Selina Concise Chapter 8 Electricity ICSE Solutions Class 8 Physics
Exercise 8 A
A. Objective Questions
1. Write true or false for each statement:
(a) A fuse wire has a high melting point.
Answer
(b) Flow of protons constitutes electric current.
Answer
(c) Silver is an insulator of electricity.
Answer
(d) S.I. unit and commercial unit of electrical energy are same.
Answer
(e) Overloading of electric current in circuits can lead to short circuiting.
Answer
(f) Our body can pass electricity through it.
Answer
(g) All metals are insulators of electricity.
Answer
(h) The earth wire protects us from an electric shock.
Answer
(i) A switch should not be touched with wet hands.
Answer
(j) AH electrical appliances in a household circuit work at the same voltage.
Answer
(k) In a cable, the green wire is the live wire.
Answer
(l) A fuse is connected to the live wire.
Answer
(m) A switch is connected to the neutral wire.
Answer
2. Fill in the blanks
(a) The unit in which we pay the cost of electricity is _____.
(b) The electrical energy consumed in a house is measured by _____.
(c) In a household electrical circuit, the appliance are connected in _____ with the mains.
(d) A switch is connected to the _____ wire.
(e) The red colour insulated wire in a cable is the _____ wire.
(f) One kilowatt-hour is equal to ______ joule.
(g) A fuse wire should have low ______.
(b) The electrical energy consumed in a house is measured by kWh meter.
(c) In a household electrical circuit, the appliance are connected in parallel with the mains.
(d) A switch is connected to the live wire.
(e) The red colour insulated wire in a cable is the live wire.
(f) One kilowatt-hour is equal to 3.6 × 106 joule.
(g) A fuse wire should have low melting point.
3. Match the following
|
Column A |
Column B |
|
(a) Electric power |
(i) Volt |
|
(b) kWh |
(ii) joule |
|
(c) Electric current |
(iii) volt × ampere |
|
(d) Electric energy |
(iv) Watt |
|
(e) watt |
(v) ampere |
|
(f) potential difference |
(vi) electrical energy |
Answer
|
Column A |
Column B |
|
(a) A stone at a height |
(iii) Maximum displacement |
|
(b) A moving hall |
(v) 1/time period |
|
(c) Energy |
(ii) amplitude |
|
(d) Power |
(i) frequency |
|
(e) watt |
(iv) presence of other frequencies |
|
(f) potential difference |
(vi) electrical energy |
4. Select the correct alternative
(a) All wires used in electric circuits should be covered with
1. colouring material
2. conducting material
3. an insulating material
4. none of the above
(b) Electric work done per unit time is
1. electrical energy
2. electric current
3. electric voltage
4. electrical power
(c) One kilowatt is equal to
1. 100 watt
2. 1000 watt
3. 10 watt
4. none of these
(d) Fuse wire is an alloy of
1. tin-lead
2. copper-lead
3. tin-copper
4. lead-silver
(e) A fuse wire should have
1. a low melting point
2. high melting point
3. very high melting point
4. none of the above
(f) When switch of an electric appliance is put off, it disconnects
1. the live wire
2. the neutral wire
3. the earth wire
4. the live and the neutral wire
(g) The purpose of an electric meter in a house is
1. to give the cost of electricity directly
2. to give the consumption of electrical energy
3. to safeguard the circuit from short circuiting
4. to put on or off the mains.
(h) If out of the two lighted bulbs in a room, one bulb suddenly fuses, then
1. other bulb will glow more
2. other bulb will glow less
3. other bulb will also fuse
4. other bulb will remain lighted unaffected.
B. Short/Long Answer Questions
1. From where does the electricity come to our home ?
Answer
In our homes electricity comes from the City Substation.
2. What is an electric meter ? Where is it fixed in our house ?
Answer
Electric Meter: It is energy meter or kWh meter and measures the amount of electric energy consumed in kWh for which electricity bill is paid.
It is connected through a cable after company fuse on the front or outside wall of the house.
3. State the purpose of kWh meter.
Answer
Purpose of kWh meter is to measure the electric energy consumed in kWh for which the electricity bill is to be paid. i.e. units of energy consumed.
