CLASS 7 SCIENCE · CHAPTER 3 · STUDY NOTES
Electricity: Circuits and their Components
Trace a circuit, identify each connection and explain why a lamp does—or does not—glow.
01 / EXPLORE & UNDERSTAND
Electricity helps in different ways
A fan moves air; a lamp gives light. Both use electrical energy.
1. Group devices by what they do
A kettle heats water, a motor turns a fan, and a lamp produces light. Electricity also supports communication, transport and medical equipment. One device can do several things: a television gives both light and sound.
2. Start with a small, safe source
Electricity can come from solar panels, falling water, wind and other sources. For this chapter’s investigations, use only teacher-approved low-voltage cells and components. Wall sockets, generators and opened appliances are not experiment supplies.
Your turn
Question
Why can a television belong to more than one group of electrical uses?
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02 / EXPLORE & UNDERSTAND
A torch needs a source, a path and a lamp
The switch controls a connection; it does not make the energy.
1. Identify the jobs
The cells provide electrical energy. Metal connections make a path. The lamp changes some electrical energy into light. The switch opens or closes a gap in that path. These parts work together to make the torch useful.
2. Explain the change when switched on
When the switch closes the path, current can pass through a working lamp. When it opens the path, current stops in this simple circuit. The cells may still contain stored energy even while the torch is off.
Your turn
Question
Does turning a torch off mean its cells have no stored energy left?
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03 / EXPLORE & UNDERSTAND
Find the two cell terminals
A complete circuit must connect to both terminals.
Two ends, two terminals
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1. Read the marks
An electric cell has a positive (+) and a negative (−) terminal. On the familiar cylindrical cell shown in NCERT, the raised metal cap is positive and the flat metal end is negative. Other cell shapes can look different, so check their markings.
2. Match the cell to the device
A cell is a portable source of electrical energy. Cylindrical cells are used in many clocks and torches; button cells fit small devices. Use the correct type and orientation shown on a device rather than assuming that every cell is interchangeable.
Your turn
Question
Why is one wire touching only the positive terminal not enough to run a lamp?
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04 / EXPLORE & UNDERSTAND
Join cells in the correct order
Follow the connections, not just where the cells sit.
Positive joins the next negative
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1. Positive joins negative
In the series arrangement shown in the book, the positive terminal of one cell connects to the negative terminal of the next. The two remaining outer terminals connect to the circuit. Reversing one cell changes how the cells work together.
2. Side by side can still be series
A holder can use metal strips to join cells even when they are side by side. Check the + and − marks and the metal connections. The physical layout alone does not tell you the electrical connection.
A little deeper: Why more cells are not always better
In a series combination of suitable similar cells, the supply voltage adds. A component must be matched to that supply; extra cells can damage it. More cells do not automatically guarantee a longer running time. In everyday language, “battery” can also mean a single packaged cell.
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Question
Two cells sit side by side. Must they be connected in a different way from end-to-end cells?
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05 / EXPLORE & UNDERSTAND
Inside an incandescent lamp
The thin filament is the part that becomes hot and glows.
Look inside the lamp
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1. Trace the two different contacts
One supporting wire joins the metal tip at the bottom; the other joins the metal case around the base. The contacts are separated by insulation. Current must pass from one contact through the filament to the other contact.
2. Understand a fused lamp
If the filament breaks, the conducting path inside the lamp is broken. Even correctly connected outer wires cannot bridge that internal gap. The lamp will not glow, so checking connections alone may not solve the problem.
Your turn
Question
Why does touching both wires to the metal case fail to send current through the filament?
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06 / EXPLORE & UNDERSTAND
The current needs a complete path
Trace one continuous route through the lamp.
Trace the complete route
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1. Trace the outside path
Start at the cell’s positive terminal. Follow a wire to one lamp contact, through the filament, out through the other contact and along a wire to the cell’s negative terminal. A working lamp with a suitable supply can glow when this path is complete.
2. Use the current-direction convention
We describe conventional current in the outside circuit as travelling from the cell’s positive terminal towards its negative terminal. An incandescent lamp can work either way around. The important point is that the path passes through its filament.
A little deeper: Energy is transferred; current is not used up in the lamp
The cell supplies energy that the lamp transfers as light and heat. Do not imagine the lamp using up all the current so that nothing returns along the other wire. The complete circuit is needed for a steady current.
Your turn
Question
A wire comes loose anywhere in this single loop. What happens to the lamp?
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07 / EXPLORE & UNDERSTAND
An LED has a direction
Changing the connections can stop it glowing.
Compare the LED connections
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1. Compare it with a filament lamp
An LED does not use a glowing filament. On the untrimmed LED shown in the book, the longer lead is positive and the shorter lead is negative. After leads are cut, length alone is not a reliable guide; check the component’s markings or information.
2. Match the polarity and supply
For the chapter’s suitable low-voltage arrangement, connect the LED’s positive side towards the supply’s positive terminal and its negative side towards the negative terminal. Reverse connection normally prevents it glowing. Use a teacher-prepared, current-limited LED circuit so excessive current does not damage it.
Your turn
Question
Why might an LED stay off after its connections are reversed even though a filament lamp would still glow?
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08 / EXPLORE & UNDERSTAND
A switch opens or closes a gap
Closed means a connected path. Open means a broken path.
