Secondary 4 Pure Physics Energy Power Quiz

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Secondary 4 Pure Physics Quiz - Energy Power

Name: ____________________ Class: __________ Date: __________ Score: ________ / 45

Duration: 60 minutes
Total Marks: 45
Instructions: Answer all questions. Show all working clearly. Use $g = 10\text{ m/s}^2$ where necessary.

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Section A: Conceptual Foundations (Questions 1-5)

1. State the Principle of Conservation of Energy. [2]
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2. Distinguish between "work done" and "power" in terms of their physical definitions. [2]
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3. A ball is thrown vertically upwards. Describe the energy transfers that occur from the moment it leaves the hand until it reaches its maximum height. [2]
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4. Why is the efficiency of a real-world machine always less than 100%? [2]
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5. State two ways to increase the power output of an electric motor without changing the voltage supply. [2]
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Section B: Calculations & Applications (Questions 6-15)

6. A bullet of mass $15\text{ g}$ is fired from a rifle at a speed of $400\text{ m/s}$. Calculate the kinetic energy of the bullet. [2]
   
   
   Answer: ____________________

7. An electric hoist lifts a load of $200\text{ kg}$ through a vertical height of $12\text{ m}$ in $15\text{ s}$. Calculate the useful work done by the hoist. [2]
   
   
   Answer: ____________________

8. Following Question 7, calculate the average power output of the hoist. [2]
   
   
   Answer: ____________________

9. A tea kettle contains $0.8\text{ kg}$ of water at $20^\circ\text{C}$. Calculate the thermal energy required to raise the temperature of the water to $100^\circ\text{C}$. (Specific heat capacity of water = $4200\text{ J/kg}\cdot\text{K}$) [3]
   
   
   Answer: ____________________

10. A cup of hot coffee at $85^\circ\text{C}$ cools down to $30^\circ\text{C}$ over 30 minutes. If the mass of the coffee is $0.2\text{ kg}$, calculate the total loss of thermal energy. (Assume coffee has the same specific heat capacity as water) [3]
    
    
    Answer: ____________________

11. An electric motor is rated at $500\text{ W}$ input power. If its efficiency is $72\%$, determine the useful power output of the motor. [2]
    
    
    Answer: ____________________

12. A car of mass $1200\text{ kg}$ accelerates from rest to $20\text{ m/s}$. Calculate the change in its kinetic energy. [2]
    
    
    Answer: ____________________

13. A crane lifts a $50\text{ kg}$ crate at a constant speed of $0.5\text{ m/s}$. Calculate the power exerted by the crane. [2]
    
    
    Answer: ____________________

14. A spring is compressed by $0.05\text{ m}$, storing $2.5\text{ J}$ of elastic potential energy. Calculate the force required to compress the spring by this distance. [2]
    
    
    Answer: ____________________

15. A light bulb is rated $60\text{ W}, 240\text{ V}$. Calculate the electrical energy consumed by the bulb if it is left on for 5 hours. [3]
    
    
    Answer: ____________________

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Section C: Synthesis & Analysis (Questions 16-20)

16. A $0.5\text{ kg}$ block slides down a frictionless slope from a height of $3\text{ m}$. Calculate its velocity at the bottom of the slope using the principle of conservation of energy. [3]
    
    
    Answer: ____________________

17. A pump delivers $100\text{ kg}$ of water per minute to a tank $20\text{ m}$ above the ground. Calculate the minimum power required by the pump. [3]
    
    
    Answer: ____________________

18. An electric heater has a power rating of $2\text{ kW}$. It is used to heat $1.5\text{ kg}$ of water. If the heater is $90\%$ efficient, calculate the time taken to raise the water temperature from $25^\circ\text{C}$ to $75^\circ\text{C}$. [4]
    
    
    Answer: ____________________

19. A roller coaster car of mass $600\text{ kg}$ starts from rest at the top of a hill (Height $A = 40\text{ m}$). It descends to a lower point (Height $B = 15\text{ m}$). Calculate the velocity of the car at point $B$, assuming no energy is lost to friction. [4]
    
