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DSE Combined Science (Physics) Practice Paper: Energy and Heat (12 Questions, 50 Marks, 75 Minutes)
DSE Combined Science (Physics): Energy and Heat
DSE Combined Science (Physics): 12 original questions, 50 marks, 75 minutes on energy, power, efficiency, specific heat capacity and latent heat.
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Paper details Level: S6 | Subject: Combined Science (Physics) | Time: 75 minutes | Marks: 50 | Questions: 12
Section A (Q1-Q6): Energy, Work and Power
Answer all questions. Show your working.
| # | Question | Answer and marking notes |
|---|---|---|
| 1 | A ball of mass 0.50 kg is released from rest at a height of 3.2 m. Air resistance is negligible and g = 9.81 m s-2. (a) Find its speed just before it reaches the ground. (b) Find the work done by gravity on the ball during the fall. | (a) v = sqrt(2gh) = sqrt(2 x 9.81 x 3.2) = sqrt(62.784) = 7.9 m s-1 (2 marks) (b) W = mgh = 0.50 x 9.81 x 3.2 = 15.7 J (2 marks) |
| 2 | A car of mass 1200 kg accelerates from 8.0 m s-1 to 20.0 m s-1. Find the increase in its kinetic energy and state the work done by the net force. | Increase in KE = 0.5 x 1200 x (20.0^2 – 8.0^2) = 600 x 336 = 201,600 J (3 marks) By the work-energy theorem the work done by the net force equals this increase, i.e. 201 600 J (1 mark) |
| 3 | A student of mass 60 kg climbs a staircase to a floor 12 m higher in 15 s. Take g = 9.81 m s-2. Find the average power he develops against gravity. | Work done against gravity = mgh = 60 x 9.81 x 12 = 7,063.2 J (2 marks) Average power = W / t = 7063 / 15 = 471 W (2 marks) |
| 4 | A crane lifts a load of 250 kg through 8.0 m at constant speed in 20 s. Its input power is 1.2 kW. Find the efficiency of the crane. | Useful work output = 250 x 9.81 x 8.0 = 19,620 J (2 marks) Input energy = 1 200 x 20 = 24,000 J (1 mark) Efficiency = 19620 / 24000 x 100% = 81.8% (1 mark) |
| 5 | An immersion heater of power 250 W heats 0.80 kg of oil for 240 s. The specific heat capacity of the oil is 2.0 x 10^3 J kg-1 K-1. Assuming no heat loss, find the temperature rise of the oil. | Energy supplied = Pt = 250 x 240 = 60,000 J (2 marks) From E = mcDT, DT = 60000 / (0.80 x 2.0 x 10^3) = 37.5 K (2 marks) |
| 6 | 0.25 kg of water at 80 C is mixed with 0.75 kg of water at 20 C. Assuming no heat loss, find the final temperature of the mixture. | Let the final temperature be T: 0.25 x 80 + 0.75 x 20 = (0.25 + 0.75) x T (2 marks) T = (20 + 15) / 1.0 = 35 C (2 marks) |
Section B (Q7-Q9): Heat Energy and Change of State
Answer all questions. State the formula used.
| # | Question | Answer and marking notes |
|---|---|---|
| 7 | Find the energy needed to melt 0.30 kg of ice at 0 C completely into water at 0 C. The specific latent heat of fusion of ice is 3.34 x 10^5 J kg-1. | E = mL = 0.30 x 3.34 x 10^5 = 100,200 J, about 1.0 x 10^5 J (4 marks) |
| 8 | How much energy is needed to heat 0.20 kg of ice from -10 C to 0 C? The specific heat capacity of ice is 2.1 x 10^3 J kg-1 K-1. | E = mcDT = 2.1 x 10^3 x 0.20 x 10 = 4,200 J (4 marks) |
| 9 | An electric kettle rated at 2 000 W heats 1.5 kg of water from 20 C to 100 C. The specific heat capacity of water is 4 200 J kg-1 K-1 and heat loss is negligible. (a) Find the energy required by the water. (b) Find the time needed. | (a) E = mcDT = 4 200 x 1.5 x 80 = 504,000 J (2 marks) (b) t = E / P = 504000 / 2 000 = 252 s, about 4.2 minutes (2 marks) |
Section C (Q10-Q12): Heat Transfer and Energy Conversion
Answer all questions. Explain your reasoning and show your working.
| # | Question | Answer and marking notes |
|---|---|---|
| 10 | A copper block of mass 2.0 kg cools from 150 C to 30 C. The specific heat capacity of copper is 390 J kg-1 K-1. (a) Find the energy released by the block. (b) Explain why a metal handle feels colder than a wooden handle at the same room temperature. | (a) E = mcDT = 390 x 2.0 x 120 = 93,600 J, about 9.4 x 10^4 J (3 marks) (b) Metal conducts heat much better than wood, so heat flows quickly from the hand into the metal, the skin surface cools faster and feels colder. Wood conducts slowly so less heat leaves the hand (2 marks) |
| 11 | A solar panel of total area 2.5 m2 receives solar radiation of power 800 W m-2 and converts it with an efficiency of 18%. (a) Find the electrical power output. (b) Find the electrical energy generated in 3.0 hours, in joules. | (a) Electrical power = 800 x 2.5 x 0.18 = 360 W (2 marks) (b) Energy = Pt = 360 x (3.0 x 3 600) = 3,888,000 J, about 3.9 x 10^6 J (3 marks) |
| 12 | A roller-coaster car of mass 400 kg starts from rest at a height of 25 m. Take g = 9.81 m s-2. (a) Find its speed at ground level if there is no friction. (b) Its actual speed is 20.0 m s-1; find the energy lost to friction. | (a) 0.5mv^2 = mgh, v = sqrt(2 x 9.81 x 25) = 22.1 m s-1 (2 marks) (b) Energy lost = mgh – 0.5mv^2 = 98,100 – 80,000 = 18,100 J, about 1.8 x 10^4 J (2 marks) |
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