Conservation and dissipation of energy

The diagrams show a spring hanging from a nail.

• diagram 1 shows the spring with no weight added

• diagram 2 shows the spring stationary, after a weight has been added

• diagram 3 shows the spring after the weight has been pulled down

weight

Diagram 1
Diagram 2
Diagram 3

Which energy store has increased for the spring in diagram 2 compared to the spring in diagram 1?

The spring is released from the position shown in diagram 3.

Describe the energy transfers that take place until the spring stops vibrating.

Shock absorbers containing springs are used on motorcycles. Shock absorbers are designed to compress and expand as the motorcycle moves across a rough surface.

A new type of shock absorber has been developed to generate electricity from the movement of the motorcycle.

This new type of shock absorber consists of magnets that slide inside a coil when the motorcycle goes over a bump.

Some of the energy that would normally be wasted can be recovered, so fuel is saved.

Which of these statements best describes the advantage of this new type of shock absorber?

Explain how this new type of shock absorber can generate electricity.

Road X has a rough surface.

Road Y has a smooth surface.

(Source: © MikhailSh / Shutterstock)
(Source: © BearFotos / Shutterstock)

Road X
Road Y

A motorcycle travels at the same speed along road X and road Y.

Explain why the new type of shock absorber will generate more electricity for this motorcycle on road X than on road Y.

An athlete jumps vertically from the ground.

The athlete has a mass of 85 kg, and leaves the ground with an upwards velocity of 4.5 m/s.

State the formula linking kinetic energy, mass and speed.

Calculate the athlete’s kinetic energy at a speed of 4.5 m/s.

At the maximum height of the jump, the athlete has a speed of 0 m/s.

State the increase in the amount of energy in the athlete’s gravitational potential energy store from the ground to the maximum height of the jump.

State the formula linking gravitational potential energy, mass, gravitational field strength and height.

Calculate the maximum height of the athlete’s jump.

Sketch a velocity‑time graph for the athlete’s jump from the ground to the maximum height of the jump. No numbers are required.

Assume there is no air resistance.

What feature of the velocity‑time graph gives the acceleration due to gravity?

Explain why the gravitational field strength on the Moon is different from the gravitational field strength on the Earth.

Air is trapped in a boiling tube by sealing the boiling tube with a rubber bung.

The boiling tube is placed in a beaker containing hot water.

Energy is transferred from the thermal store of the water to the thermal store of the air in the boiling tube with an efficiency of 16%.

The air in the boiling tube gains 1800 J of energy during this process. This is defined as the useful energy transfer.

Calculate the amount of energy wasted during this process.

Give two ways that the apparatus could be modified to improve the efficiency of the energy transfer.

This question is about generating electricity from renewable energy resources.

Photograph 1 shows a solar farm in the United Kingdom and photograph 2 shows a geothermal power station in Iceland.

(Source: © Marcin Jucha / Shutterstock)
(Source: © Javarman / Shutterstock)

Discuss the advantages and disadvantages of generating electricity using solar power and geothermal resources.

A student investigates how the amount of light affects the output voltage of a solar cell.

Photograph 3 shows how the student sets up their equipment.

The student varies the distance between the solar cell and the lamp and measures the output voltage of the solar cell at each distance.

The student’s results are shown below.

5 cm, 0.45 V 14 cm, 0.06 V

8 cm, 0.18 V 17 cm, 0.04 V

11 cm, 0.10 V 20 cm, 0.03 V

Draw a table showing the student’s results.

Give a control variable for the student’s investigation.

The student states that the solar cell could be used as a power source for the lamp.

They suggest that this would work if

• the solar cell is connected directly to the lamp

• the light from the lamp is used to produce the output power from the solar cell

Explain, with reference to the principle of conservation of energy, why the student’s suggestion would not work.

The photograph shows a toy called a marble run.

A student lifts a marble from the table to the top of the marble run at point A. They release the marble from point A and it rolls through pipes to reach the bottom of the marble run at point B.

The marble leaves the marble run at point B and rolls across the table.

