Conservation & dissipation of energy

Figure 3 shows a person using a zip wire to move from a tree to the ground.

As the person moves down the zip wire, the block moves upwards.

What happens to the gravitational potential energy of the person as the person accelerates down the zip wire?

Tick (✓) one box.

What happens to the kinetic energy of the person as the person accelerates down the zip wire?

Tick (✓) one box.

The block is 3.4 m above the ground when the person is at the bottom of the zip wire.

mass of block = 2.5 kg

gravitational field strength = 9.8 N/kg

Calculate the gravitational potential energy of the block.

Use the equation:

gravitational potential energy = mass × gravitational field strength × height

The trolley is a seat suspended from wheels which can roll along the zip wire.

When the person reaches the end of the zip wire, the person gets off the trolley.

The block falls downwards pulling the trolley back to the top of the zip wire.

maximum speed of block = 4.8 m/s

mass of block = 2.5 kg

Calculate the maximum kinetic energy of the block.

Use the equation:

kinetic energy = 0.5 × mass × (speed)²

As the trolley moves, work is done against friction.

What is the effect of this?

Tick (✓) one box.

The person oils the wheels on the trolley.

Explain how this will affect the speed of the person down the zip wire.

Figure 10 shows a child’s toy. A child pushes down on the toy to compress the spring. The spring then launches the toy into the air.

A student measured the maximum height reached by the toy. The student placed a vertical metre rule near the toy, and observed the height reached by the toy. The student repeated the experiment, observing from a different position. Figure 11 shows the toy at its maximum height and the two positions of the student.

Observing the toy from position B instead of position A affected the measurement of the maximum height reached by the toy. Explain how.

The greatest height reached by the toy was 64 cm. The gravitational potential energy of the toy at this height was 0.049 J. gravitational field strength = 9.8 N/kg Calculate the mass of the toy. Use the Physics Equations Sheet. Give your answer to 2 significant figures.

When the spring launches the toy into the air, the temperature of the air increases. Explain why the child’s toy on its own is not a closed system.

Figure 6 shows a swimmer wearing a wetsuit.

The wetsuit helps to keep the swimmer warm.

A student wrapped a thermometer in a piece of wetsuit material and placed the thermometer in water containing ice.

Figure 7 shows the apparatus.

After 30 seconds in the water the temperature of the thermometer had decreased by 7.5 °C

Calculate the average decrease in temperature each second.

The student recorded the temperature of the thermometer after 30 seconds for four materials. Each piece of material was the same size and thickness.

In each test the starting temperature of the thermometer was 21.0 °C

Table 2 shows the results.

Complete Figure 8 using the data in Table 2.

You should:

• label the y-axis

• draw the bars for materials Y and Z.

Which material is the best thermal insulator?

Give a reason for your answer.

Tick (✓) one box.

The student tested a new material with a greater thermal conductivity than material Z.

The piece of new material was the same size and thickness as the piece of material Z.

What was the temperature of the thermometer after 30 seconds?

Tick (✓) one box.

During the investigation 0.0150 kg of the ice melted. The temperature of the water and ice did not change.

specific latent heat of fusion of ice = 334 000 J/kg

Calculate the energy needed to melt the ice.

Use the equation:

energy to melt the ice = mass × specific latent heat

The student wanted to determine the density of a wetsuit material.

The student measured the length of one side of a cube of wetsuit material with:

• a micrometer

• a ruler.

Table 3 shows the results.

Complete the sentence.

Choose the answer from the box.

The results show that compared to the ruler the micrometer has a higher

Use the Physics Equations Sheet to answer questions 04.7 and 04.8.

Write down the equation that links density (ρ), mass (m) and volume (V).

The student calculated the volume of the cube of wetsuit material to be 0.186 cm³

The density of the cube was 0.300 g/cm³

Calculate the mass of the cube.

Give your answer in grams.

Solar intensity is a measure of the radiation received from the Sun at the surface of the Earth.

Figure 7 shows how the mean solar intensity changes with the distance from the equator.

The city of Athens is 4200 km from the equator.

What is the mean solar intensity in Athens?

Solar water heaters use radiation from the Sun to heat water.

The heated water is stored in a water tank.

Figure 8 shows a solar water heater on the roof of a building.

Cities closer to the equator have many more buildings with solar water heaters than cities further away from the equator.

Suggest why.

The use of solar water heaters may reduce the need to burn fossil fuels.

Complete the sentence.

Choose the answer from the box.

Burning fossil fuels contributes to global warming because there is an increase in

The efficiency of the solar water heater is 0.61

Calculate the useful power output when the total power input to the solar water heater is 1100 W.

Use the equation:

useful power output = efficiency × total power input

Different solar water heaters have different sized heating panels.

Suggest how the size of the heating panels affects the input power to a solar water heater.

Water has a high specific heat capacity.

What is meant by the specific heat capacity of water?

Tick (✓) one box.

The water tank contained 80 kg of water.

The change in thermal energy of the water was 8 400 000 J.

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

Calculate the temperature change of the water.

Use the Physics Equations Sheet.

The water tank is thermally insulated.

How does thermal insulation affect the rate of energy transfer from the water in the tank?

Tick (✓) one box.

Table 1 shows information about different materials.

