By place of use: home • school • hospital • farm • factory • market.
By what electricity is converted into: light (lamp) • heat (geyser) • sound (radio) • motion (fan, pump).
By source of supply: devices that run on cells/batteries (torch, watch, remote) and those that run on the mains socket (fridge, TV).
By necessity: essential (hospital equipment, water pump) • comfort (AC, TV) • luxury.
⚠ Caution from the book: Never perform experiments with the power supply at home or school. Use only
cells/batteries (as in torches, wall clocks, radios, remotes) for electricity experiments.
3.1 A Torchlight — Activity 3.1
A3.1
Slide the torch switch and observe. Does the lamp glow? Open the torchlight — what do you find inside? And why does the torch lamp glow in only one position of the switch?
SOLUTIONFig. 3.1 — A torchlight (from the chapter)
In one position of the switch the lamp glows; in the other position it does not glow.
On opening the torch, we find two or more electric cells inside (along with the lamp, the switch and metal strips/wires).
Why it glows in only one position: In that position the switch closes the circuit — it completes
the path so that current can flow from the cells through the lamp. In the other position the switch breaks (opens)
the circuit, leaving a gap, so no current flows and the lamp stays dark.
3.2.1 Electric cell — Activity 3.2
A3.2
Turn an electric cell around and look at it carefully. Do you notice a (+) sign and a (–) sign? A metal cap on one side and a flat metal disc on the other?
SOLUTIONFig. 3.2 — An electric cell (from the chapter)
Yes. Every electric cell has two terminals.
The small protruding metal cap is the positive (+ve) terminal.
The flat metal disc is the negative (–ve) terminal.
An electric cell is a portable source of electrical energy.
3.2.2 Battery — Activity 3.3
A3.3
In a torch we generally use more than one cell. Are those placed in any particular order? Put the cells back in a different order and reverse one — does the lamp glow?
SOLUTIONFig. 3.3 — A battery made up of (a) two cells (b) four cells (from the chapter)
Observation. The lamp glows only when the cells are placed in the order shown in Fig. 3.3. If one cell
is reversed, the lamp does not glow.
The rule. The positive terminal of one cell must be connected to the negative terminal of the next cell.
Definition. Such a combination of two or more cells is called a battery.
Why use more than one cell? Connecting more cells provides energy to the circuit for a
longer time and/or more energy.
Fascinating fact: the word “battery” is also used for a single cell — like the battery of a mobile phone.
3.2.3 Electric lamp — Activities 3.4 & 3.5
A3.4
Examine the torch lamp. Do you notice a thin wire in the middle of the glass bulb? Which part of the lamp glows? How is the filament fixed?
SOLUTIONFig. 3.4 — (a) A small incandescent lamp (b) its simplified drawing (from the chapter)
Yes, a thin wire is fixed in the middle of the glass bulb. The thin wire glows — it is called the
filament of the lamp.
The filament is attached to two thicker wires that support it.
One thick wire goes to the metal case at the base; the other goes to the metal tip at the centre of the
base. These are the two terminals of the lamp, kept apart by an insulator so that they do not touch
each other.
In an incandescent lamp, the filament gets hot and glows to produce light when current passes
through it.
Fascinating fact: If the filament breaks, the lamp is said to be ‘fused’ — the broken filament stops the
flow of current, so the lamp cannot glow.
A3.5
Take an LED and observe. Do you see any filament inside it? Is one of its two wires longer than the other?
SOLUTION
Fig. 3.5 — An LED lamp for torchFig. 3.6 — LEDs of different colours
No filament. Unlike incandescent lamps, LEDs do not have filaments.
Yes, one wire is longer than the other. An LED also has two terminals:
Longer wire → positive terminal
Shorter wire → negative terminal
A torch may use one or more LEDs, sometimes of different shapes.
3.2.4 Making a lamp glow — Activity 3.6 & Table 3.1
A3.6
Predict and then observe: in which of the six arrangements of Table 3.1 will the lamp glow?
SOLUTION — TABLE 3.1 (COMPLETED)
Fig. 3.7 — Cell holder / cell with taped wiresFig. 3.8 — Lamp holder / lamp with taped wires
S.No.
Arrangement
Prediction
Observation
Reason
1
Glows
Lamp GLOWS ✔
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 → the circuit is complete.
