Physics 0625 · Question 1[8 marks]
A vacuum flask contains 0.80 kg of hot coffee at 95 °C in a room at 20 °C. Over a 4-hour period (14,400 s), the temperature drops to 75 °C. (Specific heat capacity of coffee c = 4200 J kg⁻¹ °)
(a)[2]
Calculate the total thermal energy lost by the coffee during 4 hours.
(b)[2]
Calculate the average rate of heat loss (power loss in Watts) from the flask.
(c)[3]
Explain how each of the following flask components reduces heat loss:
(i)[1]
Evacuated space between double glass walls
(ii)[2]
Silvered coatings on glass surfaces
(d)[1]
State why the temperature of the coffee drops faster when the flask is half full than when completely full.