Energy, Work & Power (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award): Physics): Flashcards

True.

An example of energy transfer by heating is a hot drink warming up cold hands.

Kinetic energy is the amount of energy an object has as a result of its mass and speed.

True.

Any object in motion has energy in its kinetic store.

The equation for kinetic energy is:

Where:

• = kinetic energy, measured in joules (J)

• = mass, measured in kilograms (kg)

• = speed, measured in metres per second (m/s)

False.

Kinetic energy is measured in joules (J).

The kinetic energy becomes four times larger.

This is because kinetic energy is directly proportional to the square of the speed ().

Gravitational potential energy is the energy an object has due to its height in a gravitational field.

False.

Energy is transferred to an object's gravitational potential store when it is lifted.

The equation for the change in gravitational potential energy is:

Where:

• = change in gravitational potential energy, measured in joules (J)

• = mass, measured in kilograms (kg)

• = gravitational field strength, measured in newtons per kilogram (N/kg)

• = change in height, measured in metres (m)

False.

Gravitational potential energy is measured in joules (J).

Work is done against the object's weight.

This transfers energy to the object's gravitational potential store.

Energy cannot be created or destroyed; it can only be transferred from one store to another.

Work done is the product of the force applied to an object and the distance moved in the direction of the force. It is measured in joules (J) or newton metres (N m).

False.

The total amount of energy in a closed system remains constant; it cannot be created or destroyed.

False.

Work is only done when the force causes the object to move a distance in the direction of the force. If there is no movement, no work is done.

An energy flow diagram shows the energy stores and the energy transfers taking place within a system.

The equation for work done is:

Where:

• = work done, measured in joules (J) or newton metres (N m)

• = force, measured in newtons (N)

• = distance moved in the direction of the force, measured in metres (m)

The energy is related by:

total energy in = useful energy out + wasted energy

True.

When work is done, energy is transferred, so the energy transferred (J) is equal to the work done (J).

Energy is dissipated (spread out) to the surroundings by heating and radiation. This energy is often not useful, so it is described as wasted energy.

1 joule (J) is equal to 1 newton metre (N m).

True.

Work is done against the ball's weight as it is lifted, so energy is transferred to its gravitational potential store.

Power is the rate at which energy is transferred or work is done. It is measured in watts (W).

False.

Power is the rate at which energy is transferred or work is done, not the total amount of energy transferred.

False.

A car has more power if it does the same amount of work in a shorter time.

Power can be calculated using:

or

Where:

• = power, measured in watts (W)

• = work done, measured in joules (J)

• = energy transferred, measured in joules (J)

• = time, measured in seconds (s)

The energy transferred is given by:

Where:

• = energy transferred, measured in joules (J)

• = power, measured in watts (W)

• = time, measured in seconds (s)

False.

Power is measured in watts (W). Energy is measured in joules (J).

Efficiency is a measure of the useful power or energy output from a system compared to its total power or energy input. It is often expressed as a percentage.

True.

Efficiency measures how much energy is transferred usefully compared to the total input, so it also tells us how much energy is wasted.

True.

The overall efficiency of a typical thermal power station is approximately 30%.

False.

The higher the efficiency of an energy transfer, the less energy is wasted.

The equation for calculating efficiency is:

True.

Efficiency can be expressed as a ratio between 0 and 1, or as a percentage between 0 and 100%.

False.

Efficiency is a ratio, so it is a dimensionless quantity with no units.

True.

The useful and wasted energy add up to 100%, so if a transfer is 65% efficient then 35% of the energy is wasted.

The primary source of energy for most of Earth's energy resources is the Sun.

True.

The Sun heats the atmosphere, creating wind and producing waves.

False.

Geothermal energy comes from the heating of rocks by the Earth's core, not from the Sun.

Solar cells (also called photovoltaic cells) are made of semiconducting materials and produce electricity from sunlight.

True.

Solar energy is renewable because it comes from the Sun, which will continue to shine for billions of years.

Energy from the Sun is transferred to the Earth by radiation, mostly as visible light and infrared radiation.

False.

Solar panels produce no greenhouse gases or pollution once they are operating.

Solar panels transfer energy from sunlight to the thermal store of the panels, which is used to heat water.

Solar furnaces use large curved mirrors to focus sunlight onto a small area. This heats water to produce steam, which turns turbines to generate electricity.

False.

Solar farms need to be large-scale to produce large amounts of electricity, because each solar cell only produces a small amount.

The wind turns a turbine, which turns a generator, which generates electricity. Energy is transferred from the kinetic store of the wind.

Advantages of wind power include:

• renewable

• produce no greenhouse gases or pollution

• no fuel costs and low running costs once built

• the land can still be used for farming

False.

Wind power is not reliable because the wind does not blow constantly.

Fossil fuels are natural resources formed over millions of years from the remains of plants and animals.

The three main fossil fuels are:

• coal

• oil

• natural gas (mostly methane)

False.

Burning fossil fuels produces carbon dioxide, which contributes to global warming, but also other harmful gases such as sulphur dioxide, which causes acid rain.

The main uses of fossil fuels are transport, electricity generation and heating.

True.

It takes millions of years for fossil fuels to form, which is why they are considered a non-renewable energy resource.

Biofuels are made from plant matter. Ethanol or methane can be produced and used in place of fossil fuels.

The main advantages of biofuels are that they are renewable and considered carbon neutral.

Disadvantages of biofuels include:

• growing the crops uses land and resources needed for food production

• burning biofuels still releases carbon dioxide

• biofuels have a lower energy density than fossil fuels

True.

Nuclear power stations release no pollution into the atmosphere during normal operation.

Nuclear fuel, such as uranium, is a material used in nuclear reactors to release energy through nuclear fission.

Nuclear power stations use the energy released by nuclear fission to heat water and produce steam. The steam turns turbines, which turn generators to generate electricity.

Nuclear fuel is non-renewable because there is a finite supply of uranium ore in the ground.

Wave and tidal power harness the energy from the motion of waves and tides to turn turbines and generate electricity.

False.

Wave and tidal power do not produce pollution.

Energy is transferred from the kinetic store of the moving water to the kinetic store of the turbines. The turbines turn generators, which generate electricity.

Hydroelectric dams store water at a height behind the dam. When released, the water flows downhill and is used to generate electricity.

Water stored at a height has energy in its gravitational potential store. As the water flows down, this is transferred to its kinetic store. The moving water turns a turbine, which turns a generator.

Geothermal energy is energy in the thermal store of hot rocks beneath the Earth's surface, heated by tectonic activity. The rocks heat water to provide a hot water supply and to produce steam that turns turbines for electricity.

False.

Geothermal energy is a renewable resource.

A key disadvantage is that there are few suitable locations on Earth, because geothermal energy depends on tectonic activity.

False.

Geothermal power stations are usually small, so they are not suitable for large-scale electricity production.

Geothermal power stations are reliable because the rocks remain hot for thousands of years, so they can consistently produce electricity.

Nuclear fission is the process in which a large nucleus is split into smaller nuclei, releasing a large amount of energy.

Nuclear fusion is the process in which small nuclei are joined to form a larger nucleus, releasing a large amount of energy.

In nuclear fusion, small nuclei are joined to form larger nuclei. In nuclear fission, large nuclei are split into smaller nuclei.

False.

The Sun releases energy by nuclear fusion, in which small nuclei join to form larger nuclei.

Nuclear fusion requires extremely high temperatures and pressures, which can normally only be found in the core of a star.