Physics OCR Chapter 5 (Work, Energy, Power)

Work Done

Work Done (W) involves a force (F), and the distance moved in the direction (x) of that force

  • W = F x
  • J = Nm

Work Done only occurs when there is movement, energy is transferred when you do work.

Work Done = Energy Transferred

Work Done at an angle to motion

Force is often applied at an angle to motion

You work out the component of the force:   Work Done W = (Force CosØ) x X

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Conservation of Energy

Perpetual Motion = Impossible  --> Energy In/Stored = Energy Out/Stored

Energy can be converted from one form to another. However, energy cannot be destroyed. 

  • Total  Intitial Energy = Total Final Energy

Principle of Conservation of Energy: Total energy of a closed system remains constant: energy can never be created or destroyed but can be transferred from one form to another

** Potential = Stored Energy**

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Forms of Energy

Kinetic Energy - Energy due to motion of object with mass

GPE - Energy of object due to position in gravitational field

Chemical EnergyEnergy in bonds between atoms, released when atoms are rearranged 

Elastic PE Energy stored as a result of reversible change in shape

Electric PEEnergy of electrical charges due to position in electric field

Nuclear - Energy stored within nuclei of atoms

Radiant or EMF Energy in EM waves stored within oscillating electric and magnetic field

Sound - Energy of mechanical waves due to movement of atoms

Internal The sum of random potential and kinetic energies of atom in a system

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Kinetic Energy and GPE

Kinetic Energy is the energy of an object as a result of motion.

Energy K  = 1/2  x  Mass  x Velocity ^2

Gravitational Potential Energy is stored energy due to the position of the object in a gravitational field. GPE increases as objects gain height, and GPE decreases when they lose height.

Energy GPE = Mass x g x Height of free fall

Energy GPE = Work Done = Energy K

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Power and Efficiency

Energy and Power

Power is the rate at which energy is transferred,   P=W/t     P = power  W= Work Done   t= time

Measured in Joules per Second  or in Watts


Energy will be lost in other forms such as radiant energy

efficiency = useful energy / total energy x 100 

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