4. For which unit do we pay our electricity bill ?
Answer
Unit: It is board of trade unit i. e. kWh = 1000 W used in 1 hour.
5. How can you check just by seeing the meter whether the electricity is in use or not ?
Answer
If the electricity is in use, the counter fixed on the armature of the meter rotates and reads the number of rotations (A red mark is seen rotating).
6. The diagram below in figure shows the reading on the dials of a meter. State what is its
reading.
Answer
The reading is 2 7 8 9
7. One day the meter reading is found to be 7643 units while next day, it was 7657 units. What is the consumption of electricity in a day ?
Answer
Final reading: 7657 units
Initial reading: 7643 units
Consumption of electricity in a day
= Final reading-initial reading
= 7657 – 7643
= 14 units.
8. A source of potential difference V volt sends current I ampere in a circuit for time t second. Write expressions for
(a) electrical energy supplied by the source, and
(b) electrical power spent by the source.
Answer
Potential difference V is the work done in moving a unit charge
∴ Work done in moving a charge Q through pot. diff. V = QV
i.e. W = QV
but = Q/t = I or Q = It
∴ W = VIt
Work done = Electrical energy
(a) ∴ Electrical energy = VIt is the expression
(b) Power is the rate of doing Work
P = VI is the expression.
Unit in which the electricity bill is charged is board of TRADE
Unit = kWh
1 Unit = kWh = 3600000 J = 3.6 × 106 J
1 kWh= 3.6 × 106 J
10. If an appliance of power P watt is used for time t hour. How much electrical energy is consumed in kWh.
Answer
Power P = W/t = Electrical energy/t
∴ Electrical energy = P × t
11. What is an electric fuse ? State its purpose in the household electrical circuit.
Answer
Fuse: It is a safety device which is used to limit the current in an electric circuit.
Purpose of Fuse in House hold Circuit:
It safe guards the circuit and the appliances connected in the circuit from being damaged if the current in the circuit exceeds the specified value due to voltage fluctuation or short circuiting.
12. State one property of the material of a fuse wire.
Answer
Two characteristic of Fuse Wire are:
- Has low melting point.
- Has high resistance.
13. Name the material of a fuse wire.
Answer
Material used is an Alloy of Lead(50%) and Tin (50%) having low M.P. of 200°C.
14. Can we use copper wire as a fuse wire? Give reason.
Answer
Copper wire cannot be used as a fuse wire since meting point of Cu is higher and resistance is very low and current can flow through it without melting it and without breaking the circuit.
15. How does a fuse protect the electric wiring (or an appliance) from being damaged ?
Answer
1 Unit = kWh=3600000 J = 3.6 × 1066 J
Due to voltage fluctuations or insulation on the wire of cable gets worn and tom (breaks), the live wire comes in contact with neutral wire and results in Short Circuit. To prevent this damage, a fuse is used in the Live Wire. The fuse wire gets heated up to the extent that it melts, a gap is produced in the live wire and the circuit becomes incomplete, no current flows and the appliance is saved.
16. Which fuse wire is thick: 5 A or 15 A?
Answer
∴ R ∝ l/a
More the area, lesser is the resistance and more is the current flowing.
∴ 15 A fuse wire is thick.
17. Write the full form of M.C.B.
Answer
Miniature Circuit Breaker.
18. How is M.C.B. superior to the fuse wire ?
Answer
M.C.B. automatically fall down and switch off the circuit in very short time, when excessive flow of electric current in a circuit. It is raised up after the fault is rectified.
19. With which wire: live or neutral is the fuse wire connected?
Answer
Fuse is connected with Live Wire.
20. What do you mean by short circuiting of a circuit ?
Answer
Short Circuit: If the insulation on the wire of cable used in the wiring (or used with an appliance) break. The live wire comes in contact with the neutral wire. This results in a short circuit.
21. Figure shows two ways of connecting the three bulbs A, B and C to a battery. Name the two arrangements. Which of them do you prefer to use in a household circuit? Give a reason to support your answer.
Answer
- (a) is Series arrangement .
- (b) is parallel arrangement.
