Living Science Physics Class 8 ICSE Solution Chapter 8 Electricity

ICSE Living Science Physics for Class 8 Solutions Chapter 8 Electricity

Check Your Progress

Write true or false. Correct the false statements.
Question 1.
When a glass rod is rubbed with a silk cloth, the glass rod acquires a negative charge.
Answer:
False.
Correct : When a glass rod is rubbed with a silk cloth, the glass rod acquires a positive charge.

Question 2.
A metal rod can be charged if an ebonite rod rubbed with woollen cloth is brought near it.
Answer:
True.

Question 3.
A conductor held in hand can be charged by rubbing it with a silk cloth.
Answer:
False.
Correct : A conductor held in hand cannot be charged by rubbing it with a silk cloth.

Question 4.
Two balloons rubbed with hair will attract each other.
Answer:
False.
Correct : Two balloons rubbed with hair will repel each other.

Question 5.
If an object transfer its electrons to another object, it becomes negatively charged.
Answer:
False.
Correct : If an object transfer its electrons to another object, it becomes positively charged.

Check Your Progress

Fill in the blanks.

1. An electroscope can be used to detect the presence of ……………
2. The gold leaves of an uncharged electroscope …………. if a charged body is brought in contact with its brass disc.
3. A …………. is used to prevent a tall building from damage from a lightning strike.
4. ………….. is a high-energy electric discharge.
5. An electroscope can be charged by either of the two ways, …………. of ………….

Answer:

1. An electroscope can be used to detect the presence of charge.
2. The gold leaves of an uncharged electroscope diverge if a charged body is brought in contact with its brass disc.
3. A lightning conductor is used to prevent a tall building from damage from a lightning strike.
4. Lightning is a high-energy electric discharge.
5. An electroscope can be charged by either of the two ways, conduction of induction.

Think and answer

Question 1.
Why does a fuse wire has lower melting point than that of connecting wires ?
Answer:
A fuse wire has a melting point less than that of connecting wires. This is to ensure that the fuse wire melts before the connecting wires in case the circuit gets overheated due to overloading or short circuit.

Think and answer

Question 1.
What would happen if electrical appliances are not connected to the earth wire ?
Answer:
If electrical appliances are not connected to the earth wire, then in case of leakage of current from the appliance, the undesired current reaching its outer body passes through the person touching the device, and he gets an electric shock.

A. Tick the most appropriate answer.

Question 1.
Which of the following is a non-conductor of electricity?
a. silver
b. copper
c. plastic
d. aluminium
Answer:
c. plastic

Question 2.
An electroscope is an instrument used to –
a. detect the presence of charge on a body.
b. view electrons.
c. produce an electrostatic charge.
d. view atoms.
Answer:
a. detect the presence of charge on a body.

Question 3.
If an electroscope is charged by induction, it acquires –
a. the same charge.
b. an opposite charge.
c. no charge at all.
d. none of these.
Answer:
b. an opposite charge.

Question 4.
Transferring a charge by touching is called charging by –
a. conduction.
b. induction.
c. convection.
d. radiation.
Answer:
a. conduction

Question 5.
Moving charges constitute –
a. electrode.
b. electric current.
c. static electricity.
d. magnetism.
Answer:
b. electric current.

Question 6.
The colour code for a live wire is –
a. black or blue.
b. green or yellow.
c. red or brown.
d. white and black.
Answer:
c. red or brown

Question 7.
The top thick pin of a plug is always connected to the –
a. live wire.
b. neutral wire.
c. earth wire.
d. fuse wire.
Answer:
c. earth wire.

Question 8.
An electric fuse is always connected to the –
a. live wire.
b. neutral wire.
c. earth wire.
d. fuse wire.
Answer:
a. live wire.

Question 9.
The consequence of improper handling of electricity is –
a. electric shock.
b. fire.
c. both a and b
d. none of these
Answer:
c. both a and b

Question 10.
A battery is a collection of cells in –
a. series.
b. parallel.
c. random order.
d. none of these
Answer:
a. series.

