Determine the change in thermal energy of 100 g of copper (M = 63,5, Debye 348K) if it is cooled from

(a) 300 K to 4 K,

(b) 78 K to 4 K

(c) 20 K to 4 K.

If the latent heat of liquid He is 2700 J. l - 1, calculate how much He is needed to bring the cooling about in each case.

given,

mass of copper = 100 g

latent heat of liquid (He) = 2700 J/l

a) change in energy

Q = m Cp (T₂ - T₁)

Q = 0.1 * 376.812 * (300 - 4)

Q = 11153.63 J

He required

Q = m L

11153.63 = m * 2700

m = 4.13 kg

b) Q = m Cp (T₂ - T₁)

Q = 0.1 * 376.812 * (78 - 4)

Q = 2788.41 J

He required

Q = m L

2788.41 = m * 2700

m = 1.033 kg

c) Q = m Cp (T₂ - T₁)

Q = 0.1 * 376.812 * (20 - 4)

Q = 602.90 J

He required

Q = m L

602.9 = m * 2700

m = 0.23 kg