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ELECTRICITY
✔ ) the most appropriate answer.
I. Tick (✔
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 the
rate 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)
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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 electrons
flow 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. earth
6
7. red
8. overloading
6. 3.6 × 10
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9.
12.
15.
18.
short-circuit
10. electric current
potential differnce 13. leclanche
parallel circuit 16. open
negative, positive
11.
14.
17.
19.
potential difference
closed
series circuit
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, conductor
5. 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.
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6. There is no relation between the SI unit and commercial unit of
electrical energy.
Ans. There is the relation between the SI unit and commercial unit of
electrical energy, i.e., 1 Kwh = 3.6 × 106 J.
7. Electrical work done by electric current is equal to electrical power
consumed.
Ans. Electrical work done by electric current is equal to electrical energy
consumed.
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 through
it.
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) volt
2. current
(b) joule
3. resistance
(c) watt
4. electrical energy
(d) coulomb
5. electric charge
(e) ampere
6. 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
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2.
Ans.
3.
Ans.
4.
Ans.
5.
Ans.
6.
Ans.
7.
Ans.
receiving an electric shock.
We should not be barefooted when we are repairing or replacing
any electrical component.
We should not be barefooted when we are repairing or replacing
any electrical component. Because, if there is any damage or cut in
wire, the current leaks through them and can give nasty shock.
Nichrome is used in an electric iron.
Nichrome is used in an electric iron because it has high resistance
and high melting point.
Tungsten metal is used to make the filament of electric bulb.
The filament of an electric bulb is made up of tungsten metal because
it has very high resistance and very high melting point.
Inert gases like nitrogen and argon are filled in electric bulb.
Inert gases like nitrogen and argon are filled in electric bulb because
it prevents the tungsten from melting or getting oxidized in air at
high temperature.
Current carrying wires must be insulated.
Current carrying wires must be insulated because it is dangerous to
anyone to touch the current carrying wires, if it is not insulated.
Fuse wire should be made of low melting point materials.
Fuse wire should be made of low melting point materials because
when some overloading or voltage fluctuation takes place, the fuse
wire melts. This in turn breaks the electric circuit, and hence current
stops 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 good
conductors of electricity while wood is bad conductor of electricity.
2. Plastic, rubber, glass, silver
Ans. 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
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electric appliances and operated by electricity while electric fuse is
used in the electric circuit to avoid short circuit and hence, prevents
an electric fire.
4. Dry cell, solar cell, generator, electric bulbs
Ans. Electric bulbs — Dry cell, solar cell and generator, all are used to
produce electrical energy while electric bulb uses electrical energy
to glow.
VIII. In the given circuit diagrams, which circuit will (a) light up (b) not
light up? Give a reason for your answer.
Ans. Circuit (a) will not light up because rubber band is an insulator and
hence, circuit is incomplete.
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Circuit (b) will not light up because jute and thread both act as
insulators and hence, circuit is incomplete.
Circuit (c) will light up because copper wire acts as a conductor
and hence, circuit is complete.
Circuit (d) will light up because aluminium and copper wires are
conductors.
Circuit (e) will not light up because cotton thread in the circuit is
insulator.
Circuit (f) will not light up because cotton thread in the circuit is
insulator.
Circuit (g) will not light up because plastic piece in the circuit is an
insulator and hence, circuit is incomplete.
Circuit (h) will light up because aluminium and silver both are good
conductor 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 strong
current flowing through the body it produces violent muscular
contractions and the person receive a shock. These shocks can
damage 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 electric
pumps to draw water from ground to irrigate the fields.
In our homes television, computer, refrigerator are common
appliances and all of these appliances need electricity. So, we should
contribute towards conserving electricity by putting in simple efforts
like turning off lights and fans when not needed. Always use
fluorescent bulbs instead of incandescent bulbs. These are some
simple 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
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other end is connected to a large metal plate buried deep inside the
ground. This is called earthing of an appliance. Any current leaking
into the appliance flows harmlessly to the earth. Therefore, we do
not 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 cell
chemical reaction is reversible.
(b) Potential difference and electric current
Ans. Electric current is the rate of flow of charge between two bodies at
different potentials, while potential difference is the amount of work
done in bringing a unit positive charge from one point to the other.
(c) Direct current and alternating current
Ans. 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 flow
through them with ease while insulators do not allow the current to
flow through them with ease.
(e) Series and Parallel connection
Ans. 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?
Electric power (P)
Ans. Electric current (I) = Potential difference (V)
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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 effect
4. What is the relationship between heat produced, resistance, current
and time.
Ans. H = I2 Rt
5.
Ans.
6.
Ans.
7.
Ans.
8.
Ans.
9.
Ans.
10.
Where, H = heat produced, I = current, R = resistance and t = time.
Name four appliances based on the heating effect of electric current.
Many devices like electric iron, electric kettle, electric heater and
electric bulb work on the basis of the heating effects of an electric
current.
How does a fuse work?
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 a
fault in the household circuit like short- circuiting or over-loading, a
very large current flow in a household circuit, the fuse wire gets
heated and melts. This breaks the circuit and hence protects the
electric appliances.
An electrical bulb has the figures 220 V, 100 W written on it. What
does it mean?
If a bulb has the figures 220 V and 100 W, it means, this bulb is to
be used on a voltage of 220 volts and will consume electrical energy
at the rate of 100 joules per second.
What are the three wires in an electric circuit? Write their colours.
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.
Why the consumption of electricity is increasing in recent time?