As the marble rolls, energy is transferred due to the different forces acting on the marble.

Describe the energy transfers from before the student lifts the marble to when the marble reaches point B of the marble run.

The student wants to measure how much energy the marble loses as it moves from point A to point B.

The student needs to measure the speed of the marble as it leaves the marble run at point B.

Describe a method the student could use to measure this speed.

The difference in height between point A and point B is 0.21 m.

The mass of the marble is 5.5 g.

The marble leaves the marble run at point B with a speed of 0.76 m/s.

Calculate the energy lost by the marble as it rolls from point A to point B.

The diagram shows some apparatus that can be used to determine the specific heat capacity of water.

Describe how a student could use this apparatus to determine the specific heat capacity of water.

Include details of any additional equipment needed in your answer.

The table shows the student’s results.

Use the student’s results to calculate the specific heat capacity of water.

specific heat capacity of water =_........ J / kg °C

Give two reasons why the energy from the heater is not all retained in the thermal store of the water.

A model electric motor is used to lift a load through a vertical height.

The load has a mass of 400 g and gains 3.2 J of energy in its gravitational store when lifted.

State the formula linking gravitational potential energy, mass, gravitational field strength (g) and height.

Calculate the height the load is lifted.

State the amount of useful work done on the load by the motor when the load is lifted through this height.

The load is lifted at a constant speed.

Diagram 1 shows the lifting force acting on the load as it is lifted.

Draw a labelled arrow on diagram 1 to show the other force acting on the load.

Ignore the effects of air resistance.

Diagram 1

A joulemeter measures the amount of energy transferred electrically to the motor as the motor lifts the load.

The joulemeter displays a reading of 11.0 J when the load has gained 3.2 J of energy in its gravitational store.

Calculate the efficiency of the motor.

Justify why 7.8 J of energy must be dissipated into the thermal store of the surroundings as the load is lifted.

Diagram 2 is an incomplete Sankey diagram.

Complete the Sankey diagram to show the energy transferred by the motor.

The driver of a racing car makes a pit stop during a race to change the tyres on the racing car.

The area where the tyres are changed is called the pit lane.

Before entering the pit lane, the speed of the car must decrease for safety reasons.

The mass of the racing car is 830 kg.

The maximum braking force is 41 000 N.

Show that the maximum deceleration of the racing car is approximately 50 m / s².

The racing car is travelling at an initial speed of 72 m / s.

Calculate the minimum distance needed to decrease the speed of the racing car from 72 m / s to 26 m / s.

The racing car slows down using its brakes.

The brakes work using friction.

The brakes become very hot when the racing car slows down.

Using ideas about energy, explain why the brakes become hot.

The tyres of the racing car also get very hot during a race.

A mechanic has to handle the hot tyres during the pit stop.

They wear protective gloves which have several layers of insulating materials.

Explain how the layers of insulating materials in the gloves reduce the risk of the mechanic burning their hands on the hot tyres.

The photograph shows a toy car. When the toy car is pulled backwards, energy is stored in the elastic store as the rubber band is twisted.

(Source: Mama Belle and the kids/Shutterstock)

When the car is released, some of the energy from the elastic store is transferred to the kinetic store of the car.

The remaining energy is transferred into the thermal store of the surroundings.

State what is meant by the principle of conservation of energy.

The car is pulled backwards so that there is 165 J of energy in its elastic store.

When the car is released, this energy is transferred to the car’s kinetic energy store with an efficiency of 15 %.

State the formula linking efficiency, useful energy output and total energy output.

Calculate the energy transferred into the thermal store of the surroundings.

Draw a labelled Sankey diagram for this energy transfer.

The car is pulled backwards again.

When released, 45 J of energy transfers into the car’s kinetic store.

The car travels a distance of 7.5 m during this energy transfer.

State the useful work done on the car.

Calculate the mean accelerating force acting on the car.

The photograph shows a vertical drop ride at an amusement park.

The car is pulled to the top of a vertical shaft and then released from rest.