    
    Answer: ____________________

20. A $100\text{ W}$ LED lamp and a $100\text{ W}$ incandescent lamp are both used for 1 hour. The LED lamp is $80\%$ efficient in converting electrical energy to light, while the incandescent lamp is $15\%$ efficient. Calculate the difference in thermal energy dissipated by the two lamps. [4]
    
    
    Answer: ____________________

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Answer Key - Energy Power Quiz

1. Principle of Conservation of Energy: Energy cannot be created or destroyed, only converted from one form to another. The total energy in an isolated system remains constant. [2]

2. Work vs Power: Work is the product of force and distance in the direction of the force ($W=Fd$); Power is the rate at which work is done or energy is transferred ($P=W/t$). [2]

3. Energy Transfers: Kinetic energy (KE) $\rightarrow$ Gravitational Potential Energy (GPE). Some energy is also transferred to the surroundings as thermal energy due to air resistance. [2]

4. Efficiency: In real machines, some energy is always dissipated as heat/sound due to friction or electrical resistance, meaning not all input energy is converted to useful output. [2]

5. Increase Power: (1) Increase the current flowing through the motor; (2) Increase the efficiency of the motor (e.g., better lubrication). [2]

6. $KE = \frac{1}{2}mv^2 = 0.5 \times 0.015 \times 400^2 = 1200\text{ J}$ [2]

7. $W = mgh = 200 \times 10 \times 12 = 24,000\text{ J}$ [2]

8. $P = W/t = 24,000 / 15 = 1600\text{ W}$ [2]

9. $Q = mc\Delta\theta = 0.8 \times 4200 \times (100-20) = 0.8 \times 4200 \times 80 = 268,800\text{ J}$ [3]

10. $Q = mc\Delta\theta = 0.2 \times 4200 \times (85-30) = 0.2 \times 4200 \times 55 = 46,200\text{ J}$ [3]

11. $P_{out} = 0.72 \times 500 = 360\text{ W}$ [2]

12. $\Delta KE = \frac{1}{2}mv^2 - 0 = 0.5 \times 1200 \times 20^2 = 240,000\text{ J}$ [2]

13. $P = Fv = (mg)v = (50 \times 10) \times 0.5 = 250\text{ W}$ [2]

14. $W = F \times d \rightarrow 2.5 = F \times 0.05 \rightarrow F = 50\text{ N}$ [2]

15. $E = P \times t = 60 \times (5 \times 3600) = 60 \times 18,000 = 1,080,000\text{ J}$ (or $1.08\text{ MJ}$) [3]

16. $mgh = \frac{1}{2}mv^2 \rightarrow 10 \times 3 = 0.5 \times v^2 \rightarrow v^2 = 60 \rightarrow v = 7.75\text{ m/s}$ [3]

17. Mass per second = $100/60 = 1.67\text{ kg/s}$. $P = (m/t)gh = 1.67 \times 10 \times 20 = 334\text{ W}$ [3]

18. $Q = 1.5 \times 4200 \times 50 = 315,000\text{ J}$. Useful power = $0.9 \times 2000 = 1800\text{ W}$. $t = 315,000 / 1800 = 175\text{ s}$ [4]

19. $\Delta GPE = \Delta KE \rightarrow mg(h_A - h_B) = \frac{1}{2}mv^2 \rightarrow 10(40-15) = 0.5v^2 \rightarrow 250 = 0.5v^2 \rightarrow v^2 = 500 \rightarrow v = 22.36\text{ m/s}$ [4]

20. Total energy for each = $100 \times 3600 = 360,000\text{ J}$. LED thermal loss = $20\% \times 360,000 = 72,000\text{ J}$. Incandescent thermal loss = $85\% \times 360,000 = 306,000\text{ J}$. Difference = $306,000 - 72,000 = 234,000\text{ J}$ [4]