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P3 (Energy transferred by heating (Def of conduction
The transfer of heat…
P3
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Conservation Of Energy
Conservation of energy
Energy is always conserved – the total amount of energy present stays the same before and after any changes.
The pendulum
The pendulum shows the principal of conservation of energy in action. Gravitational potential energy is converted to kinetic energy and back, over and over again, as the pendulum swings
The diagram shows a pendulum in three positions - the two the ends of it's swing and as it passes through the middle point.
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Pendulum
When the pendulum bob is at the start of its swing it has no kinetic energy
because it is not moving, but its gravitational potential energy (GPE)gravitational
potential energy: The energy stored by an object lifted
up against the force of gravity. is at a maximum,
because it is at the highest point.
As the bob swings downwards it loses height. So its gravitational potential energy (GPE)
decreases. The work done on the bob by the gravitational force (weight) pulling it downwards increases its kinetic energy. The loss of GPE = the gain in KE.
At the bottom of its swing, the bob’s kinetic energy is at a maximum
and its gravitational potential energy is at a minimum -
because it is at its lowest point.
At the very top of its swing it stops for a moment. It once again has no kinetic
energy, but its gravitational potential energy is at a maximum.
As the bob swings upwards it slows down. Its kinetic energy decreases
as work is done against its weight. As it gains height the gravitational
potential energy increases again.
At all points during the swing, the total (GPE + KE) is constant.
Note that in a real pendulum the bob’s swing will become slightly lower
with each swing, because some energy is lost (dissipated, ‘wasted’)
through heating, due to air resistance.
Energy Transfers
Sankey diagrams
Sankey diagrams summarise all the energy transfers taking place in a process. The thicker the line or arrow, the greater the amount of energy involved. The Sankey diagram for an electric lamp below shows that most of the electrical energy is transferred as heat rather than light.
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Efficiency
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'Wasted' energy
Energy cannot be created or destroyed. It can only be transferred from one form to another or moved. Energy that is 'wasted', like the heat energy from an electric lamp, does not disappear. Instead, it is transferred into the surroundings and spreads out so much that it becomes very difficult to do anything useful with it.
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This means that 10% of the electrical energy supplied is transferred
as light energy (90% is transferred as heat energy).
The efficiency of the energy-saving lamp is (75 ÷ 100) × 100 = 75%. This means that
75% of the electrical energy
supplied is transferred
as light energy (25% is transferred as heat energy).
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