Thermal energy and its transferIB MYP Sciences: Subtopic test

1. 1
   A mountaineering team in Nepal carries hot soup in a vacuum flask. The flask has two glass walls with a vacuum between them. The surfaces facing the vacuum are coated with shiny silver, and the stopper is made of plastic.

   (a)

   Why is there a vacuum between the two walls of the flask?

   [1 mark]

   • AConduction and convection need particles, so they cannot transfer energy across the vacuum
   • BIt makes the flask lighter to carry
   • CIt stops all radiation from leaving the soup
   • DIt makes the soup heavier so it stays hot

   (b)

   What is the purpose of the shiny silver coating?

   [1 mark]

   • AIt conducts thermal energy away from the soup
   • BIt absorbs infrared radiation from the soup
   • CIt stops convection currents in the soup
   • DIt reflects infrared radiation back towards the soup

   (c)

   Explain why the stopper is made of plastic rather than metal.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Fatima in Dubai heats a pan of water on a hob. She notices that the handle of a metal spoon left in the pan becomes warm, and that tea leaves in the water move around in circles.

   (a)

   How is thermal energy transferred along the metal spoon?

   [1 mark]

   • AConvection
   • BConduction
   • CRadiation
   • DEvaporation

   (b)

   Which statement about temperature is correct?

   [1 mark]

   • AIt is the total thermal energy stored in an object
   • BIt is measured in joules
   • CIt is a measure of how hot an object is, not the total thermal energy stored
   • DIt is the same thing as thermal energy

   (c)

   Explain how the convection currents that move the tea leaves are formed.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Students in Lima investigate how the surface of a can affects how quickly hot water cools. They fill a matt black can and a shiny silver can with 200 g of water at 80 °C and measure the temperatures after 10 minutes. The water in the black can was at 52 °C and the water in the silver can was at 63 °C. All other conditions were kept the same.

   (a)

   Explain, using ideas about radiation, why the water in the black can cooled more than the water in the silver can.

   [3 marks]

   (b)

   Calculate the fall in temperature of the water in each can and evaluate the method, suggesting one improvement.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A housing company in Cyprus is building a new estate. Each home will have a solar water heater with a 150 kg tank. The company must decide how much money to spend on insulating the tanks and walls to reduce the families' energy bills.

   (a)

   Discuss the benefits and drawbacks of spending more on insulation for the tanks and walls, and decide whether the company should do so.

   [6 marks]

   (b)

   The solar water heater raises the temperature of 150 kg of water from 18 °C to 58 °C. The specific heat capacity of water is 4200 J/kg °C. Calculate the thermal energy transferred to the water. Overnight the tank loses 1.26 × 10⁷ J to the surroundings: calculate the percentage of the thermal energy that is lost, and outline how the loss could be reduced.

   [6 marks]

   Total for question 4: 12 marks

End of questions