Question

how to solve the following problem?

Answer 1

you need to know specific heat capacity of water

Answer 2

once you have it,

1) find how many joules the heater can supply in one hour
2) find how many joules it requires to raise the temperature of water
3) divide (1) / (2) <-- gives how many hours heater should be on.
4) multiply (3) with unit cost .08 and divide by 1000

Answer 3

specific heat is 4186 j/kgC

Answer 4

ok... good finding

Answer 5

so no of joules will be
200*4186&65=5.4*10^7 J
this is correct. ?

Answer 6

correct

Answer 7

now lets find, how many joules the heater can supply in one hour

Answer 8

once we fild that, we can divide (2)/(1) and get how many hours heater should be on. ok ?

Answer 9

once we know time the heater is on, its easy to calculate cost

Answer 10

Energy supplied by heater per hour, working at 210 V, 20 ohm resistance =
V^2/R * 60*60 = 210^2/20 * 60 * 60

Answer 11

= 7.938 * 10^6 joules per hour

Answer 12

so here is what we have so far :
1) joules required to raise temperature of water = 5.4*10^7 J
2) joules supplied by heater = 7.938 * 10^6 J per hour

Answer 13

u with me so far ?

Answer 14

yes

Answer 15

great !

Answer 16

so heater must be turned on for how many hours ?

Answer 17

its

(5.4 * 10^7) / (7.938*10^6) = 6.9 hours ?

Answer 18

ok now.

Answer 19

what is next
?

Answer 20

we have W = 2205W = 2.205 kw
time = 6.9 hours
cost per 1 kwh = .08

Answer 21

cost = 2.205 * 6.9 * .08 = 1.2 dollars

Answer 22

(we got power from W = V^2/R = 210^2/20 = 2205)

Answer 23

thanks for your help.

Answer 24

np.. .