B. Fill in the blanks.

1. Matter is made of tiny particles called …………..
2. When an atom loses electrons, it acquires a net …………. charge.
3. An electroscope can be used to test whether a body is a …………. or …………. not.
4. When a positively charged body is brought near a conductor, it induces a …………. charge on the near end of the conductor.
5. A lightning conductor is a long rod made of either …………… or ………….
6. In an electric circuit, the ………….. wire carries all the current.
7. The bigger hole in a socket called …………. saves us from getting electric shocks.
8. An electric meter measures the amount of electricity consumed in units of …………..
9. The flow of large amount of current through a circuit due to -the contact between the live and the neutral wire is called …………
10. The melting point of a fuse wire is ………….. than the melting point of the connecting wires.

Answer:

1. Matter is made of tiny particles called atoms.
2. When an atom loses electrons, it acquires a net positive charge.
3. An electroscope can be used to test whether a body is a charged or not.
4. When a positively charged body is brought near a conductor, it induces a negative charge on the near end of the conductor.
5. A lightning conductor is a long rod made of either copper or iron.
6. In an electric circuit, the live wire carries all the current.
7. The bigger hole in a socket called earth connector saves us from getting electric shocks.
8. An electric meter measures the amount of electricity consumed in units of a kilowatt-hour.
9. The flow of large amount of current through a circuit due to the contact between the live and the neutral wire is called short circuit.
10. The melting point of a fuse wire is less than the melting point of the connecting wires.

C. Write true or false. Correct the false statements.

Question 1.
Neutrons are present inside the nucleus of an atom.
Answer:
True

Question 2.
Charges can be exchanged between two bodies by friction.
Answer:
True

Question 3.
The force of attraction or repulsion between two charged bodies is called gravitational force.
Answer:
False.
Correct : The force of attraction or repulsion between two charged bodies is called electrostatic force.

Question 4.
The leaves of a gold-leaf electroscope are made of brass.
Answer:
False.
Correct : The leaves of a gold-leaf electroscope are made of gold.

Question 5.
Lightning is caused due to discharge between similarly charged clouds.
Answer:
False.
Correct : Lightning is caused due to discharge between oppositely charged clouds.

Question 6.
The cables that bring electricity to our homes consist of five wires.
Answer:
False.
Correct : The cables that bring electricity to our homes consist of three wires.

Question 7.
Electric meters measure the consumption of energy in joules.
Answer:
False.
Correct : Electric meter measure the consumption of energy in kilowatt-hours.

Question 8.
Miniature circuit breakers act as safety devices.
Answer:
True

D. Answer the following in short.

Question 1.
How is a body charged by friction?
Answer:
A body is charged by friction whenever two objects are rubbed against each other. Then, some of the free electrons get transferred from one object to the other. The object that loses electrons gets positively charged, while the one that gains electrons (lost by the first object) gets negatively charged.

Question 2.
Why does a glass rod get positively charged when rubbed with silk?
Answer:
When a glass rod is rubbed with silk, some free electrons from the glass get transferred to the silk cloth, thus making the glass positively charged and the silk negatively charged.

Question 3.
Explain why the leaves of a negatively charged electroscope collapse if a positively charged glass rod is brought near the disc of the electroscope.
Answer:
When a positively charged glass rod is brought in contact with a negatively charged electroscope, the free electrons from the leaves move towards the rod. This decrease in negative charge decreases the divergence in the gold leaves. Hence, the gold leaves collapse.

Question 4.
Define charging by conduction.
Answer:
When an uncharged object is touched with another charged object, the uncharged object, also gets charged. The total charge on the charged object gets equally distributed between the two objects in contact. The process is called charging by conduction.

Question 5.
What is meant by earthing of an electrical device?
Answer:
Earthing of an electrical device refers to the connection of the metal case of the electrical appliance to a large metal plate buried deep inside earth, via an earth wire.

Question 6.
How is a short circuit caused?
Answer:
Short circuit is caused due to the contact between the live and the neutral wire because of defective or loose connections or damaged wiring. This results in a sudden large current flowing through the circuit.

Question 7.
What is meant by overloading of an electric circuit?
Answer:
If too many devices connected to the same circuit are operated at-the same time, the wires draw a much larger current from the mains than they can handle. This is called overloading.

Question 8.
List two precautions while handling electricity.
Answer:
Two precautions while handling electricity are given below.
(a) Do not touch switches or plugs with wet hands.
(b) Put off the mains while working on the main cable wiring or installing a new device.

E. Answer the following in detail.

Question 1.
Describe the structure of an atom with the help of a diagram.

