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Energy change in a system - High demand – Questions
Q1.Figure 1 shows a kettle a student used to determine the specific heat capacity of water.
Figure 1
© vladimirkim3722/iStock/Thinkstock
The student placed different masses of water into the kettle and timed how long it took for the water to reach boiling point.
The student carried out the experiment three times.
The student’s results are shown in the table below.
Time for water to boil in seconds
Mass of water in kg 1 2 3 Mean
Mass × change in
temperature in kg°C
Energy supplied in
kJ
0.25 55 60 63 59 20 131
0.50 105 110 116 110 40 243
0.75 140 148 141 143 60 314
1.00 184 190 183 182 80 401
1.25 216 215 211 214 100 471
1.50 272 263 266 267 120 587
1.75 298 300 302 140
(a) Suggest how the student was able to ensure that the change in temperature was the same for each mass of water.
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(2)
(b) Calculate the uncertainty in the student’s measurements of time to boil when the mass of water was 1.75 kg.
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Uncertainty = ____________________ s(2)
(c) The power rating of the kettle is 2.20 kW.
Calculate the average electrical energy used by the kettle, in kJ, for 1.75 kg of water to reach boiling point.
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Average energy = ______________________ kJ(2)
(d) Use information from the table above to calculate the change in temperature of the water during the investigation.
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Change in temperature = _________________ °C(2)
(e) The student plotted a graph of energy supplied in kJ against mass × change in temperature in kg °C.
Figure 2 shows the graph the student plotted.
Figure 2
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Use data from the table above to plot the four missing points.
Draw a line of best fit on the graph.(3)
(f) Use the graph to determine the mean value of the specific heat capacity of water, for the student’s investigation.
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Specific heat capacity of water = ______________ J / kg °C(4)
(g) The student’s value for the specific heat capacity of water was greater than the accepted value.
Suggest why.
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(h) The kettle used in the experiment had a label stating that the power rating of the kettle was 2.2 kW.
The student did not measure the power of the kettle.
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Suggest why measuring the power of the kettle may improve the student’s investigation.
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(Total 17 marks)
Q2.The diagram below shows a person using a device called a jetpack. Water is forced downwards from the jetpack and produces an upward force on the person.
(a) State the condition necessary for the person to be able to remain stationary in mid-air.
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(b) The person weighs 700 N and the jetpack weighs 140 N.
(i) Calculate the combined mass of the person and the jetpack.
Gravitational field strength = 10 N/kg
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Combined mass = ___________ kg(2)
(ii) Increasing the upward force to 1850 N causes the person to accelerate upwards.
Calculate the acceleration of the person and the jetpack. Give the unit.
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Acceleration = ___________ Unit ___________(3)
(Total 6 marks)
Q3.Figure 1 shows an exercise device called a chest expander. The three springs are identical.
Figure 1
A person pulls outwards on the handles and does work to stretch the springs.
(a) Complete the following sentence.
When the springs are stretched ___________ ___________ energy is stored in the springs.
(1)
(b) Figure 2 shows how the extension of a single spring from the chest expander depends on the force acting on the spring.
Figure 2
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(i) How can you tell, from Figure 2, that the limit of proportionality of the spring has not been exceeded?
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(ii) Use data from Figure 2 to calculate the spring constant of the spring.Give the unit.
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Spring constant = ___________ Unit ___________(3)
(iii) Three identical resistors joined in parallel in an electrical circuit share the total current in the circuit.
In a similar way, the three springs in the chest expander share the total force exerted.
By considering this similarity, use Figure 2 to determine the total force exerted on the chest expander when each spring is stretched by 0.25 m.
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Total force = ___________ N(2)
(c) The student in Figure 3 is doing an exercise called a chin-up.
Figure 3
Each time the student does one chin-up he lifts his body 0.40 m vertically upwards.The mass of the student is 65 kg.The student is able to do 12 chin-ups in 60 seconds.
