Efficiency

This question is about transformers.

The diagram shows a step‑down transformer.

The table gives some data for the step‑down transformer.

State the formula linking input (primary) voltage, output (secondary) voltage and the turns ratio for a transformer.

Use data from the table to calculate the output (secondary) voltage of the transformer.

This transformer is not 100% efficient.

The efficiency of a transformer can be calculated using the formula

Calculate the efficiency of the transformer.

Explain how transformers are used in the efficient, large‑scale transmission of electrical energy.

You may draw a diagram to help your answer.

The photograph shows a wind turbine used for supplying energy on a large scale.

Describe a change in the turbine’s energy stores from when the turbine is at rest to when it is moving at full speed.

Describe a change in the turbine’s energy stores when the wind is blowing but the turbine moves at constant speed.

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

The wind transfers 1.2 × 10⁶J of energy to the turbine.

The total mass of the turbine blades is 16 000 kg.

The wind causes the turbine blades to accelerate from rest to an average speed of 7.6 m/s.

Calculate the efficiency of the turbine.

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.

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 circuit diagram shows a motor, power supply, switch and variable resistor connected in series.

The switch is open and the motor is not moving.

State how much energy is in the kinetic energy store of the motor.

The switch is closed.

The motor takes 0.42 seconds to reach its maximum speed of rotation.

The mean current in the motor is 3.9 A.

The voltage across the motor is 7.1 V.

Show that the work done on the motor during the 0.42 seconds is about 12 J.

Complete the Sankey diagram for the electric motor during the 0.42 seconds by giving the missing information.

store of surroundings

2 J

Use the Sankey diagram to calculate the efficiency of the process of bringing the motor to its maximum speed of rotation.

A winch is used to pull a truck along a horizontal road.

The winch is connected to the truck by a thick rope.

The winch does 41 kJ of useful work on the truck when the truck is pulled a horizontal distance of 15 m.

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

Calculate the force that the rope exerts on the truck.

The winch includes a small engine. The engine burns petrol to power the motor in the winch.

The winch transfers energy mechanically to the truck.

The winch has an efficiency of 25% when pulling the truck.

Draw a Sankey diagram for this energy transfer.

The winch can also be used to pull the truck uphill at a constant speed.

The table gives some energy stores.

Add one tick () to each row to show what happens to the energy in each store as the truck is pulled uphill.

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 diagram shows part of the National Grid.

There is a step-up transformer at the power station end of a transmission line and a step-down transformer at a distant town.

Explain why step-up transformers and step-down transformers are used in this way.

State the formula linking the input voltage, the output voltage and the turns ratio for a transformer.

The primary coil on a step-up transformer has 3300 turns.

Calculate the number of turns required on the transformer’s secondary coil to step up the voltage from 15 kV to 340 kV.

Transformers in the National Grid have efficiencies less than 100%.

Suggest which energy store of the transformer increases as a result of the efficiency being less than 100%.

The iron core of a transformer is an electrical conductor.

When the transformer is in use, the primary coil causes a changing magnetic field in the iron core. This causes a small current to be induced in the core.

Explain how a current is induced in the core of the transformer.

The chart shows the efficiencies of different methods of generating electricity.

The chart shows that the geothermal power station has an efficiency of 15%.

Explain what is meant by an efficiency of 15%.

A small hydroelectric power station has a useful energy output of 6.0 kJ.

Calculate the total energy input for this power station using information from the chart.

Name a fuel used in the reactor of a nuclear power station.

These sentences describe the process of nuclear fission.

Complete these sentences by writing a suitable word in each blank space.

A nucleus absorbs a_........ .

The nucleus formed splits because it is_........ .

The nucleus splits into two smaller nuclei called_........

nuclei and two or three_........ .

The energy released is transferred into the_........

energy store of the fission products.

This question is about an electric fan.

A battery supplies a voltage of 12 V and a current of 0.25 A to the fan.

The fan is switched on for 12 seconds and the fan gains 25 J in its kinetic energy store.

Calculate the efficiency of this energy transfer.

The diagram shows part of the electric motor inside the fan.

Explain why the coil starts to rotate when there is a current in the coil.

Which side of the coil moves vertically upwards?

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.