6 electricity - the assam valley school today/worksheets/7/questio… · class-vii physics question...

1Class-VII Physics Question Bank

6 ELECTRICITYI. Tick (✔✔✔✔✔) the most appropriate answer.

1. Electrical work done per unit time is(a) electrical energy (b) electrical current(c) electrical power (d) electrical circuit

2. One megawatt is equivalent to(a) 102 W (b) 106 W (c) 104 W (d) 108 W

3. Electric current is the flow of(a) Electrons (b) protons (c) neutrons (d) none of these

4. The colour of the earth wire is generally(a) red (b) blue (c) black (d) green

5. Fuse wire is a(a) tin-lead alloy (b) copper-lead alloy(c) tin-copper alloy (d) silver-lead alloy

6. A person gets a severe electric shock on touching(a) a neutral wire (b) a live wire(c) an earth wire (d) none of these

7. A bulb of 100 W means that it will consume electrical energy at therate of(a) 100 kJ per second (b) 100 kJ per hour(c) 100 joules per second (d) 100 joules per hour

8. Potential difference between any two points is measured in(a) volts (b) joules (c) coulombs (d) none of these

9. All wires used in electric circuits should be covered with(a) a coloured material (b) an insulating material(c) a conducting material (d) none of these

10. The fuse wire should have a(a) low melting point (b) high melting point(c) very high melting point (d) none of these

Ans. 1. (c) 2. (b) 3. (a) 4. (d) 5. (a) 6. (b)7. (c) 8. (a) 9. (b) 10. (a)


II. Fill in the blanks.1. When a short-circuit takes place, the fuse wire melts and breaks the

______________.2. If a person touches a live wire, he gets a severe ___________.3. The electric meters in our houses measure the electrical energy

consumed in __________.4. Charge will flow from one body to another body only when there is a

_______________________________ between them.5. The ________________ is considered to be at zero potential.6. One kilowatt-hour is equivalent to _________________ joules.7. A live wire is ________________ in colour.8. The overheating of electrical wiring in any circuit due to the flow of

large current though it is called _______________ of electric circuit.9. A flow of very large current due to the direct contact of a live and a

neutral wire is called _____________.10. The flow of charge per unit time through a conductor is called

__________.11. The level of charge per unit time on a body is called ____________

of the body.12. The voltmeter is an instrument used to measure ______________.13. The dry cell was invented by ________________.14. Electricity moves only in a ________________ circuit.15. Homes are wired in __________________.16. When we turn the switch to the off position the circuit is __________.17. When you remove a bulb from a three bulb circuit, and the other two

bulbs go off, then the bulbs must have been be connected in__________________.

18. When you attach a wire to both terminals of a dry cell, the electronsflow from the _____________ terminal to ____________ terminal.

19. _____________ and ________________ are semi-conductors.Ans. 1. circuit 2. electric shock 3. kilowatt-hour

4. differnce in the potential 5. earth6. 3.6 × 106 7. red 8. overloading


9. short-circuit 10. electric current 11. potential difference12. potential differnce 13. leclanche 14. closed15. parallel circuit 16. open 17. series circuit18. negative, positive 19. Germanium, silicon

III. Fill in the blank spaces by choosing words from the list given below.List: electric current, conducting material, conductor, cell, dynamo,power house.

1. The source of electric current for hand held transistor is __________.2. Electricity used in homes is generated in __________________.3. An electricity generating machine consists of a turbine and a

_______________.4. The substance which allows _______________ to flow through it

is called _______________.5. A closed circuit has its every part made of _______________.

Ans : 1. cell 2. power house 3. dynamo 4. electric current, conductor5. conducting material.

IV. Statements given below are incorrect. Write correct statements.1. Fuses should be connected to the neutral wire.

Ans. Fuses should be connected to the live wire.2. We should switch on the main switch if a person accidentally touches

a live wire.Ans. We should switch off the main switch if a person accidentally touches

a live wire.3. A large number of electrical appliances connected in a circuit can be

operated together.Ans. A large number of electrical appliances connected in a circuit can

not be operated together.4. We can get an electric shock if we touch an earthed appliance.

Ans. We can get an electric shock if we touch an unearthed appliance.5. Black wire is the earth wire.

