Question

9(a) Explain the terms: (i) thermal equilibrium; (ii) fundamental interval.
(b) List two uses of the hydraulic press.
(c) Name the material used to reset the steel index on the Six's maximum and minimum thermometer.
(d)(i) A nursing mother prepared her baby's milk mixture at 85°C, in a feeding bottle. In order to cool it to 40°C, she immersed the bottle in an aluminium bowl of heat capacity 90 JK⁻¹ containing 500g of water at 26°C. If the mass of the mixture is 300g, calculate the specific heat capacity of the mixture. [Neglect heat losses and heat capacity of the bottle; specific heat capacity of water 4200 Jkg⁻¹ K⁻¹]
(ii)(a) Name two ways through which the bottle loses heat.
(b) Name two industrial processes in which heat exchangers are used.

Accepted Answer

(a)(i) Thermal Equilibrium: When two bodies of different temperature are in contact, a point is reached where the two bodies attain the same temperature due to equal heat exchange. At this point, we say the bodies have attained thermal equilibrium.

(ii) Fundamental interval: This is the distance between the upper fixed point and the lower fixed point. Each thermometer has 2 reference temperatures (fixed points) which are: (i) The upper fixed point which is the temperature of steam from pure water boiling at standard atmospheric pressure of 760mmHg. (ii) The lower fixed point which is the temperature of pure melting ice at the standard atmospheric pressure of 760mmHg. On a Celsius scale it is 100°F(C) and 180°F on a Fahrenheit scale.

(b) The hydraulic press is an application of the transmission of pressure in fluids. It consists of two interconnected cylinders of different cross-sectional areas. A small effort applied to the small piston results in a much greater force on the large piston, since pressure is transmitted equally throughout the liquid (Pascal's principle).
Uses: (i) Barbers' chairs (ii) Hydraulic brakes / vehicle lifts and jacks

(c) In the Six's thermometer, the indexes made of steel have light springs that hold them in position in the stem, and these can be reset with a magnet brought into contact with the mercury.

(d)(i) Heat gained by mixture, q = m꜀C꜀ΔT꜀, where m꜀=0.3kg (mass of mixture), C꜀=specific heat capacity of mixture(unknown), ΔT꜀=(85-40)=45°C
q = 0.3×C꜀×45 = 13.5C꜀

Heat gained by aluminium bowl = mᵦCᵦΔTᵦ; heat capacity of bowl = mᵦCᵦ = 90 JK⁻¹, ΔTᵦ=(40-26)=14°C
Heat gained by bowl = 90×14 = 1,260 J

Heat gained by water: mw=0.5kg, Cw=4200 Jkg⁻¹K⁻¹, ΔTw=(40-26)=14°C
Heat gained by water = 0.5×4200×14 = 29,400 J

Heat lost by mixture = heat gained by bowl + heat gained by water
13.5C꜀ = 1,260+29,400 = 30,660
C꜀ = 30,660/13.5 = 2,271.1 Jkg⁻¹K⁻¹

(ii)(a) Two ways the bottle loses heat: (i) Conduction (ii) Convection

(b) Two industrial processes where heat exchangers are used: (i) Air-conditioning/Refrigeration (ii) Space heating/Natural gas processing

WAEC Physics 2016 Theory — Question 9

Sign up free to unlock