22. How are the electrical appliances connected in a house circuit: in series or in parallel? Give reason.
Answer
Electrical appliances in a house are connected in parallel as :
- Each appliance gets full voltage as resistance in parallel becomes low.
- If fuse of one appliance is fails it does not effect the working of other appliances.
- We can light one bulb without lighting the other bulbs.
23. In the household electric circuit, if one bulb is fused in a room, the other bulbs keep glowing. Explain the reason.
Answer
In the house hold electric circuit the appliances are connected in parallel and each appliance has an independent path for current and work at the same voltage. Hence, if one bulb in a room is fused, the other bulbs keep glowing.
24. State the voltage at which electricity is supplied to our houses.
Answer
Electricity voltage is supplied to our houses at 220 volt.
25. Draw a labelled diagram with the necessary switches to connect a bulb, a fan and a plug socket in a room with the mains. In what arrangement will you connect them to the mains ?
Answer
26. State the colour coding of the three wires in a cable used for
Answer
Colour coding of three wires:
27. Why is the metal covering of an electrical appliance earthed?
Answer
Earthing of the Appliance: To protect from the electric shock electric appliance is earthed. Sometimes due to break of insulation of wires, live wire comes in contact with the body of appliance and we get a fatal shock when the appliance is touched. If the appliance is earthed, the current will pass to the earth and we remain protected from the electric shock.
C. Numericals
1. An electrical appliance is rated as 60 W – 150 V.
(a) What do you understand by this statement ?
(b) How much current will flow through the appliance when in use ?
Answer
(a) 60 W-150 V power rating of appliance means, If appliance is put in circuit on 150 volt, it will consume 60 w electrical power i.e. electric energy consumed by it in 1 sec. is 60 J.
(b) P = VI
∴ Current will through the appliance
2. An electric iron of power 1.5 kW is used for 30 minute to press the clothes. Calculate the electrical energy consumed in (a) kilowatt hour (b) joule.
Answer
(a) P = W/t
∴ Electrical energy, W = P × t
P = 1.5 kW t = 30 minute = 30/60 = 1/2 hr
1.5 × 1/2 = 0.75 kwh
(b) 1 kWh = 3.6 × 106 J
∴ 75 kWh =0.75 × 3.6 × 106
= 2.700 × 106
= 2.7 × 106 J
3. Assuming the electric consumption per day to be 12 kWh and the rate of electricity to be? 6.25 per unit, find how much money is to be paid in a month of 30 days ?
Answer
Electric energy consumed per day = 12 kWh = 12 units
Electric energy consumed in 30 days = 12 × 30 = 360 units
Cost to be paid in 1 month = 360 × 6.25 = ₹ 2250
4. In a premise 5 bulbs each of 100 W, 2 fans each of 60 W, 2 A.Cs each of 1.5 kW are used for 5 h per day. Find :
(a) total power consumed per day,
(b) total power consumed in 30 days,
(c) total electrical energy consumed in 30 days.
(d) the cost of electricity at the rate of ₹ 6.25 per unit.
Answer
Power consumed by
5 bulbs = 5 × 100 = 500 W
2 fans = 2 × 60= 120 W
2 A.C = 2 × 1.5 × 1000 = 3000 W
(a) Total power consumed per day = 3620 W
(b) Total power consumed in 30 days = × 30 = 108.6 kw
(c) Electric energy is used for 5 h per day.
Total electrical energy consumed in 30 days.
(d) Cost of electricity = P × t
= 108.6 × 5
= 543 kWh
= 543 × 6.25
= ₹ 3393.75
Exercise 8 B
A. Short Answer Questions
1. Write true or false for each statement:
(a) The number of electrons and protons in an atom are same.
Answer
True
(b) If the charge is not in motion, we call it static electricity.
Answer
True
(c) Human body is a conductor of electricity.
Answer
True
(d) When an ebonite rod is rubbed with fur, the electrons move from ebonite to fur.
Answer
False
Correct: When an ebonite rod is rubbed with fur, the electrons move from fur to ebonite.
(e) When, glass rod is rubbed with dry silk cloth, the electrons move from glass to silk.
Answer
True
(f) The cap of gold leaf electroscope is made of copper.
