Efficiency

The diagram shows a power station connected to a town by the national grid.

Power Station Transformer Pylons and Wires Transformer
Town

The national grid uses alternating voltage.

A simple circuit powered by a battery uses direct voltage.

Explain the difference between alternating voltage and direct voltage.

The table shows the current in one of the wires in the national grid at two different voltages.

The wire has a resistance of 2.0 .

Calculate how much power is dissipated from the wire at 11 000 V.

Use the equation: power = current² resistance

Give your answer to 2 significant figures and in MW.

The national grid transfers 8 000 kJ of energy from the power station every second.

When the voltage across the wires is 400 000 V, the transformers and wires dissipate 160 kJ of this energy every second.

Calculate the efficiency of the energy transfer at 400 000 V.

Explain how step-up transformers allow the national grid to transfer energy more efficiently.

A UK mains plug has 3 wires: live, neutral and earth.

Describe the function of the neutral wire.

The national grid uses alternating voltage.

A simple circuit powered by a battery uses direct voltage.

Explain the difference between alternating voltage and direct voltage.

The table shows the current in one of the wires in the national grid at two different voltages.

The wire has a resistance of 2.0 .

Calculate how much power is dissipated from the wire at 11 000 V.

Use the equation: power = current² resistance

Give your answer to 2 significant figures and in MW.

The national grid transfers 8 000 kJ of energy from the power station every second.

When the voltage across the wires is 400 000 V, the transformers and wires dissipate 160 kJ of this energy every second.

Calculate the efficiency of the energy transfer at 400 000 V.

Explain how step-up transformers allow the national grid to transfer energy more efficiently.

A UK mains plug has 3 wires: live, neutral and earth.

Describe the function of the neutral wire.

This is a diagram of a transformer.

Calculate the power output from the secondary coil.

Use the equation: power = potential difference current

Name the type of transformer shown in the diagram.

Where can this type of transformer be used in the national grid?

Tick (✓) one box.

The energy input to the primary coil is 200 kJ.

The energy output from the secondary coil is 180 kJ.

Calculate the efficiency of the transformer.

This question is about transformers.

Complete the sentences to describe how transformers are used in the national grid.

A step‑up / step‑down transformer increases current / potential difference at the power station.

A step‑up / step‑down transformer increases / decreases potential difference locally for domestic use.

A student researches three different transformers A, B and C.

Transformer A is used to decrease a potential difference from 240 V to 20 V.

The current in the primary coil is 0.8 A.

Calculate the current in the secondary coil.

Use the Equation Sheet.

Transformer B is used in a lighting circuit.

The energy input to the primary coil is 175 J.

The energy output from the secondary coil is 140 J.

Calculate the efficiency of the transformer.

Use the Equation Sheet.

A lamp is connected to the secondary coil of transformer C.

The current in the lamp is 0.25 A. The resistance of the lamp is 800 .

The lamp is at its normal brightness.

Calculate the output power of transformer C.

Use the Equation Sheet.

A student calculates the efficiency of an energy transfer.

The input energy is 1600 J. The useful output energy is 1200 J.

Which answer shows the correct calculation?

Use the Equation Sheet.

The manufacturer says, ‘The transformer is 98% efficient.’

Explain what this statement means.

The diagram shows a power station called a ‘combined heat and power’ (CHP) station.

Why is a CHP station is more efficient than a coal fired power station?

A Less energy is wasted in a CHP station.

B A CHP station produces renewable energy.

C The homes and offices contain insulation.

D There is more input of chemical energy in a CHP station.

A student drops a ball onto a hard floor. The mass of the ball is 0.06 kg.

The ball bounces once and the student catches the ball.

The graph shows how the velocity of the ball changes after it is dropped.

State a point on the graph where the gravitational store of the ball has a minimum value.

Choose from A, B, C, D or E.

State the point on the graph where the kinetic store of the ball has a maximum value.

Choose from A, B, C, D or E.

Calculate the kinetic energy of the ball at 0.4 seconds. Use the Data Sheet.

Calculate the potential energy of the ball at A.

Use the graph and the equation: potential energy = mass × height × gravitational field strength

Gravitational field strength = 10 N / kg.

The potential energy of the ball at E is 0.45 J.

Calculate the efficiency of the ball bounce. Use the Data Sheet and your answer to (c)(i).

Radio waves are electromagnetic waves and are used for communications.

Suggest why gamma rays are not used for communications.

Name one other electromagnetic wave that is used for communications.

State one use of this electromagnetic wave in communications.

A TV transmitter emits electromagnetic waves with different frequencies.

Table 15.1 gives information about the waves emitted.

Using data from Table 15.1 describe the relationship between frequency and wavelength.

How many significant figures are used in each value in Table 15.1 ?

Calculate the speed of the wave ARQ B.

Use the equation: wave speed = frequency × wavelength

Speed =........ m / s

A TV transmitter has a useful output energy transfer of 100 kJ in 1 second.

The input energy is 250 kJ in 1 second.

Calculate the efficiency of the TV transmitter.

Radio waves are electromagnetic waves and are used for communications.

Suggest why gamma rays are not used for communications.

Name one other electromagnetic wave that is used for communications.

State one use of this electromagnetic wave in communications.

A TV transmitter emits electromagnetic waves with different frequencies.

Table 11.1 gives information about the waves emitted.

Using data from Table 11.1 describe the relationship between frequency and wavelength.

How many significant figures are used in each value in Table 11.1 ?

Calculate the speed of the wave ARQ B.

Use the equation: wave speed = frequency × wavelength

Speed =........ m / s

A TV transmitter has a useful output energy transfer of 100 kJ in 1 second.

The input energy is 250 kJ in 1 second.

Calculate the efficiency of the TV transmitter.

Which row in the table is correct?

Use the equation: efficiency = useful output energy transfer / input energy transfer

How can the efficiency of an energy transfer be increased?

A Decrease the total energy input.

B Decrease the wasted energy.

C Increase the total energy input.

D Increase the wasted energy.

15* A turbine contains a rotating blade.

Fig. 15.1 is a diagram of a wind turbine and Fig. 15.2 is a diagram of a water (tidal) turbine.

A volume of air (wind) or water (tidal) flows through the turbine every second.

Some of the kinetic energy of the air or water is transferred.

This table gives information about wind and tidal turbines:

Compare the advantages and disadvantages of using each type of turbine.

Use kinetic energy calculations to support your answer.

Calculate the efficiency of this light bulb.

Use the equation: efficiency = useful output energy transfer ÷ input energy transfer

Energy is transferred when a machine is switched on.

• 1000 J is the input energy transfer

• 750 J is useful output energy transfer

Use the equation: Efficiency = Useful output energy transfer ÷ Input energy transfer

Calculate the efficiency of this machine.

A 0

B 0.25

C 0.75

D 750