Conservation & dissipation of energy

A student investigated the thermal conductivity of different metals.

Figure 2 shows some of the apparatus used.

Each rod was made from a different metal.

The metal rods are joined at the centre.

The size of each metal rod was the same.

This is the method used.

1. Attach a drawing pin to the end of each metal rod using wax.

2. Light the Bunsen burner.

3. Start a stopclock.

4. Record the time taken for the wax to melt and for the drawing pin to fall off each metal rod.

Table 1 shows the results.

Complete Figure 3.

You should:

• label the x-axis

• plot the data from Table 1 as a bar chart.

Which type of metal in Table 1 had the greatest thermal conductivity?

Give a reason for your answer.

Tick () one box.

The thermal energy transferred to the nickel rod was 132 J.

The temperature change of the rod was 20 °C.

mass of the nickel rod = 0.015 kg

Calculate the specific heat capacity of nickel.

Use the equation:

Use the Physics Equations Sheet to answer questions 02.4 and 02.5.

Write down the equation which links energy transferred (E), power (P) and time (t).

At the end of the investigation, the Bunsen burner was turned off.

As the nickel rod cooled to room temperature, the rod transferred 132 J of energy to the room.

The mean power transfer was 0.33 W.

Calculate the time taken for the energy transfer.

Figure 10 shows a toy which is hung from a support.

When the handle is pulled down and then released, the toy bounces up and down.

When the handle is pulled down, the spring extends.

The extension of the spring is 0.35 m.

The spring constant of the spring is 5.6 N/m.

Calculate the elastic potential energy stored by the stretched spring.

Use the equation:

elastic potential energy = 0.5 × spring constant × (extension)²

Use the Physics Equations Sheet to answer questions 06.2 and 06.3.

Which equation links gravitational field strength (g), gravitational potential energy (E_p), height (h) and mass (m)?

Tick () one box.

When the handle is released, the toy moves upwards through a height of 0.60 m.

The toy has 0.294 J of gravitational potential energy in its highest position.

gravitational field strength = 9.8 N/kg

Calculate the mass of the toy.

Figure 11 shows the toy in three different positions.

The toy is released from position A and moves upwards.

The toy is moving upwards in position B.

The toy is at maximum height in position C.

Between position A and position B, the extension of the spring decreases.

How does the elastic potential energy stored by the spring change as the extension decreases?

Tick () one box.

Between position B and position C, the toy slows down.

How does the kinetic energy of the toy change as the toy slows down?

Tick () one box.

How does the gravitational potential energy of the toy change as the toy moves from position B to position C ?

Tick () one box.

The toy moves up and down many times before eventually stopping.

Which two of the following are reasons why the toy eventually stops?

Tick () two boxes.

In a ride at a theme park, a person is strapped into a pod that is attached to two stretched bungee cords.

The bungee cords behave like springs.

Figure 7 shows a person using the ride.

How is the extension of each bungee cord calculated?

Tick () one box.

Before the pod is released, the extension of each bungee cord is 7.5 m.

spring constant of the bungee cord = 800 N/m

Calculate the elastic potential energy stored in each stretched bungee cord.

Use the equation:

elastic potential energy = 0.5 × spring constant × (extension)²

The maximum speed of the pod is 15 m/s.

The mass of the pod is 240 kg.

Calculate the maximum kinetic energy of the pod.

Use the equation:

kinetic energy = 0.5 × mass × (speed)²

Use the Physics Equations Sheet to answer questions 03.4 and 03.5.

Which equation links gravitational field strength (g), gravitational potential energy (E_p height (h) and mass (m)?

Tick () one box.

The pod has 24 000 J of gravitational potential energy when at its maximum height.

The mass of the pod is 240 kg.

gravitational field strength = 9.8 N/kg

Calculate the maximum height reached by the pod.

Why is the maximum gravitational potential energy of the pod less than the initial elastic potential energy of the bungee cords?

Tick () two boxes.

In a ride at a theme park, a person is strapped into a pod that is attached to two stretched bungee cords.

The bungee cords behave like springs.

Figure 10 shows a person using the ride.

Which energy store increases as the bungee cords are stretched?

When the pod is released, the pod accelerates upwards.

Before the pod is released the extension of each of the two bungee cords is 8.0 m.

The spring constant of each bungee cord is 735 N/m.

The mass of the pod is 240 kg.

gravitational field strength = 9.8 N/kg

Calculate the maximum height reached by the pod.

Use the Physics Equations Sheet.

The actual maximum height reached by the pod will be lower than the correct answer to Question 06.2

Explain why.

A student investigated how different insulating materials affect the energy transfer from bottles of very hot water.

