Heat

Touch cannot reliably tell how hot an object is, so we measure its degree of hotness, called temperature, with a thermometer. A clinical thermometer measures body temperature and reads from 35 degrees C to 42 degrees C; it has a kink that stops the mercury from falling back on its own. The normal temperature of the human body is about 37 degrees C. A laboratory thermometer usually reads from -10 degrees C to 110 degrees C. Digital thermometers are now preferred because mercury is toxic. Heat flows from hotter to colder objects in three ways. In conduction, heat passes through a solid from particle to particle, so metals such as copper and aluminium are good conductors, while wood and plastic are insulators. In convection, heated liquid or air rises and cooler fluid takes its place, which causes sea breezes by day and land breezes at night. In radiation, heat travels without a medium, as from the Sun. Dark surfaces absorb more heat, so we wear light clothes in summer and air-trapping woollens in winter.

Detailed Worksheet: Heat Section A - Definitions (10 marks) 1. What is temperature? Name the instrument used to measure it and its unit. (2 marks) 2. What are conductors and insulators of heat? Give two examples of each. (2 marks) 3. Define conduction, convection and radiation. (2 marks) 4. What is a sea breeze and a land breeze? (2 marks) 5. Why does a clinical thermometer have a kink? What is its range? (2 marks) Section B - Calculations and Applications (15 marks) 6. Compare a clinical and a laboratory thermometer under these heads: range, use, kink, and how they are held while reading. (3 marks) 7. The temperatures of a patient recorded every 4 hours were 38.2, 38.8, 39.4, 38.6, 37.8 and 37.0 degrees C. (i) What was the highest temperature? (ii) By how much was it above normal? (iii) When did it return to normal? (3 marks) 8. Convert using F = (9/5) x C + 32: (i) 37 degrees C (ii) 100 degrees C (iii) 0 degrees C. Also convert 104 degrees F to degrees C. (3 marks) 9. Explain why (i) saucepans have plastic or wooden handles (ii) we feel warmer in a woollen sweater (iii) it is advised to wear light-coloured clothes in summer. (3 marks) 10. State the mode of heat transfer in each case: (i) a metal spoon in hot tea getting hot (ii) water in a pan heating evenly (iii) feeling warm near a fire (iv) sea breeze (v) heat from the Sun reaching the Earth (vi) ironing clothes. (3 marks) Section C - Diagrams (10 marks) 11. Draw labelled diagrams of a clinical thermometer and a laboratory thermometer, showing the bulb, kink (where present), scale and mercury thread. (4 marks) 12. Draw labelled diagrams showing the direction of air movement in (i) a sea breeze during the day (ii) a land breeze at night. (3 marks) 13. Draw a diagram of the activity using a beaker of water and a crystal of potassium permanganate to show convection currents. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Paheli put one hand in hot water and the other in cold water for a few minutes, then put both in lukewarm water. (i) What did each hand feel? (ii) What does this show about our sense of touch? (iii) What should be used instead? (iv) Name the precautions while using a laboratory thermometer. (5 marks) 15. Houses in hot regions often have thick walls, and are painted white outside. (i) Why are the walls thick? (ii) Why are they painted white? (iii) Why are ventilators placed near the roof? (iv) Which mode of heat transfer does each feature reduce or use? (5 marks) 16. A student heats one end of a metal rod with wax pieces stuck along it at equal distances. (i) In what order do the wax pieces fall? (ii) Which mode of heat transfer is shown? (iii) What would happen with a wooden rod? (iv) Why are cooking pans made of metal? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Never hold a thermometer by its bulb and wash a clinical thermometer with antiseptic before and after use.