0625 · 1.7Energy, work and power

Energy stores and transfers

0:00A cyclist rolls downhill, speeds up, then brakes to a stop. Energy explains every stage, including why the brakes become warm. The energy has not disappeared when the bicycle stops moving. By following where energy is stored and how it is transferred, we can understand motion, machines and electricity generation. In this video, we will connect energy stores, conservation of energy, work, efficiency and power. We will calculate changes in energy and interpret diagrams showing where energy goes.

0:28We will also compare the resources used to generate electricity. The calculations and ideas required for Extended students will be clearly identified as Supplement content. You only need a few starting ideas. Mass is measured in kilograms, force in newtons, distance in metres and time in seconds. Weight is the gravitational force acting on an object. Energy and work done are both measured in joules, because work done measures energy transferred.

0:54Start by separating energy stores from energy transfers. A store describes where energy is held within a system. Kinetic energy is associated with motion, while gravitational potential energy is associated with position in a gravitational field. A moving bicycle has kinetic energy, and raising a book increases its gravitational potential energy. Chemical energy is stored in fuels, food and batteries. Elastic strain energy is stored when an object is stretched or compressed.

1:22Nuclear energy is associated with atomic nuclei, while electrostatic energy is associated with the arrangement of electric charges. Internal energy is associated with the microscopic motion and interactions of particles within a substance. The syllabus also calls this thermal energy. Heating a substance generally increases its internal energy, although the detailed particle explanation belongs in a later topic. A stretched spring and a raised book can both transfer stored energy when released.

1:50The important habit is to identify the store before describing the transfer. Energy can be transferred by forces doing mechanical work. When you lift a bag, your force transfers energy to its gravitational potential store. Electrical currents transfer energy by electrical work, as in a motor connected to a battery. Heating transfers energy from a hotter object to a colder object. Energy can also be transferred by electromagnetic waves, sound waves and other waves.

2:17Sunlight transfers energy to an absorbing surface, and sound transfers energy away from a vibrating loudspeaker. Water waves can transfer energy to a device that generates electricity. Light and sound describe transfers here, rather than additional energy stores. Consider an electric kettle as a simple example. An electrical current transfers energy to the heating element by electrical work. Energy is then transferred from the element to the water by heating.

2:42Some energy also reaches the surroundings, so not all the input produces the intended increase in the water’s internal energy.

Conservation and mechanical work

2:50The principle of conservation of energy states that energy cannot be created or destroyed. Energy can be transferred between stores, but the total energy remains constant. In a complete energy account, the input equals all the outputs and any increase in stored energy. This remains true even when a machine is inefficient. Our diagram shows one hundred joules entering a device. Seventy joules provide the useful output, while thirty joules are transferred to the surroundings.

3:17Adding the outputs gives the original one hundred joules. Energy described as wasted has not vanished. It has been transferred somewhere that does not serve our intended purpose. Return to the cyclist descending the hill. Gravitational potential energy decreases while kinetic energy increases. Air resistance and friction also transfer energy to internal stores in the bicycle and surroundings. During braking, mechanical work transfers much of the remaining kinetic energy to internal energy in the brakes and surroundings.

3:47Mechanical or electrical work done is equal to the energy transferred. For a constant force acting along the movement, mechanical work done equals force multiplied by distance moved in the direction of the force. A larger force transfers more energy over the same distance. The same force acting through a greater distance also transfers more energy. Imagine pushing a box with a constant force of fifty newtons through three metres in the force’s direction.

4:13First, identify the force and the distance through which that force acts. Work done equals 50 × 3, giving one hundred and fifty joules. Your force has therefore transferred one hundred and fifty joules of energy. That result does not automatically mean the box gains one hundred and fifty joules of kinetic energy. Some energy may be transferred to internal stores through friction. If the box moves at constant speed, its kinetic energy does not increase.

4:39Holding a stationary bag also does no mechanical work on the bag, because its displacement is zero.

Gravitational and kinetic energy

4:45We now move into Supplement content for Extended students. Change in gravitational potential energy equals mass multiplied by gravitational field strength multiplied by change in height. Near Earth’s surface, this follows from the work required to lift an object against its weight. Weight equals mass multiplied by gravitational field strength, and lifting through a height transfers energy against that force. Suppose a two-kilogram load is lifted through three metres at constant speed.

