Work and Energy PDF

# Work and Energy ## Objectives - To define work done. - To know the unit of work and energy. - To know that energy can be held in different energy stores. ## Work In everyday language, work might be writing an essay or digging the garden. But to scientists and engineers, work has a precise meani...

# Work and Energy ## Objectives - To define work done. - To know the unit of work and energy. - To know that energy can be held in different energy stores. ## Work In everyday language, work might be writing an essay or digging the garden. But to scientists and engineers, work has a precise meaning: work is done whenever a force makes something move. The greater the force and the greater the distance moved, the more work is done. The SI unit of work is the joule (J): 1 joule of work is done when a force of 1 newton (N) moves an object 1 metre in the direction of the force. Work is calculated using this equation: $Work Done = Force \times Distance Moved$ For example, if a 4 N force moves an object 3 m, the work done is 12 J. ## Energy A compressed spring has energy; so does a tankful of petrol. Both can be used to do work. Like work, energy is measured in joules (J). Although people talk about energy being stored or released, energy isn't a "thing". If, for example, a compressed spring stores 100 joules of energy, this is just a measurement of how much work could be done by the spring if the energy were released. Energy can be stored in different ways. These are described on the opposite page. To understand them, you need to know the following: - Moving objects store energy. (For example, a moving ball can do work by knocking something over.) - Materials are made up of particles (atoms and molecules) that are constantly in motion. For example, in a solid, the particles are vibrating. If the solid is heated, its temperature rises, and the particles move faster. ## Energy Stores It sometimes helps to add a label to the word energy to describe how it is being stored. Here are the names used to describe the different **energy stores**: - **Kinetic energy** This is energy stored by an object because of its motion. "Kinetic" means "moving". - **Gravitational potential energy** This is energy stored by objects lifted upwards against the force of gravity. It is released when they fall downwards. - **Elastic (strain) energy** A stretched rubber band stores energy, so does a compressed spring. - **Chemical energy** This describes energy stored in the chemical bonds between atoms. Fuels store energy in this way, so do batteries and foods. It can be released by chemical reactions - a fuel burning, for example. - **Electrostatic energy** If electric charges attract each other but are held apart, they store energy. For more on the forces between charges, see spread 8.1. - **Nuclear energy** An atom has a nucleus at its centre. This is made up of particles bound together by strong forces. In some atoms, if the particles become rearranged or the nucleus splits, stored energy is released. - **Thermal energy** In all materials, the particles are moving (see opposite page), so they store energy. When a hot object cools, its particles slow down, so some of the stored energy is released - people commonly call it heat. Thermal energy is related to **internal energy**. For more on this, see spread 5.1. - **Magnetic energy** If two magnets attract each other but are held apart, they store energy. For more on the forces between magnetic poles, see spread 9.1. In many of the above, objects or particles store energy because of their **position**. The general name for this is **potential energy**. ## Energy Pathways Everywhere around us, things are rising, falling, speeding up, slowing down, heating, cooling, charging, discharging, burning... In other words, energy is being transferred (shifted) from one store to another. Here are the four pathways it can take: - Mechanically, by a force moving something. - Electrically, by a current. - By heating because of a temperature difference. - By radiation such as light waves and sound waves. ## Work and Energy Transferred In the diagram on the right, a brick is dropped. As it falls to the ground, it loses height and speeds up. Before it falls, it has 20 J of energy in its gravitational potential energy store. When it is about to strike the ground, all 20 J has been transferred to its kinetic store (assuming no air resistance). Looked at in another way, 20 J of work has been done by the force of gravity. Whenever work is done: $Work Done = Energy Transferred$ ## Units - Mass is measured in kilograms (kg). - Force is measured in newtons (N). - Weight is a force. - Work is measured in joules (J). - Energy is measured in joules (J). ## Useful Equations - $Average Speed = {Distance Moved\over Time Taken}$ - $Acceleration = {Gain In Speed \over Time Taken}$ - $Force = Mass \times Acceleration$ - $Weight = Mass \times g$ (g = 10 N/kg) - $Work Done = Force \times Distance Moved$ - $Work Done = Energy Transferred$ ## Efficiency and Power ## Objectives - To explain what efficiency is. - To know and be able to use, the equation for calculating power. ## Force, Work, and Energy Essentials - Work is measured in joules (J). - Energy is measured in joules (J). - $Work Done = Energy Transferred$ - Force is measured in newtons (N). - $Work Done = Force \times Distance Moved$ ## Engines and Motors Engines and motors do work by making things move. Petrol and diesel engines need the energy stored in their fuel. Electric motors rely on the energy transferred from a battery or generator. The human body is also a form of engine. It needs the energy stored in food. ## Efficiency An engine does useful work with some of the energy supplied to it, but the rest is wasted as thermal energy (heat). The efficiency of an engine can be calculated as follows: $Efficiency = {Useful Work Done \over Total Energy Input}$ or $Efficiency = {Useful Energy Output \over Total Energy Input}$ For example, if a petrol engine does 25 J of useful work for every 100 J of energy supplied to it, then its efficiency is 1/4, or 25%. In other words, its useful energy output is 1/4 of its total energy input. ## Inputs and Outputs In any system, the total energy output must equal the total energy input. That follows from the law of conservation of energy. Therefore, the equations on the right could also be written with "total energy output" replacing "total energy input". ## Power A small engine can do just as much work as a big engine, but it takes longer to do it. The big engine can do work at a faster rate. The rate at which work is done is called the **power**. The SI unit of power is the **watt (W)** A power of 1 watt means that work is being done (or energy transferred) at the rate of 1 joule per second. Power can be calculated as follows: $Power = {Work Done \over Time Taken}$ or $Power = {Energy Transferred \over Time Taken}$ ## The Horsepower (hp) The horsepower (hp) is a power unit which dates back to the days of the early steam engines: 1 hp ≈ 746 W (about 3/4 kilowatt). ## Diagram of Energy Transfers The diagrams in your text show the transfer of energy from fuel to electricity in a power plant. The steps look like this: Fuel -> Fuel Burners or Nuclear Reactor -> Thermal Energy -> Steam -> Boiler -> Turbines -> Rotation -> Generator -> Electricity You can also think about energy transferring from a stretched rubber band to a ball: Elastic energy -> Kinetic energy -> Thermal energy (when the ball hits the ground)