TEMPERATURE AND HEAT

4th WEEK


1) Hydrogen may be liquefied at-253˚C under the pressure of 20 atm. What is temperature on the Fahrenheit scale?
tf = 1.8tc + 32
=1.8(-253)+32
= - 391˚


2) A rod 3m long is found to have expanded 0.091cm in length for a temperature rise of 60˚C. what is α for the material of the rod?

α = 1/L∘ (∆L)/(∆T)
= (0.091x 10⁻²m)/((3m)(60k))
=5.1 x 10⁻⁶K⁻ ¹




3) Starting at 20˚C, how much heat is required to heat 0.3kg of aluminum to its melting point and then to convert it all liquid?

 Q = mc ∆t + mL. the specific heat of aluminum c is 0.22 kcal/kg. ˚C, heat of fusion L is 76.8 kcal/kg, and the melting point is 660˚C

Q = 0.3 (0.22) (660˚-20˚) + 0.3 (76.8)
= 42.24 + 23.04
=65.3kcal


4) A certain 6-g bullet melts at 300˚C and has specific heat capacity of 0.20cal/g. ˚C and heat of fusion of 15cal/g. how much heat to melt the bullet if it is originally at 0˚C?

 The bullet must first be heated to 300˚C and then melted.
 Heat needed = (6g) (0.20cal/g.˚C) (300-0)˚C + (6g) (15cal/g)
= 450cal, or 1880J


5) Refer to prob (4) .what is the slowest speed at wich the bullet can travel if it is to just melt when suddenly stopped?
 1880J is to be supplied as
K= (mv²)/2
1880J =(1/2)(0.006kg)v²min
So that
Vmin= 790m/s.