NCERT Solutions for Class 7 Science Chapter 3 Electricity Circuits and Their Components (PDF) – 2027

More uses that can be added to Nihal’s lists:

Other ways of grouping the uses: by place (home, school, hospital, farm, factory); by what electricity is changed into (light, heat, sound, motion); by the source used (cells and batteries, or the wall socket); by the user (household, farming, industry, medicine).

Because in that (ON) position the switch completes the circuit. The switch closes the gap, so current flows from the + terminal of the cell, through the lamp, back to the – terminal, and the lamp glows. In the other (OFF) position the switch leaves an air gap, the circuit is broken, no current flows and the lamp does not glow.

Yes. The lamp is at the front of the torch, behind the glass or plastic window, inside a shiny curved reflector; it may be a small bulb or one or more LEDs. The switch is the small sliding (or push) button on the side of the body of the torch.

Yes, in one position of the switch the lamp glows; when the switch is slid back, it goes off.

The position in which the lamp glows is called the ON position of the switch; the other position is the OFF position.

Inside we find two or more electric cells, placed one behind the other — the + cap of one cell touching the – disc of the next. We also see a spring at the back that presses the cells together, metal strips or wires that carry the current to the lamp, the lamp with its reflector at the front, and the switch.

Yes. A + sign is marked at one end of the cell and a – sign at the other end. They name the two terminals of the cell — the positive (+ve) terminal and the negative (–ve) terminal.

Yes. The small protruding metal cap is the positive (+) terminal of the cell and the flat metal disc is its negative (–) terminal. So even if the printed signs wear off, the raised cap tells us which end is positive.

Yes. The cells are placed so that the positive terminal of one cell touches the negative terminal of the next cell. Such a combination of cells is called a battery. If one cell is turned round, the two cells oppose each other and the lamp does not glow.

The lamp glows in only one of the arrangements — the one in which the + terminal of one cell touches the – terminal of the other.

When the lamp glows, the cells are in the order shown in Fig. 3.3 — the positive terminal of one cell is connected to the negative terminal of the next cell.

Yes. Inside the small glass bulb there is a very thin, coiled wire held in the middle, supported at its two ends by two much thicker wires.

What you see, from outside inwards:

• A clear glass bulb.
• Two thick wires rising from the base — they only hold and feed the thin wire.
• The thin wire between them, usually wound like a tiny spring.
• A ring of insulator at the base that keeps the two terminals from touching.

Only the thin wire inside the glass bulb glows. This glowing thin wire is called the filament of the lamp.

The thick supporting wires, the glass bulb and the metal base stay dark and comparatively cool.

The filament is fixed between two thicker wires that support it (Fig. 3.4a). One thick wire is connected to the metal case at the base of the lamp and the other to the metal tip at the centre of the base. These form the two terminals of the lamp, and an insulator keeps them from touching each other (Fig. 3.4b).

No. Unlike an incandescent lamp, an LED has no filament (Fig. 3.6). Inside its small coloured dome there is only a tiny piece of material joined to the two wires; it gives out light directly when current passes through it.

Yes, one wire is longer than the other. The longer wire is the positive (+) terminal of the LED and the shorter wire is its negative (–) terminal. The longer wire must be joined to the + terminal of the battery.

Use one simple rule to predict: the lamp glows only if one of its terminals is joined to one terminal of the cell and its other terminal to the other terminal of the cell — so that current has an unbroken loop to travel round.

On actually making the connections, the lamp glows in the arrangements at S.No. 1 and S.No. 6 and does not glow in the rest. So only those two bulbs are to be coloured yellow.

Yes. Where the lamp glows (S.No. 1 and 6), one terminal of the lamp is joined to one terminal of the cell and the other terminal of the lamp to the other terminal of the cell. This makes a complete path for the current. In the others, either a joint is left open (S.No. 2, 3) or both lamp terminals are joined to the same cell terminal (S.No. 4, 5), so no current flows.

The terminal of the holder that is connected to the positive terminal of one cell is the positive terminal of the battery. The terminal connected to the negative terminal of the other cell is the negative terminal of the battery.

Yes, the LED glows. The + terminal of the battery is joined to the longer (+) wire of the LED and the – terminal to the shorter (–) wire. Current can pass through the LED in this direction, so it glows.

No, the LED does not glow this time. Now the + terminal of the battery is joined to the shorter (–) wire of the LED. Current can pass through an LED in one direction only, so no current flows and the LED stays dark.

Even when they lie side by side, the cells are joined + terminal of one cell to – terminal of the next. A thick wire or metal strip inside the battery compartment makes this connection, and ‘+’ and ‘–’ symbols printed inside show how each cell must be placed.

A switch completes or breaks the circuit. In the ON position its metal part closes the gap: the circuit is closed, current flows and the torch lamp glows. In the OFF position it leaves an air gap: the circuit is open, no current flows and the lamp does not glow.

No, the lamp does not glow.

In Fig. 3.12a the free end of the safety pin is turned away, so it does not touch the second drawing pin. There is a gap between the two drawing pins, and air fills that gap.

This is the OFF position of the switch, and we say the circuit is open.

Yes, now the lamp glows.

The safety pin is made of metal, a good conductor. When its free end touches the second drawing pin, it closes the gap and completes the path.

Yes. Each component is shown by a simple symbol (Table 3.2), and a drawing of a circuit made with these symbols is called its circuit diagram. In the cell symbol the long line is the + terminal and the short line the – terminal; in the LED symbol the triangle points in the direction in which current can flow.

