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S.I. unit of electric work → Joule (J) 

1J = 1volt × 1coulomb 

1Joule : “Is the amount of work done when a charge of one coulomb flows through a conductor at a pot. diff. of 1volt.”

Material used for fuse is ‘Alloy of lead and Tin’ having low melting point and high resistance.”

The unit for electric work in SI system is  Joule.

Electric fuse : “Is a safety device which is used to limit the current in an electric circuit. The use of a fuse safe guards the circuit and the appliances connected in that circuit from being damaged.” 

(a) Electric energy : E = P × t = VIt 

“Electric energy is the product of power and time.” i.e. electric energy consumed by an electrical appliance is produced of ‘power rating’ and ‘time’ for which it is used. 

(b) Unit of electric energy → Joule (J) 

S.I. unit of electric work → Joule(J) 

1J = 1 volt × 1 coulomb 

1Joule : “Is the amount of work done when a charge of one coulomb flows through a conductor at a pot. diff. of 1 volt.” 

(c) Standard Commercial unit of Electric Energy : Kilo-watt-Hour 1kwH : “Is the amount of electrical energy consumed when an electrical appliance having a power rating of 1 kilowatt is used for 1 hour.” 

Electric work : “Electric work is said to be done when an electric charge flows through a conductor, at some potential difference.”

∵ v =w/Q

∴ w = vQ

Two bigger units of electric work : 

1. Kilo – Joule(kJ) = 10³J 

2. Mega-joule(Mj) = 10⁶J

The metallic part of an electrical appliance is earthed.

The domestic electric meter measure, the electric energy in kWh. 1kWh = 3.6 × 10⁶J

Three factors which determine heat produced in a conductor : 

Heat produced = Energy Work done W=H=I²Rt 

∴ Three factors are I — current 

R — Resistance of conductor 

t — time for which current flows.

Here, R = 75 Ω, 

i = 2A, 

t = 2 minutes = 2 × 60 = 120s 

Now, Heat produced 

H = i² Rt 

= 2 × 2 × 75 × 120J = 36000J

The switches should not be touched with wet hand otherwise we may receive a shock.

When an earthed appliance gets short circuited, then current from its metal body flows into the earth. Since earth does not offer any resistance, therefore magnitude of the current in the circuit of a short-circuited appliance suddenly rises to a very high value. 

This rise in magnitude of current in turn overloads the circuit, and hence. The fuse in that circuit melts.

So that ‘The fuse may melt first, before current reaches the appliance”.

According to new convention, the colour of live wire is  brown.

The switch should always be connected to the live wire, so that current is cut off to that appliance to which it is connected.

Function of a switch : 

Function of a switch is to connect or disconnect an electric appliance in the electric circuit.

(a) The colour of 

1. Live wire —– Brown 

2. Neutral wire —– Light Blue . 

3. Earth wire —– geen or Yellow 

(b) Position of

1. Earth pin —– is Long and Thicker so that it should not enter live or neutral cylinder of socket. 

2. Live pin —– L is on the right when earth pin goes into E, live pingoes into L. 

3. Neutral-pin —– N is on the left.When earth pin goes into E, live wire in L automatically neutral goes into N. 

(c) Earth-pin is made thicker, so that it should not enter other cylinder of socket and helps in putting right pin in right place. 

Longer —– So that earth connection is done first. This ensure the user will not get a shock.

The required relation is P = v²/p where P is power, V the potential difference and R is the resistance.

According to old convention, the colour of neutral wire is black.

(a) live wire

(c) live wire

Two systems of power distribution. 

(i) Tree system Advantages 

1. All appliances are connected parallel and can be independently switched on and switched off. 

2. The neutral wire and earth wire is common to all circuits. 

3. From the main distribution board, circuits are taken out for different rooms. 

Disadvantages : 

1. It is expensive and takes a long time to install. 

2. It requires plugs and sockets of different current value for different appliances depending upon their power rating. 

3. When a fuse in one particular line melts. It disconnects all appliances in that circuit. 

(ii) Ring system : Advantages : 

1. Every appliance has its own fuse and can be handled without disturbing other appliances. 

2. Length of wire used is very small. 

3. It is easier to install and maintain.

Even if switch is in off position the appliance will be at the potential of live wire and current will flow through the appliance.

Disadvantages in series circuit :

1. Over all resistance increases and current from the power supply is low. 

2. If one electrical appliance stops working, due to some defect, then all other appliances also stop working. 

Two advantages of parallel wiring : 

1. Electrical appliances get same voltage (220V) as that of the power supply line. 

2. If one electrical appliance stops working due to some defect, then all other appliances keep working normally. 

(a) parallel circuit

When we connect the earth wire to the metal case of the electrical appliance by using three pinplug. The metal casing of the appliance will now always remain at zero potential of the earth, this will avoid the risk of shock if we touch the metal body as the current passes directly to earth through the earth wire. 

When we connect the earth wire to the metal case of the electrical appliance by using three pinplug. The metal casing of the appliance with now always remain at zero potential of the earth, this will avoid the risk of shock if we happen to torch the metal body directly to earth through the earth wire.

1. Electric fuse. 

2. It works on the heating effect of electric current.

V = 24V,

R = 8Ω

(i) P = V²/R = (24 x 24)/8 = 72W

(ii) Current flowing P = VI

72 = 24I

I = 72/24 = 3A

VIt = w

24 x I x (30 x 60) = 8640

I = 8640/(24 x 30 x 60) = 0.2A

(i) Power P = I²R, R = 40Ω

I = 4A

P = 4 x 4 x 40 = 640W

(ii) P.d. at its ends V = p/I = 640/4 = 160V

Heat Energy given out E = 5760J

t = 1min. = 60s. p.d. V = 24V

Resistance R = V²t/E = (24 x 24 x 60)/5760 = 6Ω

R = 6Ω.

w = Energy released = VIt

w = 220 x 5 x 60 = 66000J

R ∝ 1/πr²  

R∝1/I

∴ I∝ πr² i.e. more radius, more current.

so 20A wire will be thicker, as it carries large current.

5A wire is thicker as resistance is α 1/a Thicker the wire less is resistance and electrons can run freely with less collisions.

(b) (V²/20) x t

Given : R = 20, I = 2.5A

t = 5minutes = 5 x 60 = 300s

Quantity of heat produced is given as

H = I² x R x t

= (2.5)² x 20 x 300

∴ H = 37500J

P = 500W,V = 200V

Current I = P/V = 500/200 = 2.5

∴ I = 2.5A

Energy draw E = 43200J,

t = 4min. = (4 x 60)s.

I = 6A

R = E/I²t = 43200/(6 x 6 x (4 x 60) = 5Ω

(b) electric energy

1kilowatt hour

1000 watt hour 

1000J/S x (60 x 60s) = 3600000J

1kwh = 3.6 x 10⁶J

Purpose of Fuse : 

1. The fuse limits the current in electric circuit and acts as a safety device. 

2. It saves us and protects us and our expensive appliances when circuit gets over loaded or higher current flows as the fuse blows off and current supply stops. Material used for making fuse wire is Lead and Tin alloy.

For a fuse, higher the current rating, thicker is the fuse wire.

(i) Copper cannot be used as fuse wire because : 

(a) it has high melting point 

(b) low resistivity 

(ii) Alloy of lead and tin.