Power

Two students do an experiment to determine their power when running up a set of steps.

The diagram shows how they set up their experiment. Not all of the steps are shown in the diagram.

This is the students’ method.

• student A stands with a stopwatch at the top of the steps

• student A starts timing on the stopwatch and shouts “go” at the same time

• student B begins to run up the steps when she hears student A shout

• student A stops timing when student B reaches the top of the steps

The students repeat their method two more times.

Give a reason why the times recorded may not be accurate.

There are 24 steps in total and each step has a height of 19 cm.

Student B has a mass of 67 kg.

Show that student B gains about 3000 J of energy in her gravitational store when she runs up the set of steps.

The table shows the times recorded for student B.

Calculate the mean time from this data.

Give your answer to three significant figures.

Calculate the mean power of student B transferring energy to her gravitational store as she runs up the steps.

The students extend their investigation by calculating the power for students of different masses running up the steps.

The graph shows their results.

The students conclude that the greater the mass of the person, the greater the power of the person when running up the steps.

Comment on the students’ conclusion.

The photograph shows the tidal power station across the estuary of the river Rance in France.

The diagram shows a simplified view of the tidal power station.

At high tide, water is trapped behind the tidal barrier.

At low tide, the trapped water is released through the turbine shaft. This causes a turbine to spin, which generates electricity.

Give two advantages of generating electricity using tidal power.

Give two disadvantages of generating electricity using tidal power.

At low tide, water flows through the turbine shaft.

As the height of the trapped water in the river decreases, water falls through a height of 8.0 m to reach the turbine shaft.

Calculate the energy transferred from the gravitational store of the water when 1.0 kg of water flows through the turbine shaft.

Use the formula

change in gravitational potential energy = mass × g × height

The tidal power station has a maximum power output of 240 000 kW when water falls through a height of 8.0 m.

Calculate the mass of water flowing through the turbine shaft in 1 second when the power station is operating at maximum power.

Assume the power station is 100% efficient.

A family has a television set.

The television set has a low power mode called standby.

When on standby, the power rating of the television set is 0.27 W.

Calculate the energy transferred to the television set on standby in 12 hours.

In normal use, the current in the television set is 0.31 A.

Explain how a fuse works to protect the television set if there is a fault.

Explain why a 13 A fuse is not an appropriate choice of fuse to use in the plug of this television set.

Kori Nuclear Power Plant in South Korea is one of the world’s largest nuclear fission power stations.

The reactors at Kori use nuclear fission to generate electricity.

The products released during nuclear fission have high energy in their kinetic store.

Give a product of nuclear fission.

Give two disadvantages of using nuclear fission to generate electricity.

Kori has a maximum power output of 7.49 × 10⁹W.

State what is meant by the term power.

Calculate the minimum time taken for Kori to transfer 6.4710¹⁴J of energy.

A student heats a sample of water.

The student measures the temperature of the sample of water during heating.

The sample of water has a mass of 0.45 kg.

Calculate the energy required to increase the temperature of the water from 16 °C to 100 °C.

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

The diagram shows the temperature-time graph for the sample of water.

Use the graph to determine the time taken from when the water started to boil to when the water stopped boiling.

time taken =_........ minutes

The heater used to heat the water has a power rating of 2200 W.

Calculate the energy required to boil all of the water.

Give a reason why the liquid water should be stirred during the heating.

When water boils, the liquid water becomes a gas called steam.

Describe the changes in arrangement and motion of the molecules in liquid water and the molecules in steam.

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 photograph shows the Three Gorges Dam in China, one of the largest hydroelectric power stations on Earth.

State two advantages of generating electricity using hydroelectric power stations.

Water is held in a reservoir on one side of the dam.

The water flows through turbines to a river on the other side of the dam.

The water level of the river is lower than the water level of the reservoir.

State which energy store of the water decreases as the water flows from the reservoir to the river.

How is energy transferred from the dam to homes and factories in China?

The Three Gorges Dam has a larger maximum output power than any other hydroelectric power station on Earth.

State what is meant by the term power.

The mean energy transferred by the dam each day is 9.710¹⁴ J.

Calculate the mean output power of the dam.

The maximum output power is 22 500 MW.

Which of these is the same as 22 500 MW?

Suggest why the Three Gorges Dam does not always operate at its maximum output power.

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.

Stearic acid is a solid at room temperature and melts at a temperature of 69 C.

A student investigates the specific heat capacity of solid stearic acid.

He heats a sample of stearic acid from room temperature using an electrical heater and measures its temperature using a data logger and temperature sensor.

The graph shows his results.

The student starts heating the stearic acid at 130 seconds.

Determine the time it takes to heat the stearic acid until it reaches its melting point.

The stearic acid gains 39 kJ of energy during the time it is heated to its melting point.

Calculate the mean (average) power of the heater.

Give a unit with your answer.

[assume no energy lost to surroundings]

The mass of stearic acid used in the investigation is 0.45 kg.

The stearic acid gains 39 kJ of energy when it is heated to its melting point.

Calculate the specific heat capacity of stearic acid.

You should use data from the graph in your calculation.

specific heat capacity =_........ J/kgC