Qn:3 poed olech Sutaue QUESTION 3 The heater element of an electric kettle has a

Question

Qn:3

poed olech Sutaue QUESTION 3 The heater element of an electric kettle has a constant resistance of 105? and applied voltage of 240V rature of one litre of water from 15°C to 90°C Calculate time taken to raise the tempe assuming that only 80% of the power input tothe kettle is usefully employed. If the water equivalent of the kettle is 150g, find how long it will take to raise a second litre of water through the same temperature range immediately after the first time. 3.1 (5) 3.2 (5) [10] QUESTION 4 Calculate the time taken for a 25kW furnace having an overall efficiency of 80% to melt 20kg of particular metal with the initial t metal is 896J/chg.; the melting point of the same metal is 657°C and its specific latent heat of fusion 402000J/kg. [10]

Explanation / Answer

Given,

R = 150 Ohm ; V = 240 V

3.1)T1 = 15 deg ; T2 = 90 deg

eta = 80 %

We know that, P = I^2 R = V^2/R

P = 240^2/150 = 384 W

We know that,

Qw = 1 x 95 - 15 = 75 kcal

Wk = 0.15 x 90-15 = 11.25 kca;

Qtot = 75 + 11.25 = 86.25 kcal

we have eta = 80% = 0.8

Q(tot) = 0.8 x 384 x t/4200 = 0.073 t

t = 86.25/0.073 = 1180 s

Hence, t = 1180 s = 19.65 min

In this case, t = 75/0.073 = 1027.4 s

Hence, t = 1027.4 s = 17.12 min

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