2
Does not glow
Does NOT glow ✘
A wire end is left free — the circuit is incomplete, so no current flows.
3
Does not glow
Does NOT glow ✘
Both terminals of the cell and of the lamp are not properly joined — the path is broken.
4
Does not glow
Does NOT glow ✘
The two wires do not connect the two different terminals of the cell to the two terminals of the lamp → open circuit.
5
Does not glow
Does NOT glow ✘
The lamp is connected on one side only — current has no complete path to flow through the filament.
6
Glows
Lamp GLOWS ✔
Again a complete circuit — the two terminals of the lamp are connected to the two terminals of the cell.
Conclusion: The lamp glows only in arrangements 1 and 6. The difference is simple —
in these two the circuit is complete.
3.2.5 An electrical circuit
Q
What is an electrical circuit? What is the direction of electric current in it? Does it matter which terminal of an incandescent lamp goes to the positive terminal of the cell?
SOLUTIONFig. 3.9 — An electrical circuit (from the chapter)
An electrical circuit is a setup that provides a complete path for electric current to flow through the
lamp. The lamp glows only when current passes through the circuit.
The direction of electric current is taken to be from the positive terminal to the negative terminal of the
electric cell.
For an incandescent lamp it does NOT matter which of its terminals connects to the positive or the negative
terminal of the cell. It will glow as long as the circuit is complete and current flows through the filament.
For an LED it DOES matter — see Activity 3.7 below.
A3.7
How will you decide which is the positive terminal of the two-cell battery in the holder? Connect the LED as in Fig. 3.10c and then interchange the wires (Fig. 3.10d). Does the LED glow in both cases?
SOLUTIONFig. 3.10 — Making an LED glow (from the chapter)
Positive terminal of the battery: the terminal of the holder that is connected to the
positive terminal of one cell is the positive terminal; the one connected to the negative terminal of the
other cell is the negative terminal.
Fig. 3.10(c) — LED GLOWS ✔. Here the LED’s longer wire (+) is joined to the battery’s
positive terminal and its shorter wire (–) to the battery’s negative terminal.
Fig. 3.10(d) — LED does NOT glow ✘. The wires have been interchanged, so the LED is
reverse-connected.
Reason:Current can pass through an LED in one direction only. So an LED must always be
connected the right way round to make it glow.
DD
Dive Deeper: Sometimes cells are placed side by side in a device. How are the terminals connected then?
SOLUTIONCells placed side by side in a battery compartment (from the chapter)
Inside the battery compartment there is a thick wire or a metal strip that connects the
positive terminal of one cell to the negative terminal of the next — exactly the same rule as for cells placed
end to end.
To help you put the cells in correctly, the ‘+’ and ‘–’ symbols are printed inside the compartment.
3.2.6 Electric switch — Activities 3.8 & 3.9
A3.9
How does a switch turn the torchlight on or off? Does the lamp glow in Fig. 3.12a? Does it glow when the safety pin touches the other drawing pin (Fig. 3.12b)?
SOLUTION
Fig. 3.11 — A switch (a) ‘OFF’ (b) ‘ON’Fig. 3.12 — Circuit with the switch (a) ‘OFF’ (b) ‘ON’
Position
What happens at the switch
Circuit
Lamp
OFF (Fig. 3.12a)
The safety pin does not touch the second drawing pin — there is a gap.
Open
Does NOT glow ✘
ON (Fig. 3.12b)
The safety pin touches both drawing pins and closes the gap.
Closed
GLOWS ✔ (current flows from + to – terminal of the cell)
A switch is a simple device that either completes or breaks a circuit. It can be placed
anywhere in the circuit. The switches used at home work in the same way, though they look different.
3.3 Circuit diagrams — Table 3.2 & Activity 3.10
A3.10
Using the symbols of Table 3.2, draw the circuit diagrams of the circuits in Fig. 3.12a and Fig. 3.10c.
SOLUTIONTable 3.2 — Electrical components and their symbols (from the chapter)Fig. 3.13 — Positive and negative terminals in the symbols of (a) a cell (b) an LED
In the symbol of a cell, the long line = positive terminal and the short line = negative terminal.
In the symbol of an LED, the triangle points in the direction in which current can flow, and the two
arrows show that the LED emits light.