5:11The question gives gravitational field strength as ten newtons per kilogram. Its weight equals 2 × 10, giving twenty newtons. Its gravitational potential energy increase equals 2 × 10 × 3, giving sixty joules. The lifting force does sixty joules of mechanical work on the load. Because the speed stays constant, its kinetic energy does not change. If the load later falls through the same height, its gravitational potential energy decreases by sixty joules.

5:41Always use the change in height, rather than an unrelated height above the ground. The second Supplement equation is kinetic energy equals one half multiplied by mass multiplied by speed squared. Squaring means multiplying the speed by itself before completing the calculation. Doubling mass doubles kinetic energy at the same speed. Doubling speed gives four times the kinetic energy, because the speed is squared. Take our two-kilogram load moving at four metres per second.

6:10First, square four to obtain sixteen. Then multiply one half by two by sixteen, giving sixteen joules. The load therefore has sixteen joules of kinetic energy. Forgetting to square the speed would wrongly give four joules. Conservation of energy connects these equations. If the load falls from rest and air resistance is negligible, its gravitational potential energy decrease equals its kinetic energy increase. For our three-metre fall, the final kinetic energy would be sixty joules.

6:38With air resistance, some energy reaches internal stores, so the final kinetic energy is smaller.

Efficiency, Sankey diagrams and power

6:45Efficiency describes how much of the input becomes useful output. A useful output depends on the device’s purpose. Heating water is useful for a kettle, while warming the surroundings is usually unwanted. High efficiency means a large fraction of the input serves the intended purpose, not that the device creates extra energy. For Extended students, a Sankey diagram represents these transfers using arrows whose widths are proportional to energy or power.

7:11The input arrow divides into useful and wasted output arrows. The widths of the outgoing arrows must add to the incoming width. Read the labels and arrow widths together when interpreting the diagram. Our motor receives one hundred joules and transfers twenty joules to the surroundings. It supplies eighty joules to a lifting mechanism, which transfers another twenty joules to the surroundings. The load gains sixty joules of gravitational potential energy.

7:35The complete account is sixty useful joules plus forty wasted joules, matching the original input. The Supplement efficiency equation is useful energy output divided by total energy input. Multiply that fraction by one hundred per cent to express efficiency as a percentage. For this complete system, divide sixty by one hundred to obtain zero point six. Multiplying by one hundred per cent gives an overall efficiency of sixty per cent.

8:00The lifting mechanism alone has a different efficiency because its input is eighty joules. Dividing sixty by eighty and multiplying by one hundred per cent gives seventy-five per cent. Using the original one hundred joules would describe the whole system instead. Always establish exactly which device or stage the question is asking about. Power is work done per unit time, and also energy transferred per unit time. Power equals work done divided by time, or energy transferred divided by time.

8:30Its unit is the watt, meaning one joule per second. Two motors can transfer the same energy, but the faster motor has greater power. Suppose a motor transfers six hundred joules in five seconds. Divide six hundred by five to obtain one hundred and twenty watts. If another motor transfers the same energy in ten seconds, its power is sixty watts. The first motor transfers energy twice as quickly, but it does not transfer twice as much energy overall.

8:56Extended students also need efficiency expressed using power. Percentage efficiency equals useful power output divided by total power input, multiplied by one hundred per cent. A motor receiving two hundred watts and delivering one hundred and twenty useful watts has sixty per cent efficiency. The remaining eighty watts represent energy transferred to unwanted outputs each second.

Electricity generation resources

9:18Energy resources provide energy that we can transfer into useful forms, including electrical output. A turbine rotates when moving steam, water or air pushes its blades. The turbine turns a generator, which generates electrical power. Several resources use the same turbine and generator arrangement, but supply the movement differently. In a fossil-fuel power station, burning coal, oil or natural gas releases energy from a chemical store. Heating in a boiler produces steam, which drives a turbine connected to a generator.

9:49Fossil fuels can provide large outputs when needed, provided fuel is available. They are non-renewable, and combustion releases carbon dioxide and can produce air pollutants. Biofuels contain chemical energy stored in recently living material, such as wood or plant-derived fuels. They can be burned for heating or used in a boiler, turbine and generator system. They are renewable if replacement material is grown sustainably, and stored fuel can provide reliable output.

10:15However, combustion releases carbon dioxide, while growing crops can require land, water and resources. A hydroelectric dam stores water above a lower level. As water descends, gravitational potential energy is transferred, and flowing water drives a turbine connected to a generator. Output can respond quickly to demand while sufficient water is available. Suitable sites can provide large-scale renewable generation, but drought reduces availability and reservoirs can flood habitats and settlements.

