Law of conservation of energy

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The Swinging Pendulum

You’ve seen a pendulum that swings back and forth. It is one of the simplest mechanical devices to demonstrate the Law of Conservation of Energy.

A pendulum swings from a high point at one end to another high point at the other end. In doing so it converts Potential Energy into Kinetic Energy and then back again into potential energy and so on and on. The motion only starts when the pendulum is raised at one end, giving it the required Initial potential energy. In other words, energy cannot be created from anything – it needs to be provided from another form of energy – the force of our hand lifting the pendulum, in this example.

When the pendulum is released, this potential energy converts into kinetic energy of motion until the force of gravitation slows down the pendulum at the other end. At this point, the kinetic energy has fully converted into potential energy, and the cycle repeats. In a frictionless environment, this motion goes on forever. In an environment with friction, such as on earth, the pendulum eventually slows down due to the loss of both these energies (potential and kinetic) and their transformation into sound and heat due to the friction with air particles.

This simple experiment demonstrates that one form of energy is always transformed into another form but is never created nor destroyed.

The Law of Conservation of Energy states that the energy in any system can neither be created nor destroyed. Instead, energy is either converted from one form to another or it is transferred from one system to another.

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It is the most important principle of physical sciences, which consequently gave us the First Law of Thermodynamics. The key message is that energy cannot be created from anything – you have to obtain it from somewhere. And this obtained energy is never destroyed – it is only converted to some other form, including heat. Some examples –

  • A gas turbine converts chemical energy in a fuel into thermal (heat) energy. The heat energy is then converted into mechanical energy to do some useful work, like powering an airplane. The total amount of energy from start to end of the working system remains the same.
  • Turning on a switch converts electrical energy into light energy – the light energy is not simply created out of nothing.
  • In any chemical reaction, the bonds between atoms in the reactants are broken and the atoms are bonded in a new way to create the resulting products. The destruction of bonds consumes energy while the creation of new bonds releases energy. However, there is no change in the total amount of energy as a result of a chemical reaction.

 In a cuckoo clock, the raised weights have potential energy. Their descent due to the force of gravity sets the pendulum in harmonic motion which powers the hands of the clock. When the raised weights reach the bottom, their potential energy is exhausted and the clock stops working.

Check Point

  1. A pendulum held high at one end possesses ______ energy.
  2. When the pendulum is released from a high end, ______ energy converts into ______ energy.
  3. Energy can neither be ______ not ______. It can only be _______ into another form.
  4. The first law of _______ is a version of the law of conservation of energy.
  5. When you flick a switch of a bulb, ______ energy converts into ______ energy.

Answer Key

  1. Potential
  2. Potential, Kinetic
  3. Created, Destroyed, Converted
  4. Thermodynamics
  5. Electrical, Light

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