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Energy Energy Wednesday, 24 May 2017 • Objective: – Identify the different forms of energy – Explain work and power COPY Energy Energy is the capacity to do work Feynmann Definition 'There is a fact, or if you wish a law, governing all natural phenomena that are known to date. There is no exception to this law - it is exact so far as is known. The law is called conservation of energy. It says that there is a certain quantity, which we call energy, that does not change in the manifold changes which nature undergoes. That is a most abstract idea, because it is a mathematical principle; it says that there is a numerical quantity, which does not change when something happens. It is not a description of a mechanism, or anything concrete; it is just a strange fact that we can calculate some number and when we finish watching nature go through her tricks and calculate the number again, it is the same.' – (R.P. Feynman, Tbe Feynman Lectures on Physics Volumes 1-3) In other words… • Energy is not a physically measurable quantity but rather a mathematical idea. COPY Energy • There are two types of energy forms. One is active energy, the other is potential energy. • Active energy is energy that has effects that are easily seen or detected. • Potential energy is energy that is stored and only have effect when released. COPY Types of energy Active energy Potential energy Kinetic Energy Chemical potential energy Solar energy Elastic potential energy Electrical energy Gravitational potential energy Sound energy Nuclear energy COPY Some meanings… • Kinetic energy – The energy of an object when it is moving • Chemical potential energy – Energy released by chemical reactions, eg from food, fuels and explosives COPY More meanings… • Elastic potential energy – Energy stored when a spring or rubber band is stretched or compressed COPY More meanings… • Gravitational potential energy – The energy an object gains when it is lifted up. This energy is released when it falls down again. COPY More meanings… • Nuclear Energy – Energy released in nuclear reactors or atomic bomb explosions. Solar energy is also produced from nuclear energy in the sun. COPY Measuring Energy • Energy is measured in Joules (J), or Kilojoules (kJ). 1000J= 1kJ Kinetic e.g. moving objects Kinetic Sound e.g. music Kinetic Chemical Potential e.g. batteries, food Potential Heat e.g. hot stuff Kinetic Light e.g. traffic lights Kinetic Elastic Potential e.g. spring Potential Nuclear Potential e.g. atomic bomb, inside the sun Gravitational Potential e.g. object held high Potential Electric Potential e.g. electricity Potential Magnetic Potential e.g. magnets Potential COPY Calculating Energy • Ep=mgh • Gravitational potential energy = mass x gravity x height • Ek=1/2mv2 • Kinetic energy = 1/2 x mass x velocity squared COPY Energy Conservation Energy cannot be created or destroyed. It can only be changed from one form to another For example… • When climbing on a diving board… – You use your chemical potential energy and turn it into kinetic energy as you move – The kinetic energy become gravitational potential energy when you are off the ground – When you dive your gravitational potential energy is transformed into kinetic energy again. – The kinetic energy is turned into heat and sound. Energy Interactive Cloze COPY Work • Work is done when a force moves an object. • If work is done to an object then the object gains energy. The energy gained equals the work done. • So… Both work and energy are measured in joules COPY Calculating work done • Work = Force x Distance • W=Fd • Where – W is the work in Joules – F is the force in Newtons which moves the object – d is the distance the object moves in metres COPY Calculating work W F d Write the formula for… • W= • F= • d= COPY An example question • A toy car is pushed 2 m. The work done on the car is 800J. Calculate the size of the force which moved it. – d= 2m – W=800J – F=? • F=W/d =800/2 =400N Power • Power is the rate at which work is done. • Power is measured in watts, W Power • Power is calculated using: Power= Work done Time taken P=W/t Sample Problem (power) • A runner can do 12000J of work in one minute. What is the power output? P=W/t W= 12000J t=60s P=? P=12000/60 P=200W