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Calculate the power developed by the student.
Gravitational field strength = 10 N/kg
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Power = ___________ W(3)
(Total 10 marks)
Q4.The figure below shows a rollercoaster.
The rollercoaster car is raised a vertical distance of 35 m to point A by a motor in 45 seconds.
The mass of the rollercoaster is 600 kg.
The motor has a power rating of 8 000 W.
(a) Calculate the percentage efficiency of the motor.
Gravitational field strength = 9.8 N / kg.
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Efficiency = __________________ %
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(5)
(b) The rollercoaster rolls from point A to point B, a drop of 35 m.
Calculate the speed of the roller coaster at point B.
Assume that the decrease in potential energy store is equal to the increase in kinetic energy store.
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Speed at point B = __________________ m / s(6)
(Total 11 marks)
Q5.Under the same conditions, different materials heat up and cool down at different rates.
(a) What is meant by specific heat capacity?
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(b) ‘Quenching’ is a process used to change the properties of steel by cooling it rapidly.
The steel is heated to a very high temperature and then placed in a container of cold water.
(i) A metalworker quenches a steel rod by heating it to a temperature of 900 °C before placing it in cold water. The mass of the steel rod is 20 kg.
The final temperature of the rod and water is 50 °C.
Calculate the energy transferred from the steel rod to the water.
Specific heat capacity of steel = 420 J/kg °C.
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Energy transferred = ____________________ J(3)
(ii) The temperature of the steel rod eventually returns to room temperature.
Compare the movement and energies of the particles in the steel rod and in the air at room temperature.
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(iii) When the steel rod is being quenched, the temperature of the water rises to 50 °C. After a few hours the water cools down to room temperature.
Some of the cooling of the water is due to evaporation.
Explain in terms of particles how evaporation causes the cooling of water.
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(Total 12 marks)
Q6.Figure 1 shows the design of a playground ride.
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A large wooden block rests on ropes. The ropes are attached to a metal frame.
Children sit on the wooden block.
When the wooden block is moved to the left and released it moves to and fro.
When the wooden block returns to the point of release it has completed one cycle.
(a) Identify two possible hazards of the ride in Figure 1.
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(b) The designer of the ride wants to know if the ride has the same time period as a pendulum of the same length.
The designer used a model of the ride and a pendulum as shown in Figure 2.
The designer measured the time taken to complete 10 cycles for different lengths of both the model ride and the pendulum.
The results for the model ride are shown in Table 1.
Table 1
Length Time for 10 cycles in seconds Mean time
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in metres periodin secondsFirst time Second
time Third time Mean
0.100 6.36 6.37 6.29 6.34 0.63
0.150 7.76 7.74 7.80
0.200 8.97 8.99 8.95 8.97 0.90
The results for the pendulum are shown in Table 2.
Table 2
Lengthin metres
Time for 10 cycles in seconds Mean timeperiod
in secondsFirst time Second time Third time Mean
0.250 10.00 10.04 10.02 10.02 1.00
0.300 10.99 11.01 10.94 10.98 1.10
0.350 11.88 11.83 11.87 11.86 1.19
(i) Complete Table 1, giving values to an appropriate number of significant figures.
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(ii) The investigation already includes repeated readings.
Suggest one improvement that could be made to this investigation.
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(iii) The designer reads in an Advanced Physics textbook that:‘The square of the time period, T, for a simple pendulum is proportional to its length, l.’
T 2 ∝ l
Would the model ride have the same time period as a simple pendulum of the same length?
Use one row of data from Table 1 and one row of data from Table 2 to work out your answer.
State your conclusion.
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(c) The ride was redesigned and built to make it safer.
The wood was moving at maximum speed. The maximum kinetic energy of the wood was 180 J.
A parent applied a force to the wood and stopped it in a distance of 0.25 m.
Calculate the force required.
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Force = _______________ N(3)
(Total 12 marks)
Q7.An old house has a cast iron radiator. The radiator has hot water inside it.
