IIT JEE , AIEEE Forum - Physics , Thermal Physics

aditi_g

a coil takes 15 min to boil a certain amount of water another coil takes 20 min to boil the same process. Find time taken to boil the same amount of water whn both coils are connected in series... ans given is 35 min but im getting 8.6 min...

the prob is that im using the formula H= i^2 Rt coz they are connected in series but they are using H=V^2/Rt..

plzz post the complete solution

aditi_g

sumone plzz help

rishipratimm

as to which formula is to be used,i think the problem is arising because u r considerin dat current flowin thru the coils in each case is same....which is not the case ,as the resistances are different,so whn connected across the same voltage,they will have diffrnt values of current flowing across different combinations....
here,as we are connecting the diffrnt combinations of resistances to the same potential difference,that is ,voltage,we have to use the formula H=V^2/R T as in this case, we hv to deal wd only one variable term,theresistance,while the voltage value remains const.....
bt in d othr formula,we hv both I and R which both vary in diffrnt cases,so it shud nt b used...
hope u gt it...

RyuAmakusa

it is not something like series this formula.u have to understand y if u do so then there will be no prob. in series connection the currevt is same so u us ei^2rt.in parallel v is const. so the other formula.is this case as rishipratimm has rightly said the current is not same bcos the resistance is changing

rishipratimm

Considerin the 2 cases separately,
H = V^2*T(1)/R(1)=V^2*T(2)/R(2)
So,we get?.
T(1) = H R(1)/ V^2
T(2) = H R(2)/ V^2

THUS, T(1) / T(2) = R(1)/ R(2) = 20/15 = 4/3
This implies R(1) = 4x, R2=3x
In series combination,
R(s) = R(1) + R(2)=7x

SO,H = V^2*T(s)/R(s)
Which gives T(s) = H R(s) / V^2

So,we get T(s) / T(1)= R(s) / R(1) = 7x/4x = 7/4
Thus, T(s) = (7/4)* T(1) = (7/4)*20 = 35

hope u get it...

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