Skip to content
CNMS ICSE - 7 Std
Share
Explore

Answers to textbook exercises

Prepared by: learnloophq@gmail.com

Chapter: 07. Electricity

⁠

CONNECT (Page 140)

Write ‘E’ for the appliances which work directly on electricity and ‘C’ for the appliances which work on cells or batteries.
1. Grandfather clock:
C (Works on cells/batteries)
2. Food processor:
E (Works directly on electricity)
3. Tablet device:
C (Works on a rechargeable battery/cell)
4. Brick fireplace:
Neither (It is a traditional fireplace using wood logs, not an electrical appliance)
5. Computer setup (Monitor, keyboard, and mouse):
E (Works directly on electricity)
6. Flashlight:
C (Works on cells/batteries)
7. Ceiling fan:
E (Works directly on electricity)
8. Close-up of a human eye:
Neither (It is a biological organ, not an electrical appliance)
9. Camouflage plastic container:
Neither (It is a non-electrical storage box)
10. Square object with orange/white concentric circles (Induction cooktop/Hot plate):
E (Works directly on electricity)
⁠

SCIENCE TALK (Page 141)

What is the most important electrical appliance according to you? Present your views in front of the class.
According to me, the most important electrical appliance is the refrigerator. It plays a critical role in daily life by keeping food fresh, preventing spoilage/waste, and safely storing temperature-sensitive medicines, which directly impacts human health and survival.
⁠

REASON CORNER (Page 142)

What can cause the potential difference to increase or decrease?
The potential difference in a circuit can be altered by changing the voltage source. It can be increased by adding more cells in a series combination or using a higher-voltage battery. It can be decreased by removing cells from the series, using a depleted/discharged cell, or configuring cells in parallel.
⁠

SCIENCE TALK - 21st CENTURY TRIALS (Page 145)

Solar energy can be used to generate electricity. But it comes with its own challenges. How convenient is solar energy in generating electricity when compared to other traditional means of energy such as fossil fuels? Hold a debate in class on this topic.
While solar energy is highly sustainable, eco-friendly, and reduces the carbon footprint compared to burning fossil fuels, it poses challenges in daily convenience. It depends entirely on weather conditions and daylight hours (unreliable at night or on cloudy days), requires expensive battery storage systems for uninterrupted supply, and needs significant physical space for panels. In contrast, traditional fossil fuels provide a continuous, reliable, and easily dispatchable supply of electricity, but at the cost of high environmental degradation.
⁠

MY QUESTION TIME (Page 147)

Frame a question on the topic ‘Sources of Electricity’ and discuss its answer.
Question: Why are secondary cells preferred over primary cells for devices like mobile phones and laptops?
Answer: Secondary cells are rechargeable because the chemical reactions inside them are reversible. This makes them highly suitable, cost-effective, and environmentally friendly for high-drain devices like mobile phones, which require frequent recharging, whereas primary cells cannot be recharged and must be discarded after a single use.
⁠

CHECK YOUR PROGRESS (Page 147)

Choose the correct option.
1. An atomic or nuclear power plant uses nuclear energy to generate
a. electric current.
2. How many subatomic particles are there in an atom?
b. three (protons, electrons, and neutrons)
3. Which of the following is not a primary cell?
a. lead accumulator (it is a secondary, rechargeable cell)
⁠

REASON CORNER (Page 152)

Which are brighter—bulbs connected in series or bulbs connected in parallel?
Bulbs connected in parallel are brighter. In a parallel circuit, each bulb gets the full, original voltage of the power source. In a series circuit, the total voltage is shared and divided among the bulbs, which reduces their individual brightness.
⁠

REASON CORNER (Page 154)

Based on the above activity, which of these lists contains materials that are only insulators?
c. plastic, rubber, wood
⁠

CHECK YOUR PROGRESS (Page 156)

Choose the correct option.
1. Electric wires are always covered with insulators to make them
b. safe.
2. Which of the following is a source of electric current in an electric circuit?
c. cell/battery
⁠

SCIENCE TALK (Page 156)

What are the precautions to be taken in case of a lightning strike? Find out and share your information in class.
The essential precautions to take during a lightning strike include:
Immediately seek shelter inside a fully enclosed building or a metal-topped vehicle (avoid open structures, tents, or tall trees).
Unplug electrical appliances and stay away from corded phones, as lightning can travel through electrical wiring.
Do not take a bath, shower, or use running tap water since plumbing pipes can conduct electricity.
If caught outdoors in an open area with no shelter", perform the “lightning crouch”: squat low with your feet together, head tucked between your knees, and hands covering your ears. Do" not lie flat on the ground.
⁠