The car then falls freely because of the force of gravity.

Calculate the speed of the car when it has fallen 18 m.

State the formula linking kinetic energy, mass and speed.

The mass of the car and its passengers is 2.1 tonnes.

Calculate the kinetic energy of the car when it has fallen 18 m.

[1 tonne = 1000 kg]

The actual speed of the car when it has fallen 18 m is lower than the value calculated in (a).

Describe the energy transfers occurring from immediately before the car was released to when the car has fallen 18 m.

Refer to stores and transfers in your answer.

This question is about a parachutist.

A parachutist leaves a helicopter that is hovering above the ground.

The parachutist is initially at rest and falls vertically downwards.

Calculate the speed of the parachutist after they have fallen through a distance of 1300 m.

Ignore the effect of air resistance.

When the parachutist is much nearer to the ground, they open their parachute.

The parachutist slows down.

Explain the change in speed of the parachutist.

Use ideas about forces in your answer.

It is observed that from when the parachute opens to just before the parachutist touches the ground, the GPE store and the KE store of the parachutist both decrease, yet energy is still conserved.

Justify these observations.

A racing car uses an engine and an electric motor to power its wheels.

The electric motor can only operate at maximum power for part of each lap of the race.

The electric motor has a maximum power output of 1.210⁵W.

The electric motor transfers 4.010⁶J of energy when it operates at maximum power.

Calculate the amount of time the electric motor can operate at maximum power.

The diagram shows a simplified view of a direct current (d.c.) electric motor.

State what is meant by the term direct current.

Explain the purpose of the brushes and the commutator in the d.c. motor.

The electric motor in the racing car can also be used as a generator to charge a battery pack. It is known as a motor generator unit (MGU).

The rotating wheels of the car cause the magnets in the MGU to spin near a coil of wire.

This induces a current in the coil when the coil is connected to the battery pack.

Name the energy store of the battery pack that increases when the battery pack is being charged.

Explain why the kinetic energy store of the car must decrease when the MGU charges the battery pack.

The diagram shows a ball at the bottom of a ramp. The ball moves up the ramp with an initial speed v.

State the formula linking gravitational potential energy (GPE), mass, gravitational field strength (g) and height.

The ball has a mass of 14 g.

Show that when the ball reaches a height of 29 cm above the bottom of the ramp, the GPE gained by the ball is about 4 10⁻²J.

At the bottom of the ramp, the 14 g ball has a kinetic energy (KE) of 5.1 10⁻²J.

Calculate the speed of the ball at the bottom of the ramp.

The ball is stationary when it reaches the top of the ramp. There is a difference between the KE of the ball at the bottom of the ramp and the GPE of the ball at a height of 29 cm above the bottom of the ramp.

Explain why the work done by friction between the ball and the ramp is equal to this difference.

A student uses a small iron nail and a beaker of water to estimate the temperature of a flame. The diagram shows some of the equipment used.

This is the student’s method

• place a thermometer into the beaker of water

• record the temperature of the water

• heat the nail in the flame for a long period of time

• quickly move the nail into the beaker of water

• record the highest temperature of the water

State a safety precaution the student should take.

While in the water, the thermal energy store of the nail decreases and the thermal energy store of the water increases until their temperatures are the same.

The energy lost from the nail is equal to the energy gained by the water.

The table shows the student’s results.

Show that the energy gained by the water is approximately 3000 J.

Calculate the temperature change of the nail.

Give a reason why the calculated temperature change of the nail is lower than the actual temperature change.

A student uses this apparatus to investigate the force needed to pull a block along a ramp.

State the formula linking work done, force and distance moved in the direction of the force.

The student pulls the block 0.89 m along the ramp with a force of 26 N.

Show that the work done on the block by the 26 N force is about 23 J.

The block has a mass of 1.3 kg and moves vertically upwards 0.11 m.

Calculate the increase in the gravitational potential energy (GPE) store of the block between the bottom and the top of the ramp.

Explain why the work done to move the block along the ramp is greater than the increase in the GPE store of the block.

