Efficiency and powerIB MYP Sciences: Subtopic test

1. 1
   A student in Lagos compares a filament lamp with an LED lamp. Both are connected to the mains. After a few minutes the filament lamp is too hot to touch, but the LED lamp is only warm.

   (a)

   Which equation gives the efficiency of a device?

   [1 mark]

   • AUseful energy output × total energy input
   • BTotal energy input - useful energy output
   • CWasted energy output / total energy input × 100%
   • DUseful energy output / total energy input × 100%

   (b)

   What happens to the wasted energy from the lamps?

   [1 mark]

   • AIt spreads out into the surroundings, mainly by heating
   • BIt is destroyed
   • CIt is stored in the lamp for later use
   • DIt is transferred to the lamp as extra light

   (c)

   Explain why the filament lamp is less efficient than the LED lamp.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A construction company in Mumbai uses a crane powered by an electric motor to lift steel beams to the top of a tall building.

   (a)

   A motor transfers 4800 J of energy in 12 s. What is its power?

   [1 mark]

   • A57 600 W
   • B0.0025 W
   • C400 W
   • D4812 W

   (b)

   A second motor must transfer the same amount of energy as the first. Which change would give it a greater power?

   [1 mark]

   • ATransferring the energy over a longer time
   • BTransferring the energy in a shorter time
   • CUsing a heavier beam
   • DWasting more energy

   (c)

   In one lift the motor takes in 6000 J of electrical energy and transfers 4200 J as gravitational potential energy of the beam. Calculate the efficiency of the motor.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Students in Cape Town compare two electric kettles, P and Q. Each kettle heats 1.0 kg of water through 20 °C. Kettle P has a thick plastic body and kettle Q has a thin plastic body. Kettle P took in 100 kJ of electrical energy and kettle Q took in 120 kJ. In both cases the water gained 84 kJ of energy.

   (a)

   State a hypothesis that the students could test about the two kettles, giving a scientific reason.

   [3 marks]

   (b)

   Calculate the efficiency of each kettle and evaluate the hypothesis using your results and the way the investigation was carried out.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A city council in Seoul is deciding whether to replace 20 000 filament street lamps with LED lamps. LED lamps cost more to buy, but they use less electricity and last longer. Filament lamps are cheap to buy but have to be replaced often.

   (a)

   Discuss the advantages and disadvantages of replacing the filament lamps with LED lamps, and decide whether the council should do it.

   [6 marks]

   (b)

   To give 4 J of light every second, a filament lamp takes in 80 J of electrical energy each second and an LED lamp takes in 5 J each second. Calculate the efficiency of each lamp and the energy wasted each second, then calculate how much energy per second the council would save across all 20 000 lamps.

   [6 marks]

   Total for question 4: 12 marks

End of questions