EASY
12th West Bengal Board
IMPORTANT
Earn 100

An electric kettle takes 4 A when connected to a circuit of 220 V potential difference.

(ii) Find the time required to melt and boil away one kilogram of ice at 0°C assuming a loss of generated heat by 40 per cent due to conduction, radiation etc.,

Important Questions on Thermal Effect and Chemical Effect of Current

MEDIUM
12th West Bengal Board
IMPORTANT
An electric kettle takes 4 A when connected to a circuit of 220 V potential difference. 

(iii) Also calculate the cost of electrical energy consumed during the time if the cost of one kilowatt-hour of energy is 20 paise only. Given latent heat of vapourisation of water at 100°C is 540 cal/gm.

HARD
12th West Bengal Board
IMPORTANT
A copper wire having cross-sectional area 0.5 mm2 and a length of 0.1 m is initially at 25°C and is thermally insulated from the surrounding. If a current of 10 A is set up in this wire :

(i) Find the time in which the wire will start melting. The change of resistance with temperature of the wire may be neglected.

Given, for copper density =9×103 kg/m3, sp. heat =9×10-2Cal/kg°C, melting point =1075°C resistivity =1.6×10-8 Ωm.

HARD
12th West Bengal Board
IMPORTANT
A copper wire having cross-sectional area 0.5 mm2 and a length of 0.1 m is initially at 25°C and is thermally insulated from the surrounding. If a current of 10 A is set up in this wire :

(ii) What will this time be if the length of the wire is doubled.

Given, for copper density =9×103 kg/m3, sp. heat =9×10-2Cal/kg°C, melting point =1075°C resistivity =1.6×10-8 Ωm.

HARD
12th West Bengal Board
IMPORTANT
The walls of a closed cubical box of edge 50 cm are made of material of thickness 1 mm and thermal conductivity 4×10-4 cal/degcms. The interior of the box is maintained at 100°C above the outside temperature by a heater placed inside the box and connected across 400 V d.c. Calculate the resistance of the heater.
EASY
12th West Bengal Board
IMPORTANT
A town situated 20 km away from a power plant generating power at 440 V, require 600 kW of electric power at 200 V. The resistance of the two wire line carrying power is 0.4 Ω per km. The town gets power from the line through a 3000-200 step down transformer at substation in the town. (i) Find the power losses in the form of heat.
MEDIUM
12th West Bengal Board
IMPORTANT
A town situated 20km away from a power plant generating power at 440V, requires 600kW of electric power at 200V. The resistance of the two wire line carrying power is 0.4Ωperkm. The town gets power from the line through a 3000-220V step down transformer at a substation in the town. (i) Find the line power losses in the form of heat. (ii) How much power must the plant supply, assuming there is negligible power loss due to leakage?
EASY
12th West Bengal Board
IMPORTANT

An electric bulb when connected across a power supply of 220 V, consumes a power 60 W.

A If the supply drops to 180 V, then what is the power consumed?

B If the supply is suddenly increased to 240 V, then what is the power consumed?

HARD
12th West Bengal Board
IMPORTANT

In the circuit shown in Fig. 6.18 the 5 Ω resistance develops 10.24 cal/s due to the current flowing through it. Calculate the heat developed per second in the 2Ω resistance.

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