Which material in Table 1 is the best thermal insulator?

Tick (✓) one box.

An air source heat pump transfers energy from the air outside a building to increase the temperature of the air inside the building. Figure 13 shows an air source heat pump.

The compressor is connected to the mains electricity supply. The pipe in the heat pump contains a substance called coolant. In the evaporator, energy is transferred from the air outside the building to the liquid coolant. The temperature of the coolant increases and it evaporates.

Explain what happens to the internal energy of the coolant as its temperature increases.

What name is given to the energy needed to change the state of the liquid coolant?

What happens to the mass of the coolant as it evaporates and becomes a vapour? Tick (✓) one box.

The compressor increases the density and temperature of the coolant vapour inside the pipe. Explain why the pressure in the pipe increases.

The condenser transfers energy from the coolant to the air in the building. When the total energy input to the heat pump system is 1560 kJ the temperature of the air in the building increases from 11.6 °C to 22.1 °C. The efficiency of the heat pump system is 87.5%. The mass of the air inside the building is 125 kg. Calculate the specific heat capacity of the air in the building. Give your answer in standard form.

The air in the building gains 400 J for every 100 J of energy transferred from the mains electricity supply to the compressor. An advertisement claims that the heat pump system has an efficiency of 400%. Explain why the advertisement is not correct.

An eco-house is designed to be environmentally friendly. Figure 4 shows a picture of an eco-house.

The solar panels and a wind turbine are used to generate electricity for the eco-house. Solar and wind are both renewable energy resources. What does renewable energy resource mean? Tick (✓) one box.

Biomass, nuclear and natural gas are three other energy resources. Complete the table to show whether each energy resource is renewable or non-renewable.

Moving air makes the wind turbine spin. The wind turbine generates electricity which is used to charge a battery. Complete the sentences. Choose answers from the box.

energy.

battery.

The roof of the eco-house is covered with soil. Covering the roof with soil decreases the thermal conductivity of the roof. What are the advantages of having a roof with a lower thermal conductivity? Tick (✓) two boxes.

The average power transferred to the solar panels by sunlight is 26 000 W Calculate the average energy transferred to the solar panels in 30 seconds. Use the equation: energy transferred = power × time

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

The solar panels on the roof of the eco-house have an efficiency of 0.15 The average power input to the solar panels is 26 000 W Calculate the average useful power output from the solar panels.

Explain why it is a good idea for the eco-house to have both a wind turbine and solar panels.

Ice cream is made by cooling a mixture of liquid ingredients until they freeze.

Which statement describes the motion of the particles in solid ice cream? Tick (✓) one box.

How do the kinetic energy and the potential energy of the particles change as a liquid

Figure 6 shows a bowl used for making ice cream.

The walls of the bowl contain a liquid coolant. The bowl is cooled to –20 °C before the mixture is put in the bowl. The bowl causes the mixture to cool down and freeze.

Explain why the different thermal conductivities of metal and plastic are important in the design of the bowl.

The liquid coolant has a freezing point below –20 °C Explain one other property that the liquid coolant should have.

The initial temperature of the mixture was +20 °C. The mixture froze at –1.5 °C. A total of 165 kJ of internal energy was transferred from the mixture to cool and freeze it. specific heat capacity of the mixture = 3500 J/kg °C specific latent heat of fusion of the mixture = 255 000 J/kg

Calculate the mass of the mixture. Give your answer to 2 significant figures.

A hybrid car has an electric motor and a petrol engine.

Petrol is a non-renewable energy resource. What is meant by a non-renewable energy resource?

The electric motor in the car is powered by a battery. To charge the battery, the car is plugged into the mains supply at 230 V The power used to charge the battery is 6.9 kW Calculate the current used to charge the battery.

Mains electricity is an ac supply. Explain the difference between direct and alternating potential difference.

The cable used to connect the car to the mains electricity supply has a low resistance. Explain why it is better to use a cable with a low resistance than to use a cable with a high resistance.

Figure 5 shows a girl skateboarding on a semi-circular ramp.

The girl has a mass of 50 kg

Calculate the gravitational potential energy (g.p.e.) of the girl at the top of the ramp.

Use the equation:

g.p.e. = mass × gravitational field strength × height

gravitational field strength = 9.8 N/kg

The girl has a speed of 7 m/s at the bottom of the ramp.

Calculate the kinetic energy of the girl at the bottom of the ramp.

Use the equation:

kinetic energy = 0.5 × mass × (speed)²

Not all of the g.p.e. has been transferred to kinetic energy.

Which two statements explain why?

Tick two boxes.

Explain how lubricating the wheels of the skateboard can increase the speed of the girl.

Use ideas about energy in your explanation.

Figure 7 shows cavity wall insulation being installed in the wall of a house.

Explain how the wall reduces unwanted energy transfers.

The cavity insulation was tested.

• The heating inside the house was switched off.

• The temperature inside the house was measured every 20 minutes for 2 hours.

Table 4 shows the results.

Determine the temperature inside the house after 30 minutes.

Figure 8 shows the gas boiler used to heat the house.

Describe how different energy stores are changed by the boiler.

To heat the house, the boiler transfers 15 MJ of energy in 10 minutes. Calculate the power of the boiler. Write any equation that you use.