Energy

Energy is a fundamental quantity that every physical system possesses. Energy of physical system in a certain given state is defined as the amount of work W needed to change the state of the system from some initial state (called reference state or reference level) to the given state.

Transfer of energy

Work

Main article: mechanical work.

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Work is a defined as a path integral of force F over distance s:

Related Topics:
Path integral - Force

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: W = int mathbf{F} cdot mathrm{d}mathbf{s}

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The equation above says that the work (W) is equal to the integral of the dot product of the force (mathbf{F}) on a body and the infinitesimal of the body's position (mathbf{s}).

Related Topics:
Dot product - Force - Infinitesimal - Position

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Heat

Main article: Heat.

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Heat is an amount of energy which is usually linked with a change in temperature or in a change in phase of matter. In chemistry, heat is the amount of energy which is absorbed or released by a given chemical reaction.

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The relationship between heat and energy is similar to that between work and energy. Heat flows from areas of high temperature to areas of low temperature. All objects (matter) have a certain amount of internal energy that is related to the random motion of their atoms or molecules. This internal energy is directly proportional to the temperature of the object. When two bodies of different temperature come in to thermal contact, they will exchange internal energy until the temperature is equalised. The amount of energy transferred is the amount of heat exchanged. It is a common misconception to confuse heat with internal energy, but there is a difference: the change of the internal energy is the heat that flows from the surroundings into the system plus the work performed by the surroundings on the system. Heat Energy is transferred in three different ways: conduction, convection and/or radiation.

Related Topics:
Conduction - Convection - Radiation

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Conservation of energy

The first law of thermodynamics says that the total inflow of energy into a system must equal the total outflow of energy from the system, plus the change in the energy contained within the system. This law is used in all branches of physics, but frequently violated by quantum mechanics (see off shell). Noether's theorem relates the conservation of energy to the time invariance of physical laws.

Related Topics:
Thermodynamics - Off shell - Noether's theorem

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An example of the conversion and conservation of energy is a pendulum. At its highest points the kinetic energy is zero and the potential gravitational energy is at its maximum. At its lowest point the kinetic energy is at its maximum and is equal to the decrease of potential energy. If one unrealistically assumes that there is no friction, the energy will be conserved and the pendulum will continue swinging forever. (In practice, available energy is never perfectly conserved when a system changes state; otherwise, the creation of perpetual motion machines would be possible.)

Related Topics:
Pendulum - Friction - Perpetual motion

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Another example is a chemical explosion in which potential chemical energy is converted to kinetic energy and heat in a very short time.

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