Photograph supplied by PhotoObjects.net/Thinkstock
(a) The table shows how the power output of the radiator varies with the temperature difference between the hot water and the air temperature of the room.
Temperature difference in °C 10 20 30 40 50 60
Power output in W 400 900 1480 2200 3050 4000
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(i) Describe fully the relationship between temperature difference and power output.
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(ii) The house owner wants to reduce their heating bills.
Use the data in the table to advise the house owner.
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(b) The air in a room is at a temperature of 12 °C.
The house owner switches the heating on until the temperature reaches 22 °C.The amount of energy needed to raise the temperature of the air to 22 °C is 580 000 J.
The mass of air in the room is 58 kg.
Calculate the specific heat capacity of air and give the unit.
Use the correct equation from the Physics Equations Sheet.
Show clearly how you work out your answer.
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Specific heat capacity = _________________________(3)
(Total 5 marks)
Q8.A ‘can-chiller’ is used to make a can of drink colder.
The image below shows a can-chiller.
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(a) The initial temperature of the liquid in the can was 25.0 °C.The can-chiller decreased the temperature of the liquid to 20.0 °C.The amount of energy transferred from the liquid was 6930 J.The mass of liquid in the can was 0.330 kg.
Calculate the specific heat capacity of the liquid.
Give the unit.
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Specific heat capacity = _____________________ unit ________________(4)
(b) Energy is transferred through the metal walls of the can of drink by conduction.Explain how.
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(c) The energy from the can of drink is transferred to the air around the can-chiller.A convection current is set up around the can-chiller. Explain how.
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(d) The can-chiller has metal cooling fins that are designed to transfer energy quickly to the surroundings.
Give two features that would help the metal cooling fins to transfer energy quickly to the surroundings.
1. _________________________________________________________________
2. _________________________________________________________________(2)
(Total 13 marks)
Q9.During the day, the Sun transfers energy to an outdoor swimming pool.
© Volodymyr Burdiak/iStock
(a) By which method of energy transfer does the pool receive energy from the Sun?
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(b) (i) The mass of water in the pool is 5000 kg. The specific heat capacity of water is 4200 J/kg°C.
Calculate how much energy needs to be supplied to increase the water temperature by 5°C and state the correct unit.
Use the correct equation from the Physics Equations Sheet.
Give the unit.
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Energy = ______________________________(3)
(ii) The Sun supplies energy to the water in the pool at a rate of 16 kJ every second.
Calculate how much time it would take for energy from the Sun to raise the water temperature by 5 °C.
You will need to use your answer to (b)(i) and the correct equation from the Physics Equations Sheet.
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Time = _________________________ seconds(3)
(iii) On one day, the temperature of the pool is 7 °C lower than the air temperature.
The time it takes for the pool temperature to rise by 5 °C is less than the answer to part (b)(ii).
Suggest a reason why.
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(Total 8 marks)
Q10.A student investigated the specific heat capacity of five different metal cylinders.
Each metal cylinder had a mass of 2 kg.
He used an immersion heater to transfer 18 000 J of energy to each different metal cylinder.
Figure 1 shows the apparatus he used.
Figure 1
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He measured the temperature of the metal cylinder at the start and at the end of each experiment, using a thermometer.
He calculated the temperature rise of each metal cylinder.
The results are shown in Figure 2.
Figure 2
Metal cylinder
(a) Suggest an appropriate resolution for the thermometer used in the investigation.
Resolution = _______________________ °C
Give a reason for your answer.
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(b) The mass of metal and the energy transferred to each metal were control variables in the investigation.
Suggest another variable that should have been controlled.
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(c) Which metal has the highest specific heat capacity? ________________________
Give a reason for your answer.
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(d) In this investigation, 18 000 J of energy was supplied to a 2 kg cylinder of steel.
Calculate the specific heat capacity of steel.