10:44Wave devices obtain energy from the movement of water in waves. One design uses rising and falling water to force air through a turbine connected to a generator. Waves are renewable, but their strength varies with weather, making output less reliable. Suitable coastal sites are limited, and devices face storm damage and possible effects on marine habitats. Tidal power uses the regular movement of sea water caused by tides. A tidal barrage can channel water between different levels through turbines connected to generators.

11:13Tides are renewable and predictable, although electricity is not generated continuously. Suitable sites are limited, construction can be expensive, and barrages can alter estuary habitats. Geothermal resources use energy from hot rocks inside Earth. In suitable locations, heated water or steam can drive a turbine connected to a generator, or provide direct heating. Geothermal output can be reliable and is generally treated as renewable at this level. However, suitable sites are limited, drilling is expensive, and some systems release gases or affect local ground stability.

11:47Nuclear power stations release energy from nuclear fuel by fission. This energy heats water, producing steam that drives a turbine connected to a generator. Nuclear fuel is non-renewable, but stations can provide reliable, large-scale output with low operational carbon dioxide emissions. Radioactive waste requires secure management, and construction, decommissioning and accident risks must be considered. Solar cells transfer energy from sunlight directly into electrical energy. They do not need a boiler, turbine or generator.

12:17They are renewable, produce no pollution during operation, and work on scales from small devices to solar farms. However, output depends on daylight and weather, and large installations require land and materials. Solar heating panels serve a different purpose. They absorb infrared and other electromagnetic waves from the Sun to heat water. They reduce the need for other water-heating fuels and have no fuel cost during operation. Their output varies with sunlight, so storage or backup heating may be needed.

12:46They do not generate electricity. Wind is produced partly because the Sun heats Earth’s surface unevenly, creating conditions that make air move. Moving air turns turbine blades connected to a generator. Wind is renewable, uses no fuel during operation, and can supply electricity through individual turbines or large wind farms. Output varies with wind conditions, while noise, landscape changes and wildlife impacts can be disadvantages. Compare resources using renewability, availability, reliability, scale and environmental impact.

13:19Renewable means naturally replenished, not necessarily available whenever electricity is needed. Predictable tidal output is still intermittent, while stored hydroelectric water can be released when required. No resource is best in every situation, so a good comparison links each advantage to local conditions. The following source relationships are Supplement content. Radiation from the Sun is the main source of energy for all our energy resources except geothermal, nuclear and tidal.

13:47Plants store energy received from sunlight, providing biofuels and the ancient material from which fossil fuels formed. Solar heating also drives winds and the water cycle supplying hydroelectric reservoirs. Winds transfer energy into most surface waves, connecting wave power to the Sun as well. Tidal energy instead comes from gravitational interactions involving Earth, the Moon and the Sun. Geothermal energy comes from Earth’s interior, while nuclear fuel supplies energy from atomic nuclei.

14:15These are not primarily resources powered by solar radiation. Energy is released by nuclear fusion in the Sun. In fusion, light nuclei join together. This is different from the fission used in existing nuclear power stations. Research is investigating how fusion energy could produce electrical energy on a large scale. You need this distinction, but not the detailed nuclear reactions for this subtopic.

Exam practice and recap

14:38An exam-style question asks you to calculate efficiency and useful power for this motor. Percentage efficiency equals 300 ÷ 500, multiplied by one hundred per cent, giving sixty per cent. Useful power equals 300 ÷ 4, giving seventy-five watts. The energy transferred to unwanted outputs equals 500 − 300, giving two hundred joules. If asked to explain a fossil-fuel power station, describe the sequence precisely. Fuel combustion transfers energy by heating to water in a boiler.

15:10Steam drives a turbine. The turbine drives a generator, which produces electrical output. Each sentence identifies a distinct stage rather than merely saying that fuel makes electricity. Watch for three especially common mistakes. Energy is not destroyed by friction. It is transferred mainly to internal stores. Power is not an amount of energy. It is the rate of energy transfer. Efficiency cannot exceed one hundred per cent, because useful output cannot exceed the total input in a complete energy account.

15:39The central idea is to follow the energy through the whole process. Identify the stores, describe the transfer pathways, and account for useful and unwanted outputs. Work measures energy transferred, power measures how quickly it is transferred, and efficiency measures the useful fraction. These connections explain everything from a braking bicycle to a power station.