40. (II) A 2800-W oven is hooked to a 240-V source. (a) What is the resistance of the oven? (b) How long will it take to boil 100mL of water assuming 80 percent efficiency? (c) How much will this cost at 10 cents/kWh?
The resistance of the oven is 20.5 ohms
I am having trouble with the heating of the water part...any help would be great.
2006-09-16 07:47:43 · 5 answers · asked by Anonymous in Science & Mathematics ➔ Physics
(a) Resistance is given by
P=(V^2)/R gives R=20.57 ohms.
(b)It has to run 125% of time to give original heat as it is 80% efficient .Further (mass=volume*density). Again consider water starts boiling that means it reaches 100 degree centigrade...do not confuse here with (latent heat of vapourization) misspelt in above answer as latent heat of fusion...as we just reach boiling temp and no phase change.
Missing part is initial temperature of water..let it be 'x' degree centigrade , then
(V^2/R)*t*alpha=(mass=volume*density)*specific heat*Temp. difference
specific heat of water is 4.18 KJ/Kg.
Density of water=1000 Kg/m3
alpha=1.25
Put all values and manage with units:
t=(100*1000*4.18*1000*(100-x)*2*20.57)/ (1000*1.25*240*240)
(sec)
feed in 'x' initial temperature in degree centigrade.
(c)Total cost=rate*kwh
1 KWh=3.6*(10^6) Joules
So total cost=(10)*(V^2/R)*t cents
2006-09-16 09:58:31 · answer #1 · answered by Anonymous · 0⤊ 0⤋
a)
P=(V^2)/R
R=(240^2)/2800 = 20.57 ohms
b)
You are missing the initial temperature of the water, the conversion from watts to calories, and the heat of fusion of water. The latter two can be looked up, but the first has to be supplied or assumed.
2006-09-16 08:23:16 · answer #2 · answered by Helmut 7 · 0⤊ 0⤋
I looked at answering the problems. Is the water setting in the oven? What temperature is the oven set? It will take longer to heat to 100C at 150C than at 200C.
This problem, or something like it, is most likely in your text.
By the way, I validated the resistance at 20.56 ohms.
later...
2006-09-16 08:32:42 · answer #3 · answered by Stan B 2 · 0⤊ 0⤋
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2016-12-18 11:24:17 · answer #4 · answered by ? 4 · 0⤊ 0⤋
ask a geek 2 do it 4 u
2006-09-16 07:56:05 · answer #5 · answered by nat 2 · 0⤊ 1⤋