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Physics > Household Circuits > Flashcards

Household Circuits Flashcards

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1
Q

transmission of power from generating station to consumer

A

power generating station (11 kV)
grid substation (132 kV)
main substation {to heavy industry} (33 kV)
intermediate substation {to light industry} (11 kV)
city substation {to consumer} (220 V)

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2
Q

why is power generated at generating station not directly transmitted to consumers at 11 kV

A

voltage is raised to reduce the loss of energy in the form of heat in transmission line wires

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3
Q

why is ac used instead of dc

A

it is impossible to step up and step down with dc, so ac is used where there is an alternating voltage

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4
Q

how are live and neutral wires connected

A

the neutral and earth wires are connected at the local substation so that they have the same potential 0V

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5
Q

company fuse

A

before connecting the cable from a pole to the meter in a house, first a fuse of high rating is connected in the live wire at the pole

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6
Q

rating of company fuse depends on

A

load for which the connection is taken from the company

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7
Q

main switch also known as

A

earth leakage circuit breaker/ ELCB

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8
Q

ELCB

A

connected to both live and neutral wires

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9
Q

MCB

A

connected only to live wire

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10
Q

what type is main switch

A

double pole switch

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11
Q

advantage of main switch

A

breaks the connections of live and neutral wire simultaneously from main supply

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12
Q

consumer unit

A

contains a main switch/ELCB and an MCB for each circuit of the house

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13
Q

rating of MCB in ring circuit

A

30 A

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14
Q

advantages of ring system

A
  1. current from mains to the appliance has two separate paths. each appliance gets connected to the mains through a thick wire. wire of main ring is of lower current capacity than that which would be required to connect the appliance directly to the mains, and cost of wiring reduces
  2. each appliance has a separate fuse
  3. all plugs and sockets used are same size, but each socket should have its own fuse of rating suitable for appliance connected
  4. while installing new appliance, a new line up to the distribution box is not required, but can be directly connected to ring circuit in that room (precaution: total current drawn from mains by ring does not exceed rating on main fuse)
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15
Q

advantages of parallel connection

A
  1. each appliance gets 220 V
  2. operates independently and is not affected by the working of other appliances
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16
Q

disadvantages of connecting in series

A
  1. voltage gets divided, so each appliance does not operate at its rated voltage
  2. on connecting more appliances, the resistance increases
  3. affected by the working of other appliances
17
Q

define fuse

A

an electric fuse is a safety device which is used to limit the current in an electric circuit
the use of a fuse safeguards the circuit and the appliances connected in that circuit from being damaged

18
Q

principle of fuse

A

heating effect of current

19
Q

dependence of heating effect on current and radius of wire

A

directly proportional to square of current
inversely proportional to cube of radius

20
Q

material of fuse wire

A
  1. short and thin
  2. uniform area of cross section
  3. low melting point
  4. high resistance to produce sufficient heat to melt it
  5. alloy of tin and lead (melting point is 250), and specific resistance is high
21
Q

why can’t fuse be connected to neutral wire

A

when fuse blows off, current stops flowing in the circuit, but appliance is still connected to high potential point through live wire
if the appliance is touched, they get electrocuted

22
Q

rating for fuse wire in houses

A

5 A, because live wire has current carrying capacity of 5A

23
Q

where is thick fuse wire of higher rating used

A

thick fuse wire of 15 A is used in the line of power circuit for heavy current

24
Q

condition for fuse wire connected to an appliance

A

current rating of fuse must be a little more than the maximum current that can be drawn by the appliance before being overheated

25
Q

advantages of MCB

A
  1. avoids inconvenience of connecting a new fuse wire
  2. much safer due to its quick response
26
Q

define switch

A

an on-off device for current in a circuit (or in an appliance). it is connected to the live wire si

27
Q

single pole switch

A

used with an appliance to start or stop the flow of current within it
only disconnects the live wire from the appliance

28
Q

double pole switch

A

main switch at distribution board, used to switch on or off the mains
disconnects both live and neutral wires simultaneously

ex: staircase

29
Q

how is local earthing done

A

a hole 2-3 metres deep is dug
copper rod, covered by a hollow insulating pipe
thick copper plate of dimension 50cm * 50 cm is welded at lower end of rod
buried deep inside the ground
plate is surrounded by mixture of charcoal and salt to make a good contact between plate and earth
water is poured through pipe to form a conducting layer between plate and ground

30
Q

why is metallic case of appliance painted

A

it forms an insulating layer over the appliance
to connect to an earthing wire, paint must be removed from the body part to which the connection is to be made

31
Q

safety by earthing of an appliance

A

when live wire of a faulty appliance comes in direct contact with its metallic case, appliance gets high potential of live wire. person gets electrocuted.

32
Q

why are the pins of a plug splitted at the end

A

to provide a spring action so that they fit in the socket holes tightly

33
Q

why is earth pin made longer and thicker

A

long- so that earth connection is made before live connection; ensures safety in case of defective plugging
thicker- so that it is not inserted into live or neutral connection by mistake

34
Q

safety precautions for plug and socket

A
  1. hands must be completely dry
  2. check that plug pins fit into the socket tightly
35
Q

high tension wire

A

low resistance- so that heavy current can pass with less heat
large surface area- so that heat produced is radiated more quickly to surroundings

36
Q

precaution while using electricity

A
  1. one must use wires of higher current carrying capacity than total current that flows through the circuit while using all appliances simultaneously
  2. insulation must be of good quality
  3. no wet hands
  4. metallic case to be earthed
  5. proper fuse
  6. proper local earthing at kWh meter

Physics (12 decks)

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