MY QUESTION TIME (Page 156)

Frame a question on each of the following topics and discuss their answers.
a. simple electric circuits
Question: What constitutes a complete, simple electric circuit?
Answer: A complete simple electric circuit consists of a source of electric current (like a cell), conducting wires, an appliance/device (like a bulb), and a switch to turn the current on or off in a closed path.
b. circuit diagram
Question: What is the purpose of using standard symbols in a circuit diagram?
Answer: Standard symbols represent complex electrical components in a simple, universally understood way. This makes drawing, reading, and analyzing circuit designs much quicker and clearer.
⁠

PREPPING FOR PISA (Page 156-157)

Read the passage and answer the questions that follow.
1. What factor makes an MCB better than a fuse?
b. It leads to fewer power fluctuations.
2. Assertion (A): MCBs are less sensitive to current than fuse. ​Reason ®: Fuse can detect even a slight fluctuation in voltage and breaks the circuit current fast.
d. Both A and R are false.
⁠

EVACUATION (Page 157-158)

A. Choose the correct option.
1. The source of electricity in a torch is the
d. cell.
2. Electric current cannot flow through
b. wood.
3. Electric current can flow through
a. silver.
4. Which of the following is not an electrical component?
b. pen.
5. The direction of the conventional current is opposite to the direction of flow of
a. electrons.
6. Three bulbs are connected in a parallel circuit. If one bulb burns out, what will happen to the other two bulbs?
b. They will glow with the same brightness.
B. Fill in the blanks.
1. Electric current is the flow of _______________.
charges (or free electrons)
2. A combination of two or more cells is called _______________.
battery
3. Electrons revolve around the nucleus in paths called _______________.
orbits
4. A nuclear power plant uses _______________ energy to generate electric current.
nuclear
C. Name the following:
1. substances which allow current to flow through them easily _______________.
Conductors
2. centre of an atom _______________.
Nucleus
3. large number of solar cells connected together in a series _______________.
Solar panel
D. Write true (T) or false (F) against the following statements.
1. When a circuit is connected in series, if one appliance stops working, other appliances connected in the series also stop working.
T
2. The more the speed of wind, the less will be the electricity generated.
F (More wind speed generates more electricity).
3. A switch can be touched with wet hands.
F (It can cause a severe electric shock).
4. Cotton is a good conductor of electricity.
F (Cotton is an insulator).
5. An electric circuit is a closed path in which electric current flows.
T
E. Short-answer-type questions.
1. Name the sources of electricity.
The main sources of electricity include electric cells, batteries, electric generators (dynamos), solar cells, wind turbines, and the public electricity supply (mains).
2. What is an electric cell?
An electric cell is a small, portable source of electricity that converts chemical energy stored inside it into electrical energy through internal chemical reactions.
3. List the components of an electric circuit.
The components of a simple electric circuit are:
A source of electricity (cell or battery)
Conducting wires
An electrical device/appliance (such as a bulb or fan)
A switch (or plug key)
A resistor
4. What is a circuit diagram? How is a closed circuit different from an open circuit?
Circuit Diagram: A circuit diagram is a drawing that represents the actual electrical path on paper using standardized symbols for various electrical components.
Closed vs. Open Circuit: A closed circuit is a complete and continuous path where the switch is ‘ON’, allowing electric current to flow. An open circuit has a break or gap at some point (switch is ‘OFF’), which stops the flow of electric current.
5. What is a solar cell?
A solar cell is an electronic device that directly converts solar energy (sunlight) into electrical energy when light falls on it.
F. Long-answer-type questions.
1. State four precautions to be taken while using electricity.
Four essential precautions to take when using electricity are:
Never touch switches, sockets, or electrical appliances with wet hands, as water is a conductor and can cause severe shock.
Do not attempt electrical repairs yourself; always hire an authorized or trained electrician.
Ensure that all plugs fit tightly and securely in their sockets to prevent sparking and potential fires.
Always stand on a dry wooden board or a rubber mat (which act as insulators) when performing any electrical work.
2. Write a short note on a dry cell.
A dry cell is a portable chemical source of electricity. It consists of:
