Summary
Summary Energy
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When energy is transferred to an object, the energy is stored in one of the object’s energy stores: Thermal energy store Chemical energy store
Kinetic energy store Magnetic energy store
Gravitational potential energy store Electrostatic energy store
Elastic potential energy store Nuclear energy store
Energy is transferred mechanically (by a force doing work), electrically (work done by moving
changes), by heating or radiation (e.g. sound or light)
A systems is just a single object or a group of objects. When a system changes, energy is transferred.
It can be transferred into or away from the system. Closed systems are systems that neither matter
nor energy can enter or leave. The net change in the total energy of a closed system is always zero.
Work done is another ways of saying energy is transferred. Work can be done when current flows or
by a force moving an object.
Anything that is moving has energy in its kinetic energy store. Energy is transferred to this store
when an object speeds up and is transferred away from its store when an object slows down. The
energy in the kinetic energy store depends on mass and speed. The greater the speed and mass, the
more energy there will be in the kinetic energy store.
Kinetic Energy (J) = ½ x Mass (kg) x speed 2 (m/s) Ek=½mv2
Lifting an object in a gravitational field requires work. This causes a transfer of energy to the
gravitational potential energy store of the raised object. The higher the object is lifted, the more
energy is transferred to this store. The amount of energy store depend on strength, height and mass
of the gravitational field the object is in. Ep=mgh
Gravitational Potential Energy = Height (m) x Mass (kg) x Gravitational Field Strength (N/kg)
When something falls, energy from its gravitational potential energy store is transferred to its kinetic
energy store. For a falling object when there is no air resistance: energy lost from g.p.e. store=
energy gained on the kinetic energy store.
Stretching or squashing an object can transfer energy to its elastic potential energy store. So long as
the limit of proportionality has not been exceeded energy in the elastic potential energy store of a
stretched spring can be found using: Ee= ½ke2
Elastic Potential Energy (J) = ½ x Spring Constant (N/m) x Extension 2 (m)
More energy needs to be transferred to the thermal energy store of some materials to increase their
temperature than others. Materials that need to gain a lot of energy to warm up also transfer when
cooling down. Specific heat capacity is the amount of energy needed to raise the temperature of 1kg
of a substance by 1°C. ∆E = mc∆ x temperature change
Change in Thermal Energy (J) = Mass (kg) x Specific Heat Capacity (J/kg°C) x Temperature Change (°C)
The conservation of energy principle is that energy is always conserve: energy can be transferred
usefully, stored or dissipated, but can never be created or destroyed. When energy is transferred,
not all energy is transferred usefully some is always dissipated (stored in a way that is not useful.)
Power is the rate of energy transfer, or the rate of doing work. A powerful machine is not necessarily
one which can exert a strong force but one which transfers a lot of energy in a short space of time.
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