Use the correct equation from the Physics Equations Sheet.
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Specific heat capacity = _______________________ J / kg °C(3)
(Total 8 marks)
Q11.(a) Figure 1 shows a solar panel. Solar panels can be fitted to house roofs and used to
heat water for domestic hot water systems.
Use Figure 1 to explain how the design of the water pipe increases the rate of energy transfer from the Sun to the water.
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(b) Figure 2 shows a different method of heating water called a ground source heat pump. Two holes are drilled into the ground and fitted with pipes. Warm water is pumped up one pipe and waste water is returned to the ground through the other pipe. In the house, energy is transferred from the warm water by a heat exchanger.
(i) Suggest one advantage of using this method of heating water rather than using solar panels.
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(ii) A leaflet about a ground source heat pump states:
‘Ground source heat pumps are 300 – 400% efficient. For each joule of mains electrical energy the pumps use, they transfer three to four times more energy from the water’.
Two students read the leaflet.
Student A says, ‘It is incorrect to say that a device is 300 – 400% efficient’.
Student B says, ‘The statement is correct’.
Both conclusions could be considered to be correct.
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Explain why.
Student A’s conclusion ___________________________________________
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Student B’s conclusion ___________________________________________
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(iii) Domestic water enters the heat exchanger at a temperature of 7.0 °C and leaves the heat exchanger at a temperature of 55 °C.
Each day 19 000 000 joules of energy are supplied to the water passing through the heat exchanger.
Calculate the mass of water that can be heated each day.
Choose the correct equation from the Physics Equations Sheet.
Specific heat capacity of water = 4200 J / kg °C.
Give your answer to 2 significant figures.
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Mass of water = _____________________ kg(4)
(Total 12 marks)
Q12.The diagram shows a wind turbine.
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(a) The blades of the turbine are 20 metres long. On average, 15 000 kg of air, moving at a speed of 12 m/s, hit the blades every second.
Calculate the kinetic energy of the air hitting the blades every second.
Show clearly how you work out your answer.
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Kinetic energy = _________________________ J(2)
(b) Part of the kinetic energy of the wind is transformed into electrical energy.The diagram shows that, for the same wind speed, the power output of a turbine, in kilowatts, depends on the length of the turbine blades.
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Give a reason why doubling the diameter of the blades more than doubles the power output of a turbine.
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(Total 3 marks)
Q13.(a) When an object is moving it is said to have momentum.
Define momentum.
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(b) The diagram below shows one way of measuring the velocity of a bullet.
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A bullet is fired into a block of wood suspended by a long thread.The bullet stops in the wooden block.The impact of the bullet makes the block swing.The velocity of the wooden block can be calculated from the distance it swings.
In one such experiment the block of wood and bullet had a velocity of 2 m/s immediately after impact. The mass of the bullet was 20 g and the mass of the wooden block 3.980 kg.
(i) Calculate the combined mass of the block of wood and bullet.
____________________________________ Mass ____________________(1)
(ii) Calculate the momentum of the block of wood and bullet immediately after impact.
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_____________________________________ Momentum _______________(3)
(iii) State the momentum of the bullet immediately before impact.
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(iv) Calculate the velocity of the bullet before impact.
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_____________________________________ Velocity ______________ m/s(3)
(v) Calculate the kinetic energy of the block of wood and bullet immediately after impact.
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____________________________________ Kinetic energy ____________ J(3)
(vi) The kinetic energy of the bullet before the impact was 1600 joules. This is much greater than the kinetic energy of the bullet and block just after the impact.What has happened to the rest of the energy?
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(Total 13 marks)
Q14.The miners working in a salt mine use smooth wooden slides to move quickly from one level to another.
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(a) A miner of mass 90 kg travels down the slide.
Calculate the change in gravitational potential energy of the miner when he moves 15 m vertically downwards.
gravitational field strength = 10 N/kg
Show clearly how you work out your answer.