An atom consists of three sub-atomic particles-electrons (negatively charged particles protons (positively charged particles) and neutrons (neutral particles). Protons and neutrons are concentrated in the core of the atom called the nucleus and are bound by very strong forces of attraction. Electrons revolve around the nucleus in definite paths called orbits or shells.

Question 2.
What is a conductor? Explain why a conductor does not produce static electricity.
Answer:
A conductor is the substance that allow electric charges to flow through it. As a easily. A conductor cannot produce static electricity because electrons can easily flow through it. As a, result, there is no build up of charges in one place in a conductor.

Question 3.
State the basic property of electric charges. Describe an experiment to prove this property.
Answer:
The basic property of electric charges is that two electric charges exert force on each other, that is, two similar charges repel each other and two dissimilar charges attract each other.

Experiment
Aim : To show that dissimilar charges attract towards each other.
Materials required : Glass rod, silk cloth, ebonite rod, fur cloth
Procedure : Rub the glass rod with silk cloth and ebonite rod with fur cloth. Bring the two rods close to each other. You will observe that the two rods get attracted io ards each other. This is because glass rod on rubbing with silk cloth acquires positive charge ebonite rod on rubbing with fur cloth acquires negative charge.

Aim : To show that similar charges repel each other.
Materials required : Two plastic straws and dry towel.
Procedure : Take two plastic straws and rub them on a dry towel. Place one of the straws on a wooden table. Gradually take the second straw closer to the centre of the straw placed on the table. The straw on the table rolls away. This happens because on rubbing with the towel, both straws have acquired the same charge

Question 4.
What is lightning? How is it caused?
Answer:
Lightning is a high energy electric discharge between two adjacent clouds or a cloud or the ground. The phenomenon of lightning is caused by flow of electric charges in the atmosphere. It occurs because of a massive flow of electric charges from one cloud to another or from a cloud to the ground. When clouds move from one place to another, the friction with the surrounding air causes charges to develop on them.

The magnitude of charges increases as the clouds continue moving. Usually, the lower part of a cloud carries a negative charge, while the upper part carries a positive charge. When large clouds with negatively charged lower part pass over the electrically neutral ground, a positive charge is induced on the ground.

As air acts as an insulator, it does not allow these charges to flow. However, when the difference of charges, between two adjacent clouds or a cloud and the ground, becomes very large, the insulation of air breaks down and electrons flow through air to neutralize this charge difference. This generates a huge electric spark in the sky and is observed as lightning.

Question 5.
Describe the working of an electric fuse.
Answer:
A fuse is essentially a thin wire made of an alloy (60 % copper, 40 % lead) with a melting point slightly below the melting point of the connecting wires. It is fixed between two clamps of copper mounted on a porcelain grip. When the grip is inserted in the casing, the fuse wire completes the circuit.

As soon as there is overheating caused due to any of the reasons such as overloading, fluctuations in current, or short circuiting, the fuse wire melts. This breaks the circuit and disrupts the flow of current in it, thus saving a device from getting damaged or preventing a fire. The fuse wire should be replaced to reuse the fuse.

Question 6.
How is a fuse connected in an electric circuit? What is its role?
Answer:
A fuse is connected in series in an electrical circuit. It is a safety device that protects an electric circuit, or even electrical appliances in case –

1. The circuit is overloaded due to large withdrawal of current
2. If there are fluctuations of current in electricity supply and
3. If a short circuit occurs.

Question 7.
Describe the role of switches and electric meters in houses.
Answer:
A switch-is a small device used to open or close a circuit. It is always connected to the live wire. The front cover of a switch is made of an insulated substance such as bakelite or ebonite, whereas the working portion inside the cover is made of highly conducting material such as brass or, copper. When the switch is in the on mode, the metal strip inside the switch connects to the live wire and the current flows through the circuit.

When the switch is in the off mode, the connection with the live wire is disrupted and no current flows through the circuit. This is how a switch helps in making or breaking a circuit. An electric meter records the consumption of electric energy directly in kilowatt-hour (kWh) units.

Question 8.
Explain three electrical hazards.
Answer:
The three electrical hazards are :
1. Overloading : If too many devices connected to the same circuit are operated at the same time, the wires draw a much larger current from the mains than they can handle. This is called overloading. This causes heating in the circuit that can lead to sparking and electric fires.

