## Selina Concise Physics Class 8 ICSE Solutions – Electricity

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## Selina Concise ICSE Solutions for Class 8 Physics Chapter 8  Electricity

•  ELECTRICITY: “Is the rate of flow of electrons”. i = $$\overset { q }{ \underset { t }{ – } }$$
•  To keep electrons move, potential difference is needed. This is done by a cell or battery.
•  Potential difference “is the amount of work done in moving a unit positive charge from one point to other.”
Potential difference V = Work done (W) / charge moved (Q)
Or W = QV but charge = it
Hence, W = VIt or Electrical energy = VIt
•  Power is “Rate of doing work”
P = W/t = VIt/t = VI
Power is measured in watt or J S-1
1 Watt = 1 Volt × 1 Ampere
• S.I. unit of charge is coulomb (C).
S.I. unit of current is Ampere (A).
S.I. unit of P.D. is volt.
S.I. unit of electrical energy is Joule (J) and of power is watt (W)
1 kWh-3600000J = 3.6 x 106 J
•  ELECTRIC power is generated at the GENERATING STATION at 11000 volt, or 11 kV as these stations are at very far off place from areas where it is to be used. The voltage (A.C.) is of 50 HZ frequency.
• AT GRID SUB-STATION this alternating current (A.C) voltage is stepped up from 11 kV to 132 kV to minise the loss of energy in transmission line wires.
• At MAIN-SUB-STATION this voltage is stepped down from 132 kV to 33 kV and transmitted to city SUB-STATION.
•  At CITY SUB STATION, it is further stepped down from 33 kV to 220 V for supply to hourses for consumers.
•  Colour coding: Live wire — Red or Brown
Neutral—Black or light blue
Earth wire — Green or yellow
•  1 kWh = 1 unit: Power Rating on an appliance 100 W – 220 V means the appliance when worked on a 220 V will consume 100 W electricity power
• OVER LOADING: is the condition of Electric circuit, when it draws more current than it is designed for i. e. when a number of appliances are switched on at a time i.e. geyser, A.C. Electric motor etc. or a large number of plugs are put in the same socket.
•  EARTHING: is done in a house near the kWh meter. Earthing is a safety device which puts the appliance at zero potential.
•  SHORT CIRCUITING: If the insulation on the wire of cable used f in the wiring (or used with an appliance) breaks. The LIVE WIRE
COMES IN CONTACT WITH THE NEUTRAL WIRE, this result in SHORT CIRCUITING
•  FUSE: “Is a device used to limit the current in an electric circuit”. The use of fuse protects the appliance in circuit from being damaged Fuse is always connected in live wire. A fuse wire should have
(i) High resistance
(ii) Low melting point.
These days miniature circuit breakers (MCB) are used. It is AUTOMATIC breaker, when current flowing excess.
•  Appliances in a house are connected in parallel.

Test yourself

A. Objective Questions

1. Write true or false for each statement:

(a) A fuse wire has a high melting point.
Answer. False.

(b) Flow of protons constitutes electric current.
Answer. False.

(c) Silver is an insulator of electricity.
Answer. False.

(d) S.I. unit and commercial unit of electrical energy are same.
Answer. False.

(e) Overloading of electric current in circuits can lead to short circuiting.
Answer. True.

(f) Our body can pass electricity through it.
Answer. True.

(g) All metals are insulators of electricity.
Answer. False.

(h) The earth wire protects us from an electric shock.
Answer. True.

(i) A switch should not be touched with wet hands.
Answer. True.

(j) AH electrical appliances in a household circuit work at the same voltage.
Answer. True.

(k) In a cable, the green wire is the live wire.
Answer. False.

(l) A fuse is connected to the live wire.
Answer. True.

(m) A switch is connected to the neutral wire.
Answer. False.

2. Fill in the blanks

(a) The unit in which we pay the cost of electricity is kWh.
(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 x 106 joule.
(g) A fuse wire should have low melting point.

3. Match the following

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 ¡s 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.