Answer
False
Correct: The cap of gold leaf electroscope is of brass.
(g) If a glass rod rubbed with silk is brought near the cap of a negatively charged electroscope, the divergence of leaves will decrease.
Answer
True
(h) In induction, a positively charged body can make an uncharged ‘ body positively charged.
Answer
False
Correct: In induction, a positively charged body can make an uncharged body negatively charged.
(i) A lightning conductor saves the building from lighting.
Answer
True
(j) When a comb is rubbed with dry hair both comb and paper ‘ get similarly charged.
Answer
False
Correct: A comb rubbed with hair and brought near pieces of r paper attracts them, because both have dissimilar charges.
(k) A glass rod rubbed with silk repels an ebonite rod rubbed with fur.
Answer
False
Correct: A glass rod rubbed with silk attract an ebonite rod rubbed with fur.
(l) When a ebonite rod is rubbed with fur, the protons move from the ebonite rod to the fur.
Answer
False.
Correct: When a ebonite rod is rubbed with fur, the free electrons move from the fur to the ebonite rod.
(m) A conductor has a large number of free electrons.
Answer
True
(n) An ebonite rod can be charged by touching it with a charged copper rod.
Answer
False
Correct: An ebonite rod can be charged by not touching it with a charged copper rod, will be charged by induction method.
(o) To find whether a body is charged or not, an uncharged electroscope is used.
Answer
True.
(p) To find whether the charge on a body is positive or negative, an uncharged electroscope is used.
Answer
False
Correct: To find whether the charge on a body is positive or negative, an charged electroscope is used.
(q) If a negatively charged rod is brought near a negatively charged pith ball electroscope, the pith ball will be stuck with the rod.
Answer
False
Correct: If a negatively charged rod is brought near a positively charged pith ball electroscope, the pith ball will be stuck with the rod.
2. Fill in the blanks
(a) Like charges____ while unlike charges attract.
(b) Mercury is a ____ of electricity while pure water is ____ of electricity.
(c) An ebonite rod when rubbed with fur acquires the _____ charge.
(d) When an uncharged conductor is brought in contact with the disc of a gold leaf electroscope, its leaves ______
(e) Charge is shared in charging a conductor by the method of _____
Answer
(a) Like charges repel while unlike charges attract.
(b) Mercury is a conductor of electricity while pure water is insulator of electricity.
(c) An ebonite rod when rubbed with fur acquires the negative charge.
(d) When an uncharged conductor is brought in contact with the disc of a gold leaf electroscope, its leaves will remain unchanged.
(e) Charge is shared in charging a conductor by the method of conduction.
3. Match the following
4. Select the correct alternative
(a) When a glass rod is rubbed with dry silk cloth, the charge acquired by the silk cloth is
1. positive
2. negative
3. both positive and negative
4. none of the above
Answer
2. negative
(b) When an ebonite rod is rubbed with fur, the rod acquires
1. positive charge
2. negative charge
3. no charge
4. none of the above
Answer
2. negative charge
(c) When a negatively charged body is brought closer to another negatively charged body, then they will show
1. attraction
2. no effect
3. repulsion
4. none of the above
Answer
3. repulsion
(d) Charging a conductor by bringing another charged conductor close to it without touching is called
1. induction
2. conduction
3. convection
4. radiation
Answer
1. induction
(e) The factor responsible for charging a conductor is
1. transfer of protons
2. transfer of neutrons
3. transfer of electrons
4. transfer of both protons and electrons
Answer
3. transfer of electrons
(f) Two objects when rubbed together get charged. The charges on them are
1. equal and opposite
2. equal and similar
3. unequal and similar
4. unequal and opposite
Answer
1. equal and opposite
(g) When a glass rod is rubbed with silk, the glass rod and the silk get charged because
1. electrons are transferred from the silk to the glass rod
2. electrons are transferred from the glass rod to the silk
3. protons are transferred from the silk to the glass rod
4. protons are transferred from the glass rod to the silk
Answer
2. electrons are transferred from the glass rod to the silk
(h) The conductor of electricity is
1. wood
2. glass
3. ebonite
4. human body
Answer
4. human body
(i) A gold leaf electroscope is to be charged positively by conduction. For this
1. a positively charged rod is held close to near the disc of electroscope
2. a positively charged rod is placed in contact with the disc of electroscope
3. a negatively charged rod is held near the disc of electroscope
4. a negatively charged rod is touched with the disc of electroscope
Answer
2. a positively charged rod is placed in contact with the disc of electroscope
(j) A glass rod rubbed with silk is touched with the disc of a negatively charged gold leaf electroscope. The divergence of leaves will
1. decrease
2. increase
3. remain unchanged
4. first decrease and then increase.