Ans. Green wire is the earth wire.


6. There is no relation between the SI unit and commercial unit ofelectrical energy.

Ans. There is the relation between the SI unit and commercial unit ofelectrical energy, i.e., 1 Kwh = 3.6 × 106 J.

7. Electrical work done by electric current is equal to electrical powerconsumed.

Ans. Electrical work done by electric current is equal to electrical energyconsumed.

8. Pure water is a good conductor of electricity.Ans. Pure water is a bad conductor of electricity.

9. When the circuit is closed the electric current does not flow throughit.

Ans. When the circuit is open the electric current does not flow through it.10. Bulb is the source of electricity in a torch.

Ans. Cell is the source of electricity in a torch.11. Glass is a good conductor.

Ans. Glass is a good insulator.12. A cycle rickshaw works on electricity.

Ans. A cycle rickshaw works on mechanical energy.13. Silk threads are very good conductors.

Ans. Silk threads are bad conductors.

V. Match the following.1. electric power (a) volt2. current (b) joule3. resistance (c) watt4. electrical energy (d) coulomb5. electric charge (e) ampere6. potential difference (f) ohm

Ans. 1. (c), 2. (e), 3. (f), 4. (b), 5. (d), 6. (a)

VI. Give reasons for the following.1. An electric iron should be earthed.

Ans. An electric iron should be earthed because it protects the user from


receiving an electric shock.2. We should not be barefooted when we are repairing or replacing

any electrical component.Ans. We should not be barefooted when we are repairing or replacing

any electrical component. Because, if there is any damage or cut inwire, the current leaks through them and can give nasty shock.

3. Nichrome is used in an electric iron.Ans. Nichrome is used in an electric iron because it has high resistance

and high melting point.4. Tungsten metal is used to make the filament of electric bulb.

Ans. The filament of an electric bulb is made up of tungsten metal becauseit has very high resistance and very high melting point.

5. Inert gases like nitrogen and argon are filled in electric bulb.Ans. Inert gases like nitrogen and argon are filled in electric bulb because

it prevents the tungsten from melting or getting oxidized in air athigh temperature.

6. Current carrying wires must be insulated.Ans. Current carrying wires must be insulated because it is dangerous to

anyone to touch the current carrying wires, if it is not insulated.7. Fuse wire should be made of low melting point materials.

Ans. Fuse wire should be made of low melting point materials becausewhen some overloading or voltage fluctuation takes place, the fusewire melts. This in turn breaks the electric circuit, and hence currentstops flowing in the circuit.

VII. Find the odd-one out. Give reasons for your choice.1. Copper, aluminium, iron, wood, brass

Ans. Wood — Copper, aluminium, iron and brass all are the goodconductors of electricity while wood is bad conductor of electricity.

2. Plastic, rubber, glass, silverAns. Silver — Plastic, rubber and glass all are bad conductors of

electricity while silver is good conductor of electricity.3. Water heater, electric kettle, electric iron, electric fuse

Ans. Electric fuse — Water heater, electric kettle and electric iron all are


electric appliances and operated by electricity while electric fuse isused in the electric circuit to avoid short circuit and hence, preventsan electric fire.

4. Dry cell, solar cell, generator, electric bulbsAns. Electric bulbs — Dry cell, solar cell and generator, all are used to

produce electrical energy while electric bulb uses electrical energyto glow.

VIII. In the given circuit diagrams, which circuit will (a) light up (b) notlight up? Give a reason for your answer.

Ans. Circuit (a) will not light up because rubber band is an insulator andhence, circuit is incomplete.


Circuit (b) will not light up because jute and thread both act asinsulators and hence, circuit is incomplete.

Circuit (c) will light up because copper wire acts as a conductorand hence, circuit is complete.

Circuit (d) will light up because aluminium and copper wires areconductors.

Circuit (e) will not light up because cotton thread in the circuit isinsulator.

Circuit (f) will not light up because cotton thread in the circuit isinsulator.

Circuit (g) will not light up because plastic piece in the circuit is aninsulator and hence, circuit is incomplete.

Circuit (h) will light up because aluminium and silver both are goodconductor of electricity and hence, circuit is complete.