The student suggests it is better to use a less steep ramp to raise the block through the same vertical height.

Discuss an advantage and a disadvantage of using a less steep ramp.

A student uses this apparatus to investigate how the efficiency of an electric motor varies with its input voltage.

This is the student’s method.

• connect the electric motor to a d.c. power supply and a joulemeter

• attach a 1.0 kg mass to the electric motor using a length of string

• set the voltage of the power supply to 10 V and switch on the power supply

• switch off the power supply when the mass has been lifted through a distance of 50 cm

• record the input energy to the motor from the joulemeter

• calculate the energy transferred to the gravitational store of the mass

• calculate the efficiency of the motor

The student repeats this process, setting the power supply to a different voltage each time.

State how the student could improve the reliability of her results.

Give two control variables for the investigation.

Show that the gravitational store of the 1.0 kg mass increases by 5.0 J when it is lifted through a distance of 50 cm.

The table shows the student’s results.

Calculate the motor efficiency when the power supply is set to a voltage of 10 V.

Plot a graph of the motor efficiency on the y-axis against the power supply voltage on the x-axis.

Draw a curve of best fit.

Using the graph, determine the minimum power supply voltage that will allow the electric motor to operate at maximum efficiency.

Energy is transferred from the Sun to the Earth.

Nuclear fusion happens in the Sun.

Which energy store of the Sun decreases during nuclear fusion?

How is energy transferred through space from the Sun to the Earth?

Solar panels use energy from the Sun to heat water.

The total area of the solar panels is 15 m².

Each 1.0 m² of the solar panels receives 1000 J of energy per second from the Sun.

Show that the total energy transferred to the solar panels in 2 hours is about 100 MJ.

A mass of 1100 kg of cold water is in the solar panels at an initial temperature of 20C.

Calculate the final temperature of this water after it has been heated by the Sun for 2 hours.

[specific heat capacity of water = 4200 J/kg C]

final temperature _........ C

Give a reason why the actual final temperature of the water will be lower than the calculated value.

A metalworker uses a hammer of mass 1.8 kg to deform a piece of metal on an anvil.

Calculate the increase in the hammer’s gravitational potential energy (GPE) store when the hammer is raised through a height of 0.95 m above the metal.

[GPE gained = mass × gravitational field strength × height]

The metalworker moves the hammer down to strike the metal.

The hammer must have at least 21 J of energy in its kinetic energy (KE) store as it hits the metal to deform it.

Explain why the metalworker must do at least 4 J of mechanical work on the hammer to deform the metal.

Calculate the velocity of the hammer just before it strikes the metal.

[KE of hammer = 21 J]

When the hammer hits the metal, the hammer comes to rest without rebounding.

Calculate the mean (average) force needed to bring the hammer to rest in a time of 0.12 s.

A dog sits on a water‑filled bag to keep cool.

The table shows some data about the dog and the water in the bag.

The dog sits on the bag for 22 minutes.

Calculate the energy transferred from the dog to the water by heating in 22 minutes.

State an assumption you have made when calculating the energy transferred.

Calculate the temperature of the water after 22 minutes.

temperature =_........ C

Discuss why conduction is the main way that thermal energy is transferred from the dog to the water.

The photograph shows a device that can be used to ignite small pieces of combustible material.

The device consists of a cylinder containing trapped air and a piston.

A small piece of combustible material is placed at the bottom of the cylinder.

When the piston is pushed quickly into the cylinder, the trapped air is compressed and heats up.

This increase in temperature is enough to ignite the combustible material.

The piston is quickly pushed down a distance of 145 mm.

The average force exerted on the piston is 4.2 N.

State the formula linking work done, force and distance moved in the direction of the force.

Calculate the work done on the trapped air when the piston is quickly pushed down.

State the maximum amount of energy that could be transferred to the trapped air.

Using ideas about molecules, explain why the trapped air heats up.

Suggest why the trapped air does not reach a high enough temperature to ignite the combustible material if the piston is slowly pushed into the cylinder.