P1- Physics- P2.1 - 2.4 (Using Energy)

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  • Created by: Em_New99
  • Created on: 19-05-15 17:33

Forms of Energy

Key Points

  • Energy exists in different forms
  • Energy can change from one form into another form
  • When an object falls and gains speed, its gravitational potential energy decreases and its kinetic energy increases

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Different types of energy:

  • Chemical Energy- Stored in fuel (food). Energy is released when chemical reactions take place
  • Kinetic Energy- Energy of a moving object
  • Gravitational Potential Energy- Anything that can fall
  • Elastic Potential Energy- Stored in a springy object, ie- Springs, elastic and rubber bands
  • Electrical energy- Transfered by a current
  • Light Energy- From the sum and light bulbs

ENERGY CAN BE TRANSFERRED USEFULLY FROM ONE FORM TO ANOTHER, STORED OR DISSIPATED- BUT CAN NEVER BE CREATED OR DESTROYED

Energy is only useful when it can be convertedfrom one form to another

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Conservation of Energy

Key Points

  • Conservation of energy applies to all energy changes
  • Energy cannot be created or destroyed- This is called 'Conservation of energy

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Exam Tip!

Never use the term 'movement energy' in the exam; you