Open: the pin leaves a gap
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Closed: the pin connects both contacts
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1. Compare the two positions
In NCERT’s simple switch, a metal pin touches both contacts when ON. It completes the path. When the pin moves away from one contact, the air gap interrupts the path and the switch is OFF.
2. Place it in the one path
In a single-loop cell circuit, opening a switch anywhere in the loop stops current through the lamp. It does not have to sit next to the lamp. Household circuits are different systems and should only be handled by qualified adults.
Your turn
Question
Does a closed switch mean that its metal contacts are separated?
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09 / EXPLORE & UNDERSTAND
Use symbols to draw the circuit clearly
A diagram shows connections without drawing every object realistically.
A small visual dictionary
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1. Read each symbol
A cell has one long and one short line: long is positive, short is negative. A battery has repeated cell symbols. The lamp is shown by its lamp symbol, wires by lines, and an open switch by a clear gap.
2. Separate current and light arrows
The LED symbol has two small arrows pointing outwards to show emitted light. They are not extra wires. Standard symbols let people compare circuit connections even if the real components are arranged differently.
Your turn
Question
On a cell symbol, does the long line show a physically longer cell?
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10 / EXPLORE & UNDERSTAND
Test whether a material completes the path
The test object becomes part of a small cell circuit.
The spoon bridges the test gap
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1. Place the object between two free ends
First check that the cell, lamp and wires work. Then put the test object between the two free wire ends. The ends must contact the object but must not touch each other. If a suitable current passes through the object, the lamp glows.
2. Explain a result carefully
Many metals are good conductors. Dry plastic, rubber and glass are poor conductors in this test. If the lamp stays off, first check contact and equipment: a poor connection can look like an insulating material.
A little deeper: A dark lamp is not a perfect zero-current meter
A simple lamp tester only detects enough current to light its lamp. Very small currents may not make a visible glow. Pencil “lead” contains graphite, which can conduct even though it is not a metal; the result depends on the sample and tester.
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Question
Why would touching the two free wire ends together while testing plastic give a misleading result?
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11 / EXPLORE & UNDERSTAND
A wire needs metal inside and insulation outside
Different materials do different jobs in the same object.
1. Follow the intended path
Copper inside a wire carries current. Plastic or rubber around it helps keep current on that path and separates neighbouring conductors. Both are useful: a conductor makes the connection; insulation helps prevent unwanted contact.
2. Exposed ends must contact the terminals
In a teacher-prepared cell circuit, metal wire ends contact the terminals. Pressing plastic covering against a terminal does not make the same conducting connection. Damaged insulation on real appliances is a danger; tell an adult rather than trying to repair it.
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Question
Why does a plastic-coated wire conduct along its length but not through its outer covering in the normal way?
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12 / EXPLORE & UNDERSTAND
Find a fault one change at a time
A lamp that stays off can have several possible causes.
1. Check the path before replacing everything
Look for an open switch, a loose metal connection, both wires on one lamp contact or a broken filament. Also check the cell condition and, for an LED, its orientation and the correct supply. Switch the small test circuit off before changing a connection.
2. Use a known working comparison
Replace one suspected part with a suitable known working part, then test again. If several parts change together, you cannot easily tell which solved the fault. Keep a short record of the prediction, change and result.
Your turn
Question
You replace both the cell and lamp, and the circuit works. Have you shown which old part was faulty?
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13 / EXPLORE & UNDERSTAND
One loop and separate branches are different
Ask whether each lamp still has a complete path to the source.
Follow a complete route
Slide sideways to see the whole circuit.
There is one complete loop through both lamps. Both glow.
1. Consider two lamps in one loop
If two lamps are placed one after the other in a single loop, a broken filament in either lamp opens the only path. Both lamps go out. The fault affects the whole loop because there is no alternative route.
2. Then consider separate paths
If two lamps are in separate branches across a suitable source, a break in one branch need not break the other branch. Trace each complete route before deciding. The answer comes from the connections, not from whether two lamps look close together.
Your turn
Question
What question should you ask before saying a broken lamp makes every lamp go out?
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14 / EXPLORE & UNDERSTAND
Cells, solar panels and wall supplies
All can supply electrical energy, but they are not interchangeable.
1. Compare the sources
Cells provide a portable supply. A suitable solar panel can power a small motor when it receives enough light. In both cases, the motor still needs correct connections and a supply matched to its requirements.
2. Keep the wall supply separate
The chapter introduces the difference between direct current from cells and alternating current, whose direction changes repeatedly, in the usual wall supply. This does not mean all batteries are harmless or that only large devices use AC. Practise with approved small cells only.
Your turn
Question
Does a solar-powered toy fan still need a complete circuit?
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Take these ideas with you
- A cell supplies energy; a switch controls a connection.
- A single-loop circuit needs an unbroken path through both lamp contacts.
- An incandescent filament heats and glows; an LED needs the correct polarity.
- On a cell symbol, long means positive and short means negative.
- Metal conducts; insulation helps prevent unwanted contact.
- Trace connections and change one suspected part at a time.
Use your understanding in a new situation
Question
A lamp has good wires and a fresh cell, but its filament is broken. Why is the outside loop not enough?
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A student calls a switch an energy source. Correct the statement.
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Sign inNew here? Sign up freeNCERT · Chapter source PDF, pages 2, 14, 18 · Original Learnijoy explanations and practice.