Answer
1. decrease
(k) The rod in a gold leaf electroscope is made up of
1. wood
2. brass
3. glass
4. ebonite
Answer
2. brass
(l) Lightning conductor is made up of:
1. copper
2. glass
3. ebonite
4. wood
Answer
1. copper
B. Short/Long Answer Questions
1. What do you understand by electricity at rest ?
Answer
When the charges are not allowed to flow are termed as static electricity, i.e., the electricity at rest.
2. Why does a plastic comb rubbed with dry hair attract bits of paper ?
Answer
Plastic comb rubber with dry hair gets charged and attracts the bits of paper.
3.Who discovered the way of producing electricity by friction?
Answer
Philosopher Thales discovered it but it was satisfactorily explained by Dr Gilbert.
4. Name two substances which can be charged by friction.
Answer
- Glass
- Ebonite
5. What are the two kinds of charges ?
Answer
- Negative charge.
- Positive charge.
6. A glass rod is rubbed with silk. State the kind of charge acquired by each.
Answer
When a glass rod is rubbed with silk, the glass rod loses electrons to the silk piece. The glass rod has free electrons which are less tightly bound so get transferred to the silk cloth. The glass rod has deficiency of electrons and acquired a positive charge while the silk cloth acquires a negative charge. On rubbing, electrons wove from glass rod to silk
7. An ebonite rod is rubbed with fur. State the kind of-charge acquired by each.
Answer
Experiment: To Demonstrate charges are of two kinds.
- Take two glass rods A and B and charge each by rubbing them with silk.
- Take two Ebonite rods C and D and charge each by rubbing with Fur or (flannel) suspend charged rod A by a thread in stand.
- Bring charged rod B near A. A gets deflected (moves away). This show that A is repelled by B.
- Now, suspend charged rod C in a stand and bring charged rod D ‘ near C. Again repulsion is seen.
- Now, bring charged rod C near the suspended charged rod A. This time attraction is there between the two rods.
- This prove that charges are of two kinds A and B have same kind of r charge called positive charge and similar charge repels eachother where as rods C and D also have similar charge (negative charge) and repel each other.
- Rod A attracts rod C prove that opposite charges attract each other.
8. Describe an experiment to demonstrate that there are two kinds of charges.
Answer
Aim: To show that there are two kinds of charges.
Apparatus: gold leaf electroscope, glass rod rubbed with silk cloth, any two non-identical bodies say A and B.
Procedure:
(i) Charge the gold leaf electroscope positively by touching it with glass rod rubbed with silk cloth.
(ii) The gold leaf electroscope shows its charging by showing its leaves diverging.
(iii) Touch the brass cap of this electroscope with body A-Note the divergence of the gold leaves.
(iv) Now touch the brass cap with body Band note the divergence.
Observation:
In case of body A: Leaves further diverge.
In case of body B:leaves converge.
Inference:
Body A (positively charged)
Body B (negatively charged)
So, there are two types of charges.
9. How will you show that like charges repel and unlike charges attract each other ?
Answer
Experiment:
- Take a glass rod rubbed with silk piece (positively charged) Suspend it in a stand with the help of a thread.
- Bring a negatively charged Ebonite rod (rubbed with fur) near the glass rod.
- Now, inter change them i.e. suspend B and bring A near it, again attraction is there. This show that unlike charges attract each other.
Negatively charged ebonite rods
- Now, take two glass rods (positively charged) and bring them near each other, repulsion takes place.
- Similarly, if we take two -vely charged ebonite rods and suspend one of them and bring second rod near it again the suspended rod moves away showing repulsion between like charges.