Yes — drawn correctly, they come out exactly like Fig. 3.14a and Fig. 3.14b.

Metals are used because electric current flows through them easily — metals are good conductors of electricity. Most other materials do not let current pass at all, so a circuit made with them would never work.

Because plastic and rubber are insulators — current cannot pass through them. The covering keeps the current inside the metal wire and protects us from electric shock, since our body is a conductor. It also stops bare wires from touching each other.

Only if the material is a conductor. Current will pass through a ‘wire’ made of a metal such as copper, aluminium or iron, but not through one made of plastic, rubber, wood, glass, wax or paper. To find out for any material, we test it — as we do in Activity 3.11.

Yes, the lamp glows while the ends touch. Touching the two bare ends closes the gap, so the circuit is complete and current flows. This shows that the cell, the lamp and the joints are working — our conduction tester is ready.

No. The lamp glows for some objects and stays dark for others.

• It glows for the metal spoon, coin, key, pin, sewing needle, aluminium foil and pencil lead.
• It does not glow for cork, rubber, glass, the plastic scale, the wooden block, the candle, cardboard and paper.

This shows that electric current can pass easily through some materials but not through others — which is exactly the point of the activity.

A completed Table 3.3 looks like this. Rows 8 and 9 are left for objects of your own choice — two useful ones are given here.

No. The lamp glowed for some materials only. Materials through which electric current flows easily are called conductors (good conductors) of electricity; materials through which current cannot pass are called insulators (poor conductors of electricity).

Conductors: the key and other metal objects such as a coin and aluminium foil — the lamp glowed. Insulators: the wooden stick, plastic scale, glass bangle, paper strip, wax candle and rubber eraser — the lamp did not glow. So metals are conductors of electricity, and that is why wires are made of metal.

Yes. Plastic, rubber and ceramics are insulators, so current cannot pass through them. Covering the metal wire with them keeps the current inside the wire and protects us from electric shock, because our body is a conductor. For the same reason, plug tops and switches are also covered with insulators.

The incorrect statement is (i) — ‘A switch is the source of electric current in a circuit.’

A switch is not a source of current at all. The source in these circuits is the electric cell or battery; the switch only controls whether the current is allowed to flow.

The lamp will not glow if an insulator is connected between A and B — for example rubber, plastic, wood, glass, paper or wax. An insulator does not let current pass, so the circuit stays broken at the gap. (With a conductor such as a copper wire, a key or a coin, the lamp glows.)

No, the other lamp will not glow. In Fig. 3.17 the two lamps and the battery are joined in a single loop, so the current has only one path, through both filaments. A broken filament leaves a gap in this path. The circuit becomes open, no current flows anywhere in it, and so the second lamp does not glow either.

No, the lamp will not glow. The covering on the wires is an insulator. If it is not removed from the ends, the metal of the wires does not touch the terminals of the cell and the lamp. Current cannot pass through the covering, so the circuit stays open and the lamp does not glow.

A simple torch has just three components in a single loop — a battery of two cells, an electric lamp, and a switch, joined by wires.

In Fig. 3.18 the battery, S₁, L₁, S₂ and L₂ are all in one single loop, so current flows only when both switches are ON.

1. No lamp glows — S₁ is OFF, so the circuit is open.
2. No lamp glows — S₂ is OFF, so the circuit is open.
3. Both L₁ and L₂ glow — the circuit is complete.
4. No lamp glows — the circuit is open at both switches.

Possible reasons: the cell is used up (dead); the lamp is fused (broken filament) or loose in its holder; the plastic covering is not removed from a wire end; a wire is broken or has come loose; the safety pin does not really touch the second drawing pin.

To find out: test one part at a time — join the two switch wires directly, replace the cell and then the lamp with ones known to be working, and check every wire and joint.

Only in case (c) the LED will not glow. In (a), (b) and (d) the lamp or LED lights up when the switch is closed.

Use an LED. Join the longer (+) wire of the LED to one terminal of the battery and the shorter (–) wire to the other terminal. If the LED glows, the terminal joined to the longer wire is the positive (+) terminal and the other is the negative terminal. If it does not glow, interchange the connections; when it glows, identify the terminals in the same way.

(i) Items: the six cells A, B, C, D, E and F; a cell holder; a torch lamp in a lamp holder; connecting wires with the ends stripped; and one cell known to be working.

(ii) Procedure:

1. Join the cell holder and the lamp holder with wires to make a circuit.
2. Put the known good cell in the holder — the lamp must glow. This checks the circuit.
3. Put cell A in its place and see whether the lamp glows. Repeat with B, C, D, E and F, one at a time, changing nothing else.
4. If the lamp glows, the cell is working; if it does not glow, the cell is not working.

(iii) Record the results in a table. The entries below are only a sample — write what your cells actually show.

No, the LED will not glow. The black wire joins the – terminal of one cell to the – terminal of the other, so the two cells oppose each other and are not in series.

Correct connections: join the + terminal of the upper cell to the – terminal of the lower cell. Then join the free + terminal to the longer (+) wire of the LED and the free – terminal to the shorter (–) wire.

Sample answer — things I would not be able to do:

Devices that run on cells — a torch, a wall clock, a radio with cells, and a mobile phone until its battery runs down — would still work.

Circuit: join one terminal of the solar panel to one terminal of the toy fan’s motor with a wire, and the other terminal of the panel to the other terminal of the motor, making a complete loop. Keep the panel in bright sunlight — the fan runs. Cover the panel and the fan stops, which shows that the panel is the source of electrical energy.

Sample report — cells found in an electrical shop and their uses: