Energy is a peculiar beast. It can appear in many different forms, but something quite remarkable occurs if we add up all the bits of energy before we do something and compare them to all the bits of energy after we do that something. The total energy before and after will be the same. This is known as the law of the conservation of Energy. In more formal language we would say:
The law of the conservation of Energy: Energy can neither be created nor destroyed, it can only be transferred from one type to another. In other words, the total energy in a closed system is constant. (btw a closed system simply means that there are no bits of energy sneakily escaping from that system) As mentioned, energy can occur in many forms: Light or electromagnetic radiation Energy (Energy of light photon, E = hf --> covered in Quantum Physics) Elastic Energy (energy stored in an extended material, E = 1/2kx^2) Gravitational Potential Energy (the energy an object has because of its position in a gravitaional field, E = mgh) Sound Energy (although often tempting, sound energy is rarely the answer to ''where did the rest of the energy go". Although we may have very noisy system, wasted energy is invariably in the form of heat) Thermal (or heat) energy (The energy gained by heating something up, E = mcT --> covered in Thermal Physics) Nuclear Energy (Energy stored in the nucleus of an atom, Einstein's very famous equation E=mc^2 tells us how mass in the nucleus can be converted to energy) Electrical Energy (energy given to charge, E = VQ --> covered in topic on Electricity) Chemical Energy (energy stored in the bonds of molecules) Kinetic Energy (energy of a moving object, E = 1/2mv^2) We are often interested in the transfer of energy from one form to another by way of an interaction or a force. For example, if you drop an object, the weight of the object accelerates the object downwards converting Gravitational Potential Energy into Kinetic Energy. When an object slides along the floor it will slow down and eventually stop moving as the kinetic energy is converted to thermal energy by the force of friction. This leads us to the idea that: Energy Transferred = Work Done (e..g in the last example of conversion of KE to thermal energy, the energy transferred from kinetic energy and to thermal energy is equal to the work done by the force of friction) Deriving equation for Gravitational Potential Energy Energy transferred = work done Work done on lifting a mass, m through a height, h is given by: Work = Force x distance = weight x height = mg x h The gravitational energy gained = work done against gravity So GPE = mgh Deriving equation for Kinetic Energy Workdone by force in accelerating an object from rest to speed, v, is given by W = Fd but F = ma => W = mad a =(v-u)/t (but in this case u=0 so a = v/t) d = 1/2(v+u)t = 1/2vt Therefore, W = m x v/t x 1/2 vt = 1/2 mv^2 Kinetic Energy gained = work done by force in accelerating car So KE = 1/2 mv^2 Comments are closed.
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