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P1: Quiz 7 – Answers
P1: Quiz 7 – Answers
1) Power is the rate of energy transfer
Or, power is the rate of doing work
2)
a) P = ET ÷ t
P is the power in watts (W), ET is energy transferred in joules (J) and t is time in seconds (s)
b) P = WD ÷ t
P is the power in watts (W), WD is work done in joules (J) and t is time in seconds (s)
3) 15 W
4) 65 W
5) 9.75 kJ
6)
a) 3,528 J
b) 78.4 W
7) 3,000 kJ
8)
a) It will take a shorter period of time for car B to accelerate from rest to 25 m/s at full power. This is because car B has a more powerful engine, so it can transfer more energy to the car’s kinetic energy stores per second, which means that the car will accelerate quicker/ reach 25 m/s in a shorter period of time (less than the 14 seconds that it takes car A)
b) 280,000 J
c) 6.2 seconds
Or, power is the rate of doing work
2)
a) P = ET ÷ t
P is the power in watts (W), ET is energy transferred in joules (J) and t is time in seconds (s)
b) P = WD ÷ t
P is the power in watts (W), WD is work done in joules (J) and t is time in seconds (s)
3) 15 W
4) 65 W
5) 9.75 kJ
6)
a) 3,528 J
b) 78.4 W
7) 3,000 kJ
8)
a) It will take a shorter period of time for car B to accelerate from rest to 25 m/s at full power. This is because car B has a more powerful engine, so it can transfer more energy to the car’s kinetic energy stores per second, which means that the car will accelerate quicker/ reach 25 m/s in a shorter period of time (less than the 14 seconds that it takes car A)
b) 280,000 J
c) 6.2 seconds
Questions
1) Give a definition of power.
2)
a) When we use energy transfer, what is the formula for power? Give the units for all of the values as well.
b) When we use work done, what is the formula for power? Give the units for all of the values as well.
3) A motor transfers 1,800 joules of energy in 2 minutes. Work out the power of the motor.
4) An electric heater transfers 17.5 kJ of energy in 4.5 minutes. Work out the power of the electric heater. Give your answer to 2 significant figures.
5) A 25-Watt motor is on for 6.5 minutes. Calculate the energy transferred whilst the motor is on. Give your answer in kilojoules.
6) A crane lifts a 20 kg bag of cement from the ground floor to the 7th floor of a building; the 7th floor of the building is 18 metres above the ground floor. It takes the crane 45 seconds to lift the bag of cement from the ground floor to the 7th floor.
a) Work out the change in gravitational potential energy of the bag of cement. Use g as 9.8 N/kg.
b) Work out the power of the crane’s motor.
7) An engine in a car has a power of 25,000 W. How much energy is transferred whilst the car is travelling for 2 minutes at full power? Give your answer in kilojoules.
8) We have two cars; car A and car B. Both of the cars are identical, except from the fact that car A has an engine that has a power of 20,000 W and car B has an engine that has a power of 45,000 W. It takes car A 14 seconds to accelerate from rest to 25 m/s at full power.
a) Will car B be able to reach 25 m/s in a shorter or longer time when accelerating at full power? Explain your answer. You do not need to undertake any calculations
b) Work out the energy transferred to car A from the motor when it is accelerating from rest to 25 m/s. Give your answer in joules.
c) Work out how long it will take car B to accelerate from rest to 25 m/s (assume that the total amount of energy wasted by both cars during acceleration is the same). Give your answer to 2 significant figures.
1) Give a definition of power.
2)
a) When we use energy transfer, what is the formula for power? Give the units for all of the values as well.
b) When we use work done, what is the formula for power? Give the units for all of the values as well.
3) A motor transfers 1,800 joules of energy in 2 minutes. Work out the power of the motor.
4) An electric heater transfers 17.5 kJ of energy in 4.5 minutes. Work out the power of the electric heater. Give your answer to 2 significant figures.
5) A 25-Watt motor is on for 6.5 minutes. Calculate the energy transferred whilst the motor is on. Give your answer in kilojoules.
6) A crane lifts a 20 kg bag of cement from the ground floor to the 7th floor of a building; the 7th floor of the building is 18 metres above the ground floor. It takes the crane 45 seconds to lift the bag of cement from the ground floor to the 7th floor.
a) Work out the change in gravitational potential energy of the bag of cement. Use g as 9.8 N/kg.
b) Work out the power of the crane’s motor.
7) An engine in a car has a power of 25,000 W. How much energy is transferred whilst the car is travelling for 2 minutes at full power? Give your answer in kilojoules.
8) We have two cars; car A and car B. Both of the cars are identical, except from the fact that car A has an engine that has a power of 20,000 W and car B has an engine that has a power of 45,000 W. It takes car A 14 seconds to accelerate from rest to 25 m/s at full power.
a) Will car B be able to reach 25 m/s in a shorter or longer time when accelerating at full power? Explain your answer. You do not need to undertake any calculations
b) Work out the energy transferred to car A from the motor when it is accelerating from rest to 25 m/s. Give your answer in joules.
c) Work out how long it will take car B to accelerate from rest to 25 m/s (assume that the total amount of energy wasted by both cars during acceleration is the same). Give your answer to 2 significant figures.