specific heat question?

Question

1.one british thermal unit (Btu) raises the temperature of 1 lb of water by 1 ℉.how many joules are equivalent to 1 Btu?
2.a 0.5-kg steel electric kettle rated at 1200W contains 0.6kg of water at 10℃.how long does it take the water to reach 90℃?assume that there are no losses.

Answer 1

2 E=mc.deltaT
=0.6 x 4200 x (90-10)
=201600 J

E=Pt so, t=E/P
= 201600/1200
=168 secs

Answer 2

Q1.
1 kg = 2.2 lb, hence 1 lb = 1/2.2 kg
1 deg F = 5/9 deg.C

Heat needed to raise 1/2.2 kg of water by 5/9 deg.C
= (1/2.2)x4200x(5/9) J = 1060.6 J

thus 1 BTU = 1061 J (approx.)

Q2
Heat required to raise the temperature of the kettle and water from 10 deg.C to 90 deg.C
= 0.5.c.( 90-10) + 0.6x4200x(90-10) J
where c is the specific heat capacity of steel (should be given)

Since energy = power x time
Hence, 1200xt = 0.5.c.( 90-10) + 0.6x4200x(90-10)
where t is the time required

solve for t