Figure 6 shows some of the equipment used.

To prevent spillages the student used a funnel to pour very hot water into each bottle.

Why did the student use the funnel?

Tick (✓) one box.

Why did the student not use insulation for bottle A ?

Tick (✓) one box.

The student recorded how much the temperature of the water in each bottle changed in five minutes.

What equipment could the student use to measure time?

Table 4 shows the results.

Figure 6 is repeated below.

The student could not make a valid conclusion from the results about how different insulating materials affect the energy transfer.

Explain two ways that the student could improve the investigation to be able to make a valid conclusion.

Use Figure 6 and Table 4.

Figure 7 shows the graph plotted by the student.

The student should not have plotted a line graph.

What type of graph should the student have plotted?

Give a reason for your answer.

A student investigated the insulating properties of two different materials.

The same thickness of each material was used.

Figure 9 shows some of the equipment used by the student.

The student used two different types of thermometer to measure the temperature changes.

Figure 10 shows a reading on each thermometer.

What is the resolution of thermometer B ?

Tick (✓) one box.

Complete the sentence.

Choose the answer from the box.

thermometer B.

Figure 11 shows the results.

The mass of water used was 0.12 kg.

specific heat capacity of water = 4200 J/kg °C

Determine the total change in thermal energy of the water when Material X was used.

Use values from Figure 11.

Use the Physics Equations Sheet.

There is an anomalous result on Figure 11.

Draw a ring around the anomalous result.

Give two conclusions that can be made from Figure 11.

Another student investigated how the thickness of the insulation affected the rate of cooling of hot water.

Figure 12 shows some of the equipment used.

How would using thick insulation affect the rate of cooling of hot water compared with using thin insulation?

Tick (✓) one box.

Predict how using thick insulation would affect the temperature of the water after 10 minutes compared with using thin insulation.

Tick (✓) one box.

Figure 13 shows a large wind farm off the coast of the UK.

The mean power output of the wind farm is 696 MW, which is enough power for 580 000 homes.

Calculate the mean power needed for 1 home.

Give your answer in watts.

On one day the demand for electricity in the UK was 34 000 MW.

Suggest two reasons why wind power was not able to meet this demand.

Some of the energy from the wind used to rotate a wind turbine is wasted.

An engineer oils the mechanical parts of a wind turbine.

Explain how oiling would affect the efficiency of the wind turbine.

In most homes in the UK there are many different electrical devices.

Explain why people should be encouraged to use energy efficient electrical devices.

A student investigated the insulating properties of different materials. Figure 4 shows some of the equipment used by the student.

This is the method used:

1. Wrap insulating material around the can.

2. Put a fixed volume of boiling water in the can.

3. Place the lid on the top of the can.

4. Measure the time taken for the temperature of the water to decrease by a fixed amount.

5. Repeat steps 1–4 using the same thickness of different insulating materials.

Identify the independent variable and the dependent variable in this investigation. Independent variable Dependent variable

The student used two different types of thermometer to measure the temperature changes. Figure 5 shows a reading on each thermometer.

What is the resolution of thermometer B ?

Thermometer A is more likely to be misread. Give one reason why.

For one type of insulating material, the temperature of the water decreased from 85.0 °C to 65.0 °C. The energy transferred from the water was 10.5 kJ. specific heat capacity of water = 4200 J/kg °C Calculate the mass of water in the can. Use the Physics Equations Sheet.

Table 2 shows the results for two insulating materials.

Explain how the results in Table 2 can be used to compare the thermal conductivity of the two materials.

Figure 10 shows a house with a solar power system.

The solar cells generate electricity.

When the electricity generated by the solar cells is not needed, the energy is stored in a large battery.

The solar cells on the roof of the house always face in the same direction.

Explain one disadvantage caused by the solar cells only facing in one direction.

The mean current from the solar cells to the battery is 3.5 A.

Calculate the charge flow from the solar cells to the battery in 3600 seconds.

Use the equation: charge flow = current × time

Write down the equation which links efficiency, total power input and useful power output.

At one time in the day, the total power input to the solar cells was 7500 W.

The efficiency of the solar cells was 0.16

Calculate the useful power output of the solar cells.

The wasted energy that is not usefully transferred by the solar cells is dissipated.

What happens to energy that has been dissipated?

Tick (✓) one box.

Why is it unlikely that all the UK’s electricity needs could be met by solar power systems?

Tick (✓) one box.

Figure 11 shows a toy car in different positions on a racing track.

The toy car and racing track can be modelled as a closed system. Why can the toy car and racing track be considered ‘a closed system’? Tick (✓) one box.