Consumption of electricity is increasing day-by-day due to more
and more use of some electrical appliances like air conditioner,
television, computer, refrigerator etc for the comfort of human life.
So we should conserve electricity.
How can you prevent the wastage of electricity in your home?
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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 electric
current through this wire by connecting its ends to the terminals of a
lead-storage battery. After sometime we see that the wire is found to
sag. This is because the wire has expanded due to heating effect of
electric current.
This shows that, if electric current flows through a conductor, it
gets 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 following
manner.
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13.
Ans.
14.
Ans.
15.
Ans.
16.
Ans.
17.
(i) Overloading: When we connect a number of appliances in a
circuit, they draw a large current. It makes overheating of
electric wire and leads to short-circuiting and electric fires. It
is called overloading of electric circuit.
(ii) Short-circuiting: Sometimes due to damaged wiring, the live
wire and neutral wire in a circuit touch each other. It makes
resistance of the circuit almost zero and a very large current
flows through it. This sudden flow of very large current due to
sudden contact of a live and neutral wire is called shortcircuiting.
(iii) Electric shock: If anyone touches a live electric wire, he/she
gets a severe electric shock as our body is a good conductor
of electricity.
Distinguish between overloading and short-circuiting of an electric
circuit.
(i) Overloading: The overheating of electrical wiring in any circuit
due to flow of large current through it is called overloading of
electric circuit.
(ii) Shorting circuiting: It is the sudden flow of very large current
due to the direct contact of a live and a neutral wire.
Name two household appliances which produce light from
electricity.
Electric bulb and tube light are the two household appliances that
produce light from electricity.
Name two household appliances which are operated by electricity
and cool the surroundings.
Water cooler and air conditioner are the two household appliances
which are operated by electricity and cool the surroundings.
Name two industrial appliances which use electricity.
Drill machine and Welding machine are industrial appliances which
use electricity.
Name two household appliances which use electricity and are the
source of entertainment.
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Ans. Radio and television are the two household appliances which use
electricity 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 closed
or complete circuit.
If there is no complete flow of electric current in any circuit due to
some obstructions, then this type of circuit is called open or
incomplete 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 flow
through 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 this
electricity reach our homes?
Ans. On large scale, electricity is produced by turbine with the help of
water or steam. Blades of turbines are rotated with the help of water
or steam. The energy of the turbine operates the dynamo which
finally converts the mechanical energy into electric energy. Electric
energy so generated at power houses is carried by electric cables to
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25.
Ans.
26.
Ans.
27.
Ans.
the cities and villages.
What are the two kinds of charges?
There are the two kinds of charges:
(i) Positive charge
(ii) Negative charge.
A glass rod is rubbed with silk. State the charge acquired by each.
When a glass rod is rubbed with silk, it acquires, positive charge,
because electrons are less tightly bound in silk. While due to gain of
equal number of electrons, the silk becomes negatively charged.
What do you understand by the term electric current?
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 second
across the cross-section of a conductor, the cross-section being
normal to its length or to the direction of flow of current.
If a charge Q flows through the cross-section of a conductor in
time ‘t’ then
Current (I) =
28.
Ans.
29.
Ans.
30.
Ans.
31.
Ans.
32.
Ans.
charge (Q)
time (t)
In a cell, which form of energy is converted to electrical energy?
In a cell, chemical energy is converted to electrical energy.
What is electroplating?
When an electric current is passed through the electrolyte, the
positive metal ions move to the cathode and get uniformly coated
on the object kept as the cathode (negative electrode).This process
is called electroplating.
Why is electric wiring in homes connected in parallel circuits?
Electric wiring in homes are connected in parallel circuits because
if we remove a bulb, the circuits still remain closed. So the other
bulb continues to glow.
Name the condition to complete an electric circuit.
A complete circuit is formed only when both the terminals of the
battery are connected to the bulb through the conducting wires.
Name the instrument used to measure electric current.
Electric current is measured in amperes (A) by using an instrument
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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 A
Potential difference V = 12 V
Time t = 10 minutes = 10 × 60 = 600 s
Electric 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 when
it is connected to a source of 220 V?
Ans.
Power P = 1000 W
Voltage V = 220 V
Current I = ?
P
1000
=
= 4.55 A
I =
V
220
3. A current of 4 A flows through a 20 V car headlight bulb. Calculate
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∴
the electric power.
Ans.
Current I = 4 A
Voltage V = 20 V
Power P = ?
P
I =
V
P = V × I = 20 × 4 = 80 W
4. 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 Ω
Power 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 having
a resistance of 10 ohms.
Ans.
Potential difference V = 20 V
Resistance R = 2 Ω
Power P = ?
V = I×R
V
20
=
= 10 A
R
2
P = V × I = 20 × 10 = 200 W
6. 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 Ω
Time t = 30 minutes = 30 × 60 = 1800 s
Electrical energy E = ?
V = I×R
I =
I =
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V
5
=
= 2.5 A
R
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7.
Ans.
8.
Ans.
P = I × V = 2.5 × 5 = 12.5 W
E = P × t = 12.5 × 1800
= 22500 J = 22.5 KJ
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.
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 V
P = V × I = 5 × 0.5 = 2.5 W
E = P × t = 2.5 × 1800
= 4500 J = 4.5 KJ
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 energy
have been consumed during the month? Find the cost of electrical
energy consumed if 1 unit of energy costs Rs 1.20.
Initial reading of electric meter = 3396 kWh
Final 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
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