IX. Write short notes on.(a) Effect of electricity on human body.

Ans. Human body is a good conductor of electricity. When a strongcurrent flowing through the body it produces violent muscularcontractions and the person receive a shock. These shocks candamage body cells and can prove fatal at times.

(b) Conservation of electricity.Ans. In modern time, use of electricity is increasing day-by-day. Electricity

is used in homes, offices, factories and even in agriculture electricpumps to draw water from ground to irrigate the fields.In our homes television, computer, refrigerator are commonappliances and all of these appliances need electricity. So, we shouldcontribute towards conserving electricity by putting in simple effortslike turning off lights and fans when not needed. Always usefluorescent bulbs instead of incandescent bulbs. These are somesimple steps to conserve electricity.

(c) Earthing.Ans. In domestic supply, the earth wire is introduced for safety. One end

of the earth wire is connected to the electrical appliance while the


other end is connected to a large metal plate buried deep inside theground. This is called earthing of an appliance. Any current leakinginto the appliance flows harmlessly to the earth. Therefore, we donot get an electric shock if we touch such an earthed appliance.

X. Write one difference between.(a) Dry cell and storage cell

Ans. In dry cell chemical reaction is irreversible whereas, in storage cellchemical reaction is reversible.

(b) Potential difference and electric currentAns. Electric current is the rate of flow of charge between two bodies at

different potentials, while potential difference is the amount of workdone in bringing a unit positive charge from one point to the other.

(c) Direct current and alternating currentAns. Direct current is the flow of electricity in only one direction. While

alternating current reverses its direction at regular intervals of time.(d) Conductors and insulators

Ans. Conductors are the substances which allow the current to flowthrough them with ease while insulators do not allow the current toflow through them with ease.

(e) Series and Parallel connectionAns. In parallel circuit, the current flows through two or more paths while

in series circuit, the current flows in single path.IX. Answer the following questions.

1. Name three sources of electricity.Ans. Electric generator, solar cell, and dry cell are the sources of

electricity.2. What is the relationship between electric current electric power and

potential difference?

Ans. Electric current (I) = Electric power (P)

Potential difference (V)


3. List the three main effects of electric current.Ans. The three main effects of electric current are:

(i) heating effect (ii) magnetic effect (iii) chemical effect4. What is the relationship between heat produced, resistance, current

and time.

Ans. H = I2 Rt

Where, H = heat produced, I = current, R = resistance and t = time.5. Name four appliances based on the heating effect of electric current.

Ans. Many devices like electric iron, electric kettle, electric heater andelectric bulb work on the basis of the heating effects of an electriccurrent.

6. How does a fuse work?Ans. The electric fuse works on the heating effect of current. The electric

fuse is connected in the live wire of a household circuit. If due to afault in the household circuit like short- circuiting or over-loading, avery large current flow in a household circuit, the fuse wire getsheated and melts. This breaks the circuit and hence protects theelectric appliances.

7. An electrical bulb has the figures 220 V, 100 W written on it. Whatdoes it mean?

Ans. If a bulb has the figures 220 V and 100 W, it means, this bulb is tobe used on a voltage of 220 volts and will consume electrical energyat the rate of 100 joules per second.

8. What are the three wires in an electric circuit? Write their colours.Ans. All electrical circuits consists of three wires, a live wire red in colour,

a neutral wire black in colour and an earth wire green in colour.9. Why the consumption of electricity is increasing in recent time?

Ans. Consumption of electricity is increasing day-by-day due to moreand more use of some electrical appliances like air conditioner,television, computer, refrigerator etc for the comfort of human life.So we should conserve electricity.

10. How can you prevent the wastage of electricity in your home?


Ans. We can prevent the wastage of electricity in the following manner:(i) We should use tube lights in place of electric bulbs to save

electricity.(ii) At omes only those appliances should be used which are

absolutely necessary.(iii) Water should not be heated to a high temperature.(iv) Use good quality wires, as poor quality wire wastes lot of

electric energy.11. Write an experiment to show that heating effect is produced when

an electric current flows through a conductor.Ans. Electric current has heating effect. We can prove it by the following

experiment:Take a wire with two stands and clamp it tightly. Now pass electriccurrent through this wire by connecting its ends to the terminals of alead-storage battery. After sometime we see that the wire is found tosag. This is because the wire has expanded due to heating effect ofelectric current.This shows that, if electric current flows through a conductor, itgets heated up.