Electrodes: A central carbon rod with a brass cap serves as the positive terminal (anode), while the outer zinc container acts as the negative terminal (cathode).
Electrolyte: A moist paste of ammonium chloride (
math
⁠
) and plaster of Paris serves as the electrolyte.
Active Materials: The central carbon rod is surrounded by a mixture of carbon and manganese dioxide (
math
⁠
) in a muslin bag.
Working: When connected in a closed circuit, a chemical reaction takes place between the electrolyte and the electrodes, creating a potential difference that drives the flow of current. Once the chemicals are fully consumed, the cell becomes inactive and must be discarded.
3. Differentiate between
a. a primary cell and a secondary cell.
Feature
Primary Cell
Secondary Cell
Chemical Reaction
Irreversible
Reversible
Rechargeability
Cannot be recharged; single-use
Can be recharged and reused many times
Applications
Used in low-drain devices (torches, wall clocks, remotes)
Used in high-drain/electronic devices (mobile phones, laptops, cars)
Examples
Simple dry cell, Daniel cell, Leclanche cell
Lead accumulator, Lithium-ion cylindrical cells
⁠
b. a series circuit and a parallel circuit.
Feature
Series Circuit
Parallel Circuit
Current Path
Only one single continuous path for flow
Multiple branching paths for flow
Voltage
Voltage is shared/divided among components
Each component gets the full original voltage
Interdependence
Components depend on each other; if one fails, all stop working
Components are independent; if one fails, the others continue working
Switching
A single switch controls all appliances simultaneously
Separate switches can control each appliance independently
⁠
Higher Order Thinking Skills (HOTS)
1. Three bulbs X, Y and Z are connected to a cell as shown in the figure given below. If the filament of Y is broken, then which bulb(s) will glow?
a. None of the bulbs will glow. (Because the three bulbs are connected in series, breaking Y’s filament creates a gap in the only path, preventing current from completing the loop).
2. Observe the circuit given below. Will the bulb glow? Give reason.
No, the bulb will not glow. Distilled water is pure water, which does not contain dissolved mineral salts or free ions. Because of this, it is an insulator (non-conductor of electricity) and cannot conduct current to complete the circuit.
3. Rajesh made the circuit given below but the bulb did not glow. Draw the circuit with the changes required to make the bulb glow.
To make the bulb glow, Rajesh needs to implement the following changes in his circuit drawing:
Correct Battery Orientation: Connect the positive terminal of the first cell to the negative terminal of the second cell (ensure they are connected in series correctly).
Connect Both Terminals of the Bulb: Ensure one wire from the cell connects to the metal casing (side terminal) of the bulb, and the other wire connects to the metal tip at the base of the bulb.
Close the Switch: Make sure the switch is drawn in the closed (ON) position so that the circuit path is continuous.
4. Why are solar panels used in artificial satellites?
Solar panels are used in artificial satellites because sunlight is abundantly and continuously available in outer space. They convert solar energy directly into electrical energy, providing a clean, endless, and reliable power source for the satellite’s long-term operation without needing heavy fuel or maintenance.
⁠

Virtual Tour (Page 158)

1. What are the things required to make a switch?
The materials required are:
A wooden board or a piece of thermocol (base)
Two drawing pins (or metal brass paper fasteners)
A safety pin (or a paper clip)
Stripped copper connecting wires
2. What are the steps to make a switch?
The steps are:
Wrap one end of a stripped copper wire around a drawing pin and pass the drawing pin through the loop of the safety pin.
Fix this drawing pin securely into the wooden board or thermocol base.
Wrap one end of another stripped copper wire around the second drawing pin.
Fix this second drawing pin into the base at a small distance away, such that the free end of the safety pin can easily swing to touch its head.
3. What is the observation and why does it happen?
Observation: When the safety pin is rotated so that its free end touches the second drawing pin, the bulb glows. When it is turned away and does not touch the drawing pin, the bulb stops glowing.
Why it happens: When the safety pin touches the drawing pin, it acts as a closed switch, completing the electrical path (closed circuit) and allowing current to flow. Moving it away creates a gap, breaking the path (open circuit) and stopping the current.
Gallery
 
Want to print your doc?
This is not the way.
Try clicking the ··· in the right corner or using a keyboard shortcut (
CtrlP
) instead.