The car is released from rest at position A and accelerates due to gravity down the track to position B. mass of toy car = 0.040 kg vertical height between position A and position B = 90 cm gravitational field strength = 9.8 N/kg Calculate the maximum possible speed of the toy car when it reaches position B.

Figure 11 is repeated below.

At position C the car’s gravitational potential energy is 0.20 J greater than at position B. How much kinetic energy does the car need at position B to complete the loop of the track? Give a reason for your answer.

Tick () one box.

A student investigated the insulating properties of newspaper. Figure 15 shows the apparatus the student used.

The student’s results are shown in Figure 16.

Describe a method the student could have used to obtain the results shown in Figure 16.

The student could have used a datalogger with a temperature probe instead of the digital thermometer. Figure 17 shows the readings on the digital thermometer and the datalogger.

The datalogger records 10 readings every second. The student considered using a temperature probe and datalogger. Explain why it was not necessary to use a temperature probe and datalogger for this investigation.

A student investigated the insulating properties of newspaper.

Figure 1 shows the apparatus the student used.

The student’s results are shown in Figure 2.

Describe a method the student could have used to obtain the results shown in Figure 2.

The student could have used a datalogger with a temperature probe instead of the digital thermometer.

Figure 3 shows the readings on the digital thermometer and the datalogger.

The datalogger records 10 readings every second.

The student considered using a temperature probe and datalogger.

Explain why it was not necessary to use a temperature probe and datalogger for this investigation.

Figure 9 shows a person sliding down a zip wire.

As the person slides down the zip wire, the change in the gravitational potential energy of the person is 1.47 kJ

The mass of the person is 60 kg

gravitational field strength = 9.8 N/kg

Calculate the change in vertical height of the person.

As the person moves down the zip wire her increase in kinetic energy is less than her decrease in gravitational potential energy.

Explain why.

Different people have different speeds at the end of the zip wire.

Explain why.

A student investigated the thermal conductivity of different metals.

This is the method used:

1. Measure the mass of an ice cube.

2. Put the ice cube on a metal block which is at room temperature.

3. Measure the mass of the ice cube after one minute.

4. Repeat with other blocks of the same mass made from different metals.

Table 3 shows the student’s results.

The initial temperature of each ice cube was –15 °C

Why was it important that the initial temperature of each ice cube was the same?

Tick (✓) one box.

Which metal had the highest thermal conductivity?

Give a reason for your answer.

Suggest one source of random error in the student’s investigation.

An ice cube has a temperature of –15.0 °C

The total thermal energy needed to raise the temperature of this ice cube to 0.0 °C and completely melt the ice cube is 5848 J

specific heat capacity of ice = 2100 J/kg °C specific latent heat of fusion of ice = 334 000 J/kg

Calculate the mass of the ice cube.

Figure 10 shows a hot water tank made of copper.

Figure 10

Copper has a higher thermal conductivity than most metals.

How does the rate of energy transfer through copper compare with the rate of energy transfer through most metals?

Tick one box.

The tank is insulated. When the water is hot, the immersion heater switches off.

Complete the sentences.

Compared to a tank with no insulation, the rate of energy transfer from the

for longer.

Figure 11 shows how temperature varies with time for water in a tank heated with an immersion heater.

Figure 12 shows how temperature varies with time for water in a tank heated with a solar panel.

Figure 11

Figure 12

Give one advantage and one disadvantage of heating the water using solar panels rather than an immersion heater.

Use only information from Figure 11 and Figure 12.

Advantage of solar panels

Disadvantage of solar panels

During one morning, a total of 4 070 000 J of energy is transferred from the electric immersion heater.

4 030 000 J of energy are transferred to the water.

Calculate the proportion of the total energy transferred to the water.

Write down the equation that links energy transferred, power and time.

The power output of the immersion heater is 5000 W.

Calculate the time taken for the immersion heater to transfer 4 070 000 J of energy.

Give the unit.

Figure 13 shows a lift inside a building.

Figure 13

The motor in the lift does 120 000 J of work in 8.0 seconds.

Calculate the power output of the motor in the lift.

Use the equation:

The power input to the motor is greater than the power output.

Tick two reasons why.

Figure 14 shows part of the circuit that operates the lift motor.

Figure 14

The lift can be operated using either of the two switches.

Explain why.

Write down the equation that links gravitational field strength, gravitational potential energy, height and mass.

The lift goes up 14 m. The total mass of the people in the lift is 280 kg.

gravitational field strength = 9.8 N/kg

Calculate the increase in gravitational potential energy of the people in the lift.

Give your answer to 2 significant figures.