2. Short circuiting : Due to defective or loose connections or damaged wiring, the live and the neutral wires may sometimes touch each other. This results in a sudden large current flowing through the circuit. This is called short circuiting. The enormous heat produced due to a large current can damage electrical devices such as sound systems, bulbs, mixer grinders, televisions and so on, or even cause a fire.

3. Electric shocks : Due to negligence if a person touches an exposed live wire in a circuit, he will receive a severe shock. Electric shocks can often be fatal.

F. Solve the following numerical problems.

Question 1.
A fan of 60 W runs for 75 hours in a month. How much electrical energy is consumed?
Answer:
Power of the fan = 60W = 60/1000 kW = 0.06 kW
Energy consumed by the fan in 75 hours = 0.06 W x 75 h = 4.5 kWh
So, electrical energy consumed by a fan of 60 W in a month is 4.5 kWh.

Question 2.
An electric kettle rated at 2.2 kW works for 4 hours a day. Calculate the number of units consumed by it in 20 days.
Answer:
Power of an electric kettle = 2.2 kW
Electrical energy consumed by the kettle in 1 day
= 2.2 kW x 4 h = 8.8 kWh
Electrical energy consumed by the kettle in 20 days
= 8.8 kWh x 20 = 176 kWh

Question 3.
A heater is rated at 5 kW. Calculate the cost of energy consumed by it in 30 days if it is run for 2 hours daily. The cost of 1 kWh is priced at ₹ 4.20.
Answer:
Power of a heater = 5 kW
Electrical energy consumed by the heater in 1 day = 5 kW x 2 h = 10 kWh
Electrical energy consumed by the heater in 30 days = 10 kWh x 30 = 300 kWh
Cost of 1 kWh of electrical energy = ₹ 4.20
Cost of 300 kWh of electrical energy = ₹ 4.20 x 300 = ₹ 1260
Hence, the cost of energy consumed by the heater in 30 days is ₹ 1260.

Question 4.
Five LED bulbs of 8 W each, two fans of 40 W each, an electric motor of 3 kW, and a geyser of 5 kW are used for 8 hours per day by a family. Calculate the total energy consumed in 30 days and the cost of electricity used in one day at the rate of ₹5 per unit.
Answer:
Total power = 5 x 80/1000 kW + 2 x 40/1000 kW + 1 x 3 kW +1×5 kW = 8.48 kW
Electrical energy consumed in 1 day = 8.48 kW x 8 h = 67.84 kWh
Electrical energy consumed in 30 days = 67.84 kWh x 30 = 2035.2 kWh
Cost of 1 kWh of energy = ₹ 5
Cost of energy used in 1 day = ₹5 x 67.84 = ₹ 339.2

G. Answer the following.

Question 1.
Yashika was given an electroscope with its gold leaves diverged. What do the diverged leaves indicate? She was also given a rod which she did not know was charged or not. Yashika touched the rod to the cap of the electroscope to find that the divergence of the gold leaves decreased. What should she infer about the charge status of the rod?
Answer:
The diverged gold leaves of the electroscope indicates that the electroscope is charged. When the rod was brought in contact with the cap of the electroscope, the divergence of the gold leaves decreased. This means that the rod was charged with a charge opposite to that of the electroscope.

H. Draw diagrams of the following.

1. Identifying the nature of charge on an object using a positively charged electroscope.
2. Lightning Conductor
Answer:

Think and answer

Question 1.
Why does dust accumulate on ceiling fans?
Answer:
Dust accumulate on ceiling fans due to the electrical attraction between the blades and dust particles. As the blades of a fan rotate, they experience frictional forces due to the air around it. Due to this, the blades of the fan get charged and attract the dust particles in the air.

Question 2.
Why is a gold leaf electroscope enclosed in a glass case?
Answer:
An electroscope is enclosed in a glass jar to protect the gold leaves from the drafts of air current.

Question 3.
Why does current flow through the earth wire?
Answer:
In case of a fault, excessive current flows through the earth wire because it provides conducting path to the earth.

Question 4.
Why is a switch for any appliance always connected to the live wire?
Answer:
A switch must be connected to the live wire so that when it is in OFF position, then the appliance to which it is connected will be disconnected from the live wire. If the switch would be connected to the neutral wire, then the electrical appliance will be connected to the live wire even when the switch is in OFF position. This may cause an electric shock if a person touches the appliance.

Living Science Physics Class 8 ICSE Solutions