Fig. 3.14 — The required circuit diagrams: (a) with an incandescent lamp + switch (b) with an LED + battery (from the chapter)
Yes — the diagrams should look exactly like Fig. 3.14(a) and Fig. 3.14(b). A representation of an
electrical circuit using symbols is called its circuit diagram.
Dive Deeper: Bodies like the IEC, ANSI and IEEE fix standard symbols, so that engineers in every
country understand the same diagram.
3.4 Conductors and insulators — Activity 3.11 & Table 3.3
A3.11
Make a conduction tester and test different objects. Did the lamp glow for all materials? Complete Table 3.3.
SOLUTION — TABLE 3.3 (COMPLETED)Fig. 3.15 — (a) Conduction tester (b) Testing a material with it (from the chapter)
S.No.
Object
Material
Lamp glows (Yes/No)
Conclusion
1
Stick
Wood
No
Insulator
2
Scale
Plastic
No
Insulator
3
Bangle
Glass
No
Insulator
4
Paper strip
Paper
No
Insulator
5
Candle
Wax
No
Insulator
6
Key
Metal
Yes
Conductor
7
Eraser
Rubber
No
Insulator
8
Spoon / Coin / Sewing needle / Aluminium foil
Metal
Yes
Conductor
9
Pencil lead
Graphite
Yes
Conductor
10
Cork / Cardboard
Cork / Card
No
Insulator
Answers to the questions of this section
Did the lamp glow for all materials?No — it glowed only for some.
Materials through which current flows easily → good conductors (conductors of electricity) — e.g. all
metals, graphite.
Materials through which current cannot pass → insulators (poor conductors) — e.g. plastic, rubber, wood, glass,
paper, wax, ceramics.
Why are metal wires used in circuits? Because metals are conductors, so current passes through them easily.
Why are electric wires covered with plastic or rubber? Because these are insulators — the covering stops
current from reaching our body and protects us from electric shocks.
Dive Deeper:Silver, copper and gold are the best conductors, but copper is mainly used
for wires because it is cheaper and available in plenty.
Dive Deeper: Electricity from a cell/battery is Direct Current (DC); electricity from a wall socket
(power plants) is Alternating Current (AC), which can run larger appliances.
⚠ Caution: Our body is a conductor — never touch switches or plugs with wet hands.
Let Us Enhance Our Learning (Exercise)
Q1
Choose the incorrect statement.
(i) A switch is the source of electric current in a circuit.
(ii) A switch helps to complete or break the circuit.
(iii) A switch helps us to use electricity as per our requirement.
(iv) When the switch is in ‘OFF’ position, there is an air gap between its terminals.
SOLUTION
Answer: (i) A switch is the source of electric current in a circuit — this is INCORRECT.
Explanation
The source of electric current is the electric cell or battery, not the switch.
A switch only completes or breaks the circuit — (ii) is correct.
Because of it we can switch devices on and off as per our requirement — (iii) is correct.
In the ‘OFF’ position there is a gap (air gap) between its terminals, so current cannot flow — (iv) is correct.
Q2
Observe Fig. 3.16. With which material connected between the ends A and B will the lamp not glow?
SOLUTIONFig. 3.16 (from the chapter)
The lamp will not glow if an insulator is connected between A and B.
Iron key, steel spoon, copper wire, coin, aluminium foil, safety pin, pencil lead (graphite)
Conductor
Glows ✔
Reason: An insulator does not allow current to pass, so the circuit remains incomplete for the
current even though the wires are joined.
Q3
In Fig. 3.17, if the filament of one of the lamps is broken, will the other glow? Justify your answer.
SOLUTIONFig. 3.17 (from the chapter)
No, the other lamp will NOT glow.
Justification
Step 1. In Fig. 3.17 the two lamps are connected one after the other in a single loop with
the battery — current has only one path.
Step 2. A lamp with a broken filament (a fused lamp) acts like a gap in the circuit.
Step 3. The gap makes the circuit open, so no current can flow anywhere in the loop.
Step 4. Hence the second lamp, though perfectly good, also does not glow.
Q4
A student forgot to remove the insulator covering from the connecting wires while making a circuit. If the lamp and the cell are working properly, will the lamp glow?
SOLUTION
No, the lamp will not glow.
