Conservation of energy and efficiencyIB MYP Physics: Subtopic test

1. 1
   A school in Seoul is replacing its filament lamps with LED lamps. Each second a filament lamp takes in 60 J of electrical energy and gives out 3 J as light. An LED lamp takes in 8 J each second and gives out 6 J as light.

   (a)

   Which statement about energy in the lamps is correct?

   [1 mark]

   • AEnergy is used up when a lamp is switched on
   • BA lamp creates the light energy that it gives out
   • CThe total energy before is equal to the total energy after
   • DWasted energy is destroyed

   (b)

   What is the efficiency of the filament lamp?

   [1 mark]

   • A5%
   • B20%
   • C95%
   • D50%

   (c)

   Show that the LED lamp is more efficient than the filament lamp.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A motor in a warehouse in Lagos lifts boxes. Each second the motor takes in 500 J of electrical energy and the boxes gain 350 J of gravitational potential energy. The rest of the energy is wasted because of friction in the bearings.

   (a)

   How much energy is wasted by the motor each second?

   [1 mark]

   • A350 J
   • B150 J
   • C500 J
   • D850 J

   (b)

   What is the efficiency of the motor?

   [1 mark]

   • A30%
   • B0.7%
   • C143%
   • D70%

   (c)

   Suggest one way to improve the efficiency of the motor and explain how it works.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student investigates which material is the best insulator for a beaker of hot water. For each test she starts with water at 80 °C, wraps the beaker in a different material and measures the water temperature after 10 minutes. The results are: no insulation 62 °C, bubble wrap 71 °C, cotton wool 73 °C and aluminium foil 66 °C.

   (a)

   Identify the independent variable and the dependent variable, and state one control variable.

   [3 marks]

   (b)

   Calculate the fall in temperature for each test, state which material is the best insulator and explain how this links to wasted energy.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A city council in Stockholm wants to reduce the energy wasted by buildings. It is considering a law that would require thick insulation in all new homes, which would raise building costs, and a scheme to replace old appliances with more efficient ones.

   (a)

   Explain why no device can be 100% efficient even though energy is never destroyed.

   [6 marks]

   (b)

   Discuss and evaluate the council's two proposals.

   [6 marks]

   Total for question 4: 12 marks

End of questions