4th WEEK: TEMPERATURE AND HEAT

QUESTION 1

1) Hydrogen may be liquefied at-253˚C under the pressure of 20 atm. What is temperature on the Fahrenheit scale?

ANSWER

tf = 1.8tc + 32
=1.8(-253)+32
= - 391˚

QUESTION 2

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?

ANSWER

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


QUESTION 3

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?

ANSWER

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

QUESTION 4

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?

ANSWER

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

QUESTION 5

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?

ANSWER

1880J is to be supplied as
K= (mv²)/2
1880J =(1/2)(0.006kg)v²min
So that
Vmin= 790m/s.