12. What are the hazards associated with electricity?Ans. Today electricity is one of the most important and convenient sources

of energy. But its improper use makes it dangerous in the followingmanner.


(i) Overloading: When we connect a number of appliances in acircuit, they draw a large current. It makes overheating ofelectric wire and leads to short-circuiting and electric fires. Itis called overloading of electric circuit.

(ii) Short-circuiting: Sometimes due to damaged wiring, the livewire and neutral wire in a circuit touch each other. It makesresistance of the circuit almost zero and a very large currentflows through it. This sudden flow of very large current due tosudden contact of a live and neutral wire is called short-circuiting.

(iii) Electric shock: If anyone touches a live electric wire, he/shegets a severe electric shock as our body is a good conductorof electricity.

13. Distinguish between overloading and short-circuiting of an electriccircuit.

Ans. (i) Overloading: The overheating of electrical wiring in any circuitdue to flow of large current through it is called overloading ofelectric circuit.

(ii) Shorting circuiting: It is the sudden flow of very large currentdue to the direct contact of a live and a neutral wire.

14. Name two household appliances which produce light fromelectricity.

Ans. Electric bulb and tube light are the two household appliances thatproduce light from electricity.

15. Name two household appliances which are operated by electricityand cool the surroundings.

Ans. Water cooler and air conditioner are the two household applianceswhich are operated by electricity and cool the surroundings.

16. Name two industrial appliances which use electricity.Ans. Drill machine and Welding machine are industrial appliances which

use electricity.17. Name two household appliances which use electricity and are the

source of entertainment.


Ans. Radio and television are the two household appliances which useelectricity and are the sources of entertainment.

18. Name two appliances which do not need electricity.Ans. Bicycle and a diesel engine do not need electricity.

19. Name four appliances which can work with cell.Ans. Transistor, watch, mobile phone, torch are the appliances which

can work with cell.20. What do you understand by the terms (i) closed circuit (ii) open

circuit.Ans. The path along which the electric current flows is called a circuit. If

there is complete flow of current in any circuit then it is called closedor complete circuit.If there is no complete flow of electric current in any circuit due tosome obstructions, then this type of circuit is called open orincomplete circuit.

21. What is conductor of electricity? Give five examples.Ans. The substances, which allow the electric current to flow through

them with ease are called conductors of electricity. Iron, silver,aluminium, steel and impure water are conductors of electricity.

22. What is an electric circuit?Ans. The path along which the electric current flows is called an electric

circuit. It is of two types: open circuit, closed circuit.23. What is an insulator of electricity? Give five examples.

Ans. The substances which do not allow the electric current to flowthrough them with ease are called the insulators of electricity. Wood,glass, paper, plastic and rubber are insulators of electricity.

24. How do we produce electricity on large scale? How does thiselectricity reach our homes?

Ans. On large scale, electricity is produced by turbine with the help ofwater or steam. Blades of turbines are rotated with the help of wateror steam. The energy of the turbine operates the dynamo whichfinally converts the mechanical energy into electric energy. Electricenergy so generated at power houses is carried by electric cables to


the cities and villages.25. What are the two kinds of charges?

Ans. There are the two kinds of charges:(i) Positive charge (ii) Negative charge.

26. A glass rod is rubbed with silk. State the charge acquired by each.Ans. When a glass rod is rubbed with silk, it acquires, positive charge,

because electrons are less tightly bound in silk. While due to gain ofequal number of electrons, the silk becomes negatively charged.

27. What do you understand by the term electric current?Ans. The rate of flow of charge is equal to the electric current. In other

words, the current is the amount of charge flowing in one secondacross the cross-section of a conductor, the cross-section beingnormal to its length or to the direction of flow of current.If a charge Q flows through the cross-section of a conductor intime ‘t’ then

Current (I) =charge (Q)

time (t)28. In a cell, which form of energy is converted to electrical energy?

Ans. In a cell, chemical energy is converted to electrical energy.29. What is electroplating?