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Change in gravitational potential energy = _________________________ J(2)
(b) Calculate the maximum possible speed that the miner could reach at the bottom of the slide.
Show clearly how you work out your answer.
Give your answer to an appropriate number of significant figures.
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Maximum possible speed = _________________________ m/s(3)
(c) The speed of the miner at the bottom of the slide is much less than the calculated maximum possible speed.
Explain why.
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(Total 8 marks)
Q15.An electric immersion heater is used to heat the water in a domestic hot water tank.When the immersion heater is switched on the water at the bottom of the tank gets hot.
(a) Complete the following sentence.
The main way the energy is transferred through the copper wall of the water tank is
by the process of ________________________________________ .(1)
(b) The immersion heater has a thermostat to control the water temperature.
When the temperature of the water inside the tank reaches 58°C the thermostat switches the heater off. The thermostat switches the heater back on when the temperature of the water falls to 50°C.
Graph A shows how the temperature of the water inside a hot water tank changes with time. The tank is not insulated.
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Time in hours
(i) The temperature of the water falls at the fastest rate just after the heater switches off.
Explain why.
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(ii) To heat the water in the tank from 50°C to 58°C the immersion heater transfers 4032 kJ of energy to the water.
Calculate the mass of water in the tank.
Specific heat capacity of water = 4200 J/kg°C
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Mass = _______________________________ kg(3)
(iii) An insulating jacket is fitted to the hot water tank.
Graph B shows how the temperature of the water inside the insulated hot water tank changes with time.
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Time in hours
An insulating jacket only costs £12.
By comparing Graph A with Graph B, explain why fitting an insulating jacket to a hot water tank saves money.
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(Total 9 marks)
Q16.(a) Figure 1 shows the forces acting on a model air-powered rocket just after it has
been launched vertically upwards.
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(i) How does the velocity of the rocket change as the rocket moves upwards?
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Give a reason for your answer.
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(ii) The velocity of the rocket is not the same as the speed of the rocket.
What is the difference between the velocity of an object and the speed of an object?
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(b) The speed of the rocket just after being launched is 12 m / s.The mass of the rocket is 0.05 kg.
(i) Calculate the kinetic energy of the rocket just after being launched.
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Kinetic energy = _____________________ J(2)
(ii) As the rocket moves upwards, it gains gravitational potential energy.
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State the maximum gravitational potential energy gained by the rocket.
Ignore the effect of air resistance.
Maximum gravitational potential energy = _____________________ J(1)
(iii) Calculate the maximum height the rocket will reach.
Ignore the effect of air resistance.
Gravitational field strength = 10 N/kg.
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Maximum height = _____________________ m(2)
(iv) Figure 2 shows four velocity−time graphs.
Taking air resistance into account, which graph, A, B, C or D, shows how the velocity of the rocket changes as it falls from the maximum height it reached until it just hits the ground?
Write the correct answer in the box.
(1)
(c) The rocket can be launched at different angles to the horizontal.The horizontal distance the rocket travels is called the range.
Figure 3 shows the paths taken by the rocket when launched at different angles.Air resistance has been ignored.
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What pattern links the angle at which the rocket is launched and the range of the rocket?
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(Total 11 marks)
Q17.When a gun is fired, a very large force acts on the bullet for a very short time.
The change in momentum of the bullet is given by the following relationship:
force (N) × time(s) = change in momentum (kg m/s)
(a) An average force of 4000 newton acts for 0.01 seconds on a bullet of mass 50g.
Calculate the speed of the bullet. (Show your working.)
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Answer ________________________ m/s
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(4)
(b) The bullet is fired horizontally. In the short time it takes for the bullet to reach its target, its horizontal speed has fallen to 80% of its initial speed.
(i) Explain why the speed of the bullet decreases so quickly.
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(ii) Calculate the percentage of its original kinetic energy the bullet still has when it reaches its target.
(Show your working.)
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(Total 10 marks)
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