there is a gap anywhere, the circuit is open and nothing flows.
A switch simply makes or breaks that gap on purpose. Turning it on joins the two ends of the wire and completes the loop; turning it off separates them.
This is why a switch works wherever you place it in the circuit. Since the current has to pass through every part of a single loop, breaking the loop at the beginning, middle or end stops it equally well.
For example, the switch on your wall is nowhere near the bulb on the ceiling, yet it still controls it — because both sit on the same loop.
The same reasoning explains a fault: a loose wire, a burnt-out filament or a cell put in the wrong way all break the loop, and in every case the lamp stays dark.
A switch simply makes or breaks that gap on purpose. Turning it on joins the two ends of the wire and completes the loop; turning it off separates them.
This is why a switch works wherever you place it in the circuit. Since the current has to pass through every part of a single loop, breaking the loop at the beginning, middle or end stops it equally well.
For example, the switch on your wall is nowhere near the bulb on the ceiling, yet it still controls it — because both sit on the same loop.
The same reasoning explains a fault: a loose wire, a burnt-out filament or a cell put in the wrong way all break the loop, and in every case the lamp stays dark.
How do you draw and read a circuit diagram?
Drawing components realistically is slow and unclear, so scientists use standard symbols joined by straight lines.
- a cell: one long line and one short thick line, the long line being the positive terminal
- a battery: several cell symbols in a row
- a lamp: a circle with a cross inside
- an open switch: a gap with a slanting line, hinged at one end
- a closed switch: the same line lying flat, bridging the gap
- connecting wires: plain straight lines
Draw the circuit as a neat rectangle with components along its sides, and always show whether the switch is open or closed — that single detail tells the reader whether the lamp is lit.
For example, a circuit diagram of a torch is a battery, a closed switch and a lamp in one loop.
A diagram with a break in a line is not a drawing mistake; it means an open circuit. So keep your lines properly joined at the corners when you intend the circuit to be closed.
- a cell: one long line and one short thick line, the long line being the positive terminal
- a battery: several cell symbols in a row
- a lamp: a circle with a cross inside
- an open switch: a gap with a slanting line, hinged at one end
- a closed switch: the same line lying flat, bridging the gap
- connecting wires: plain straight lines
Draw the circuit as a neat rectangle with components along its sides, and always show whether the switch is open or closed — that single detail tells the reader whether the lamp is lit.
For example, a circuit diagram of a torch is a battery, a closed switch and a lamp in one loop.
A diagram with a break in a line is not a drawing mistake; it means an open circuit. So keep your lines properly joined at the corners when you intend the circuit to be closed.
How do you test whether a material conducts electricity?
Build a circuit tester: a cell, a lamp and wires with two free ends, leaving a deliberate gap. Touch the two free ends to the material being tested.
If the lamp glows, the material completed the circuit and is a conductor. If the lamp stays dark, the material is an insulator.
Conductors include copper, aluminium, iron, all other metals, and also graphite and tap water. Insulators include rubber, plastic, wood, glass, paper and dry cloth.
This knowledge is what makes appliances safe. Electric wires have a copper core to carry the current and a plastic covering so you can hold them without a shock; screwdrivers and pliers for electrical work have plastic or rubber handles; plug pins are metal while the plug body is plastic.
For example, never touching a switch with wet hands follows directly from this — tap water conducts, so wet skin gives the current a path it would not otherwise have.
If the lamp glows, the material completed the circuit and is a conductor. If the lamp stays dark, the material is an insulator.
Conductors include copper, aluminium, iron, all other metals, and also graphite and tap water. Insulators include rubber, plastic, wood, glass, paper and dry cloth.
This knowledge is what makes appliances safe. Electric wires have a copper core to carry the current and a plastic covering so you can hold them without a shock; screwdrivers and pliers for electrical work have plastic or rubber handles; plug pins are metal while the plug body is plastic.
For example, never touching a switch with wet hands follows directly from this — tap water conducts, so wet skin gives the current a path it would not otherwise have.
Exam tip
Exam tip: saying where a switch must go
Asked where to place a switch, students often answer "just before the bulb" or "next to the cell", as though the position mattered.
It does not. A switch placed anywhere in the loop controls the lamp, because breaking a single loop at any point stops the current everywhere in it.
So the correct answer names the reason, not a position: the switch may be placed at any point in the circuit, since opening the circuit anywhere breaks the complete path and stops the current. Questions of this kind are testing whether you understand the loop, and an answer without that reasoning scores poorly even if it happens to place the switch somewhere workable.
It does not. A switch placed anywhere in the loop controls the lamp, because breaking a single loop at any point stops the current everywhere in it.
So the correct answer names the reason, not a position: the switch may be placed at any point in the circuit, since opening the circuit anywhere breaks the complete path and stops the current. Questions of this kind are testing whether you understand the loop, and an answer without that reasoning scores poorly even if it happens to place the switch somewhere workable.
Did you know
Why does an LED stay cool while a bulb gets hot?
An incandescent lamp makes light by heating a filament until it glows, so heat is not a side effect — it is the method. Most of the electricity ends up as warmth rather than light.
An LED produces light directly, without needing anything to get hot. That is why an LED bulb of the same brightness feels cool, uses less electricity and lasts far longer.
An LED produces light directly, without needing anything to get hot. That is why an LED bulb of the same brightness feels cool, uses less electricity and lasts far longer.
Key takeaways
Electric circuits: quick revision
- A cell has positive and negative terminals; two or more cells joined together make a battery.
- An incandescent lamp glows by heating a filament and gets hot; an LED gives light directly, stays cool, and must have its longer leg towards the positive terminal.
- Current flows only in a closed, unbroken loop, so a lamp glows only when the circuit is complete — and a switch works wherever it sits in that loop.
- Circuit diagrams use standard symbols for cell, battery, lamp, open and closed switch and wires.
- A circuit tester identifies conductors (metals, graphite, tap water) and insulators (rubber, plastic, wood, glass), which is why wires have copper cores and plastic coverings.
You will remember all of this far better after answering five questions on it than after reading it twice.
- An incandescent lamp glows by heating a filament and gets hot; an LED gives light directly, stays cool, and must have its longer leg towards the positive terminal.
- Current flows only in a closed, unbroken loop, so a lamp glows only when the circuit is complete — and a switch works wherever it sits in that loop.
- Circuit diagrams use standard symbols for cell, battery, lamp, open and closed switch and wires.
- A circuit tester identifies conductors (metals, graphite, tap water) and insulators (rubber, plastic, wood, glass), which is why wires have copper cores and plastic coverings.
You will remember all of this far better after answering five questions on it than after reading it twice.