Ans. When an electric current is passed through the electrolyte, thepositive metal ions move to the cathode and get uniformly coatedon the object kept as the cathode (negative electrode).This processis called electroplating.

30. Why is electric wiring in homes connected in parallel circuits?Ans. Electric wiring in homes are connected in parallel circuits because

if we remove a bulb, the circuits still remain closed. So the otherbulb continues to glow.

31. Name the condition to complete an electric circuit.Ans. A complete circuit is formed only when both the terminals of the

battery are connected to the bulb through the conducting wires.32. Name the instrument used to measure electric current.

Ans. Electric current is measured in amperes (A) by using an instrument


called ammeter.34. Pick out the conductors and insulators from the following list:

(a) Brass (b) Mica(c) Air (d) Silver(e) Aluminium (f) Human body(g) Lead (h) Copper(i) Glass (j) Plastic

(k) Bakelite (i) Rubber(m) Common salt solution (n) Alcohol(o) Mercury (p) Wool(q) Leather (r) Ebonite

Ans. Conductors. Brass, aluminium, mercury, silver, copper, human body,alcohol.Insulators. Air, lead, glass, leather, mica, plastic, rubber, wool,ebonite, bakelite

X. Practice for numerical problems.1. A current of 4 A flows through a 12 V car headlight bulb for

10 minutes. How much electrical energy is consumed during this time?Ans. According to question,

Current I = 4 APotential difference V = 12 V

Time t = 10 minutes = 10 × 60 = 600 sElectric energy spent W = V × I × t

= 12 × 4 × 10 × 60= 28800 J = 28.8 KJ

2. What will be the current drawn by an electric bulb of 1000 W whenit is connected to a source of 220 V?

Ans. Power P = 1000 WVoltage V = 220 VCurrent I = ?

I =PV =

1000220 = 4.55 A

3. A current of 4 A flows through a 20 V car headlight bulb. Calculate


the electric power.Ans. Current I = 4 A

Voltage V = 20 VPower P = ?

I =PV

P = V × I = 20 × 4 = 80 W4. Calculate the electric power of an electrical appliance in which 10 A

current is flowing through a resistor of 2 ohms.Ans. Current I = 10 A

Resistance R = 2 ΩPower P = ?

V = I × R = 10 × 2 = 20 V∴ P = V × I = 20 × 10 = 200 W

5. Calculate the electric power of an electrical appliance of 20 V havinga resistance of 10 ohms.

Ans. Potential difference V = 20 VResistance R = 2 Ω

Power P = ?V = I × R

I =VR

= 202

= 10 A

P = V × I = 20 × 10 = 200 W6. Calculate the electrical energy consumed when a potential difference

of 5 V is applied across a resistor of 2 ohm for 30 minutes.Ans. Potential difference V = 5 V

Resistance R = 2 ΩTime t = 30 minutes = 30 × 60 = 1800 s

Electrical energy E = ?V = I × R

∴

I =VR

= 52

= 2.5 A


P = I × V = 2.5 × 5 = 12.5 WE = P × t = 12.5 × 1800

= 22500 J = 22.5 KJ7. A current of 0.5 A flows through a resistance of 10 ohms for half an

hour. Find the amount of electrical energy consumed by the resistance.Ans. Current I = 0.5 A

Resistance R = 10 Ω Time t = 0.5 hour = 1800 s

Electrical energy E = ?V = I × R = 0.5 × 10 = 5 VP = V × I = 5 × 0.5 = 2.5 WE = P × t = 2.5 × 1800

= 4500 J = 4.5 KJ8. An electric meter reads 3462 kWh at the end of the month while a

month ago it read 3396 kWh. How many units of electrical energyhave been consumed during the month? Find the cost of electricalenergy consumed if 1 unit of energy costs Rs 1.20.

Ans. Initial reading of electric meter = 3396 kWhFinal reading of electric meter = 3462 kWh

Total energy consumed during the month= Final reading – Initial reading= 3462 – 3396= 66 kWh = 66 units

Cost of 1 unit = Rs 1.20 Cost of 66 units = 66 × 1.20

= Rs 79.20

6 electricity - the assam valley school today/worksheets/7/questio… · class-vii physics question...

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