The wire is metal (a conductor) inside, but it is covered with plastic/rubber, which is an
insulator.
If the covering is not removed from the ends, the metal of the wire never actually touches the terminals of the
cell and the lamp.
So current cannot enter or leave the wire — the circuit remains incomplete.
That is exactly why the book says: “Remove about 1 cm of the plastic covering from both ends of each wire
to expose the metal.”
Q5
Draw a circuit diagram for a simple torch using symbols for electric components.
SOLUTION
A torch contains a battery (two cells), a switch and a lamp, joined by wires in one closed loop.
Circuit diagram of a simple torch — battery + switch + lamp in one closed loop
If the torch uses an LED instead of a bulb, replace the lamp symbol ⊗ with the LED symbol
(triangle with two arrows), taking care that its triangle points along the direction of the current, i.e. from the
+ terminal of the battery.
Q6
In Fig. 3.18, which lamp(s) will glow when —
(i) S2 ON, S1 OFF (ii) S2 OFF, S1 ON (iii) both ON (iv) both OFF?
SOLUTIONFig. 3.18 (from the chapter)
Key point: In Fig. 3.18 the battery, lamp L1, switch S2, lamp
L2 and switch S1 are all joined one after the other in a single loop. So current
can flow only if BOTH switches are ON — a gap at any one switch breaks the whole circuit.
Case
S1
S2
Circuit
Which lamp(s) glow?
(i)
OFF
ON
Open (gap at S1)
None — neither L1 nor L2 ✘
(ii)
ON
OFF
Open (gap at S2)
None ✘
(iii)
ON
ON
Closed ✔
Both L1 and L2 glow ✔
(iv)
OFF
OFF
Open (two gaps)
None ✘
Remember: a single break anywhere in a series circuit stops the current everywhere — exactly as
in Q3 with the fused lamp.
Q7
Vidyut’s circuit (Fig. 3.19) does not light the lamp even after closing the circuit. List as many possible reasons as you can. How will you find out why the lamp did not glow?
SOLUTIONFig. 3.19 (from the chapter)
Possible reasons
The cell is used up (dead) — it can no longer supply current.
The lamp is fused — its filament is broken.
The plastic covering was not removed from the ends of the wires, so metal is not touching metal.
Loose connections — the wires are not tightly fixed to the cell/lamp/drawing-pin terminals.
The lamp is not screwed properly into the lamp holder.
The safety pin does not touch the second drawing pin properly, so the switch is not really ‘ON’.
A wire is broken inside its plastic covering.
The cell is not sitting properly in the cell holder (not touching the spring/terminal).
How to find out the reason — test one thing at a time
Step 1. Press and tighten every connection and check the switch again.
Step 2. Replace the cell with a fresh cell. If the lamp glows → the old cell was dead.
Step 3. Replace the lamp with a known working lamp. If it glows → the old lamp was fused.
Step 4. Replace the wires one by one (or test each wire with the conduction tester of Fig. 3.15).
If a wire does not pass current, it is broken.
This is how scientists work — change one component at a time and observe, till the faulty part is
found.
Q8
In Fig. 3.20, in which case(s) will the lamp/LED not glow when the switch is closed?
SOLUTIONFig. 3.20 (from the chapter)
Answer: the LED will not glow in case (d).
Case
Component
Reason
Result
(a)
Incandescent lamp + cell
Circuit complete. For a lamp, polarity does not matter.
Glows ✔
(b)
Incandescent lamp + cell (cell the other way)
Still a complete circuit — the filament glows whichever way the current flows.
Glows ✔
(c)
LED + battery, correctly connected
The LED’s positive terminal is joined to the battery’s positive terminal, so current can pass through the LED.
Glows ✔
(d)
LED + battery, reverse connected
Current can pass through an LED in one direction only. Here the LED is connected the wrong way round, so no current passes.
Does NOT glow ✘
The rule to remember: An incandescent lamp works either way round; an LED works only when
its longer (positive) wire goes to the positive terminal of the battery.
Q9
Suppose the ‘+’ and ‘–’ symbols cannot be read on a battery. Suggest a method to identify the two terminals.
SOLUTION
Use an LED — because an LED allows current in one direction only, it can act as a terminal-finder.
Method (step by step)
Step 1. Take an LED (its longer wire is the positive terminal, the shorter wire is negative) and two connecting wires.
Step 2. Connect the LED to the battery through the two wires and observe.
Step 3 – If the LED glows: the battery terminal joined to the LED’s longer wire is the
positive (+) terminal, and the other one is the negative (–) terminal.
Step 4 – If the LED does not glow:interchange the two wires. Now it should glow — and again,
the terminal connected to the longer wire is the positive one.
Also useful: physically, the protruding metal cap of a cell is the positive terminal and the
flat metal disc is the negative terminal — so a careful look also helps. But for a sealed battery, the
LED test is the reliable method.
Q10
Six cells A, B, C, D, E, F — some working, some not. Design an activity to identify the working ones. (i) items required (ii) procedure (iii) carry it out.
SOLUTION
(i) Items required
The six cells (A–F)
One cell holder (or electrical tape)
One torch lamp that is known to be working, with a lamp holder
Two connecting wires (1 cm of covering removed from both ends)
(ii) Procedure
Step 1. Make a simple circuit: cell holder → wire → lamp → wire → back to the cell holder (like Fig. 3.9).
Step 2. First place a known good cell in the holder and check that the lamp glows. This proves the
tester itself (lamp + wires) is fine.
Step 3. Now put cell A in the holder (negative terminal towards the spring) and observe the lamp.
Step 4. Remove A and repeat exactly the same with B, C, D, E and F — one at a time, without changing the
lamp or the wires.
Step 5. Record the observation for each cell.
(iii) Observation table & conclusion
Cell
Lamp glows?
Conclusion
A
Yes
Working
B
No
Not working (dead)
C
Yes
Working
D
No
Not working
E
Yes
Working
F
No
Not working
Conclusion: the cells for which the lamp glows are working; those for which it does not glow are
used up. (The Yes/No entries above are a sample — write what you actually observe.) Fair test: keep the same lamp, the same wires and the same connections for every cell — change only the cell.
Q11
Using an LED that needs two cells to glow, Tanya made the circuit of Fig. 3.21. Will the lamp glow? If not, draw the wires for the correct connections.
SOLUTIONFig. 3.21 — Tanya’s circuit (from the chapter)
No — the LED will not glow.
What is wrong
Mistake 1 — the two cells are not forming a battery. In a battery, the positive terminal of one
cell must be joined to the negative terminal of the other. In Tanya’s circuit the wire joins like terminals, so the
two cells oppose each other and no current is pushed out.
Mistake 2 — the LED is connected the wrong way round. Current passes through an LED in
one direction only: its longer wire (+) must go to the battery’s + terminal and its shorter wire (–)
to the battery’s – terminal.
Correct connections
Correct wiring: cells joined + to –, then battery + → LED’s longer wire, battery – → LED’s shorter wire
Summary of the fix: (1) join the + of one cell to the – of the other to make a two-cell battery;
(2) connect the free + terminal of the battery to the LED’s longer wire; (3) connect the free – terminal to
the LED’s shorter wire. Now the LED will glow.
EP
Exploratory Projects — guidance
GUIDANCE
1. If the power supply is disrupted for two days, what could you not do?
No lights at night; no fan/cooler/AC; the refrigerator stops, so food spoils.
No water pump → no water in the taps; no geyser → no hot water.
No mixer, kettle, microwave, induction → cooking becomes difficult.
No TV, computer, Wi-Fi; mobile phones cannot be charged.
No lifts; traffic lights, hospitals and shops are affected.
Still possible: anything running on cells/batteries — torch, wall clock, radio, and a solar lamp.
2. A circuit to run a toy fan with a solar panel
Fig. 3.22 — (a) a solar panel (b) a toy fan (c) the working circuit (from the chapter)
Step 1. Take a small solar panel, a toy fan (motor with blades) and two wires.
Step 2. Join the two wires of the panel to the two wires of the motor — the panel replaces
the battery in the circuit.
Step 3. Place the panel in bright sunlight. The fan starts turning — the panel converts the
Sun’s energy into electrical energy.
Step 4. Move the panel into the shade — the fan slows down or stops. If the fan turns the wrong way,
interchange the two wires.
3. Types of cells in an electrical shop, and objects at home
Cells and batteries come in many shapes and sizes (from the chapter)
