THE NEW CAMBRIDGE ENGLISH SCHOOL [ICSE], R.P.C.
LAYOUT, BANGALORE – 104
Std.VIII : PHYSICS
TOPIC : ELECTRICITY
Answer the following
1. From where does electricity come to our homes?
The electricity reaches our homes by passing through and Grid substation,
main substation, Intermediate substation, city substation and finally to the
consumers.
2. What is an electric meter? Where is it fixed in our house?
An electric meter is a device to measure the amount of electric energy
consumed by the consumer in the unit KWh.
The electric meter is usually mounted on the front or outside wall of the
house.
3. State the purpose of KWh meter.
KWh meter measures the amount of electric energy consumed by the
consumer in the unit KWh, for which the electricity bill is paid by him to
the electricity board.
4. For which unit do we pay our electricity bill?
We pay our electricity bill through a unit called B.O.T. unit or KWh.
5. How can you check just by seeing the meter whether electricity is in use or
not?
When any electrical appliance is put on the electric current flows which
rotates the armature. The counter fixed on the armature reads the number
of rotations. The five dials on the counter read electricity consumption
directly in Kwh units. Therefore we can check just by seeing the meter
whether electricity is in use or not.
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6. Refer the diagram from text book.
The reading is as follows:
The reading on the meter is = 2789Kwh
7. One day the meter reading was 7643 units while on the following day, it was
7657 units. What is the consumption of electricity in a day?
The consumption in a day will be 7657 – 7643 = 14 Kwh
8. Question – Refer TB – Page 152
Solution : Electric power = P =
Electric energy = E = p x t
9. Question – Refer TB – page 152
The unit in which we pay the cost of our electric bill is Kwh
1 Kwhr = 1Kw x 1hr
= 1000w x 3600 sec
= 3600000 Wsec
= 3600000 J
= 3.6 x 106 J
10. Question – Refer TB = Page 152
Electrical energy = p x t Kwh
11. What is an electrical fuse? State its purpose in the household electrical
circuit?
(a) The electric fuse is a device which is used to limit the current in an
electric circuit. It safeguards the circuit and the appliances connected
in the circuit from being damaged.
(b) The purpose of a fuse wire is for the following reasons:
(i) If the insulation on the wire of cable used in wiring breaks, the live
wire comes in contact with the neutral wire which results in short
circuit which can cause burning of wires.
(ii) Due to voltage fluctuation, sometimes high current flows through
the wires that can cause burning of wires.
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12. State one property of the material of a fuse wire.
The fuse wire is made of an alloy of lead and tin. The alloy of lead and tin
has low melting point, it melts at about 200°C.
13. Name the material of a fuse wire.
Fuse wire is made of an alloy of lead and tin.
14. Can we use copper wire as a fuse wire? Give reason.
No, we cannot us copper wire as a fuse wire, because copper has a high
melting point of 180°C. So it will not melt even if the current exceeds its
safe limit.
15. How does a fuse protect the electric wiring [or an appliance] from being
damaged?
Due to voltage fluctuation, sometimes high current flows through the wires
that can cause burning of wires. To prevent this damage a fuse is
connected to the live wire of the circuit. When there is a short circuit, the
fuse wire gets heated up to the extent that it melts and makes the circuit
becomes incomplete or breaks this way, no current then flows and the
appliance or the circuit is saved.
16. Which fuse wire is thick ? : 5A or 15A. Give reason.
15A fuse wire is thicker. Higher the rating, thicker is the wire, the reason
being the dependence of thickness of fuse wire on the current rating.
17. Write the full form of MCB.
MCB stands for miniature circuit breaker.
18. How is MCB superior to fuse wire?
MCB is superior to fuse wire because when the current flow is in excess,
the MCB automatically falls down to break the electric circuit. It switches
off the circuit in a very short time [nearly 25 milli seconds] in case of short
circuiting. It is reset after the fault is rectified.
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19. With which wire live or neutral is the fuse wire connected?
The fuse wire connected in series with live wire.
20. What do you mean by short circuiting of a circuit?
When a naked live wire and a neutral wire come in contact or the live wire
and the earth wire come in contact, the major cause being the poor
insulation on wires is called short circuiting.
21. Can be solved in text book.
22. How are the electrical appliances connected in a house circuit? : in series
or in parallel? Give reason.
The electrical appliances connected in a house circuit will be in parallel
connection, because in parallel connection each appliance has an
independent path of current and works at the same voltage.
23. In the house hold electric circuit if one bulb is fused in a room, the other
bulbs keep glowing. Explain the reason.
Because in a house hold circuit each appliance has an independent path of
current. So the other bulbs keep glowing and works at the same voltage.
24. State the voltage at which electricity is supplied to our houses.
The voltage supplied to our houses is 220 volt.
25. Solution : Refer Figure : 8.14 - Page 145
26. State the colour coding of the three wires in a cable used for wiring in a
house hold electrical circuit.
The three wires used in wiring in a household electrical circuit is
Live wire – Red or brown
Neutral wire – Black or light blue
Earth wire – Green or yellow.
27. Why is the metal covering of an electrical appliance earthed?
The reason is that sometimes due to breaking of insulation of wires, live
wires come in contact with the body of the appliance, and we get a fatal
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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.
28. Define potential difference.
Potential difference is defined as the work done in moving a unit charge
from one electrode to another electrode. It is expressed in the unit volt V
V=
29. Derive P = VI
V= , W = VQ , W = VIt
I= Q = It
W = VIt
P= ,
P = VI , 1 watt = 1 volt x 1 Ampere
30. (a) Where is electric power generated?
(b) How much electric power is generated in generating station?
(c) What is the frequency the voltage generated possess?
(d) Why is the voltage raised from 11Kv to 132 KV at the generating
station?
(e) Where is the voltage reduced from 132 Kv to 33V
(f) How much voltage is reduced in main substation?
(g) How is the voltage raised or lowered in grid substation and main
substation?
(h) How much voltage is reduced when it reaches city substation.
(i) Name the device which is used to increase or decrease voltage.
(a) Electric power is generated at the power generating stations.
(b) 11 KV is generated in generating station.
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(c) Frequency is 50 Hz. The polarity at the terminals changes 100 times a
second, 50 times + and 50 times
(d) To reduce loss of energy in transmission due to heating of line wires.
(e) At the grid substation the voltage is reduced from 132 Kv to 33 Kv
(f) The voltage is reduced to 11 Kv in the main substation.
(g) The voltage is raised or lowered using step up and step down
transformers.
(h) The voltage is finally reduced to 220V when it reaches city substation.
(i) Step up or step down transformer.
31. Why is D.C. power not used in household circuits?
D.C. voltage can neither be increased nor decreased by any device.
Therefore, it is not used in household circuits.
32. Define one KWhr?
1KWh is defined as the amount of energy consumed when an electrical
appliance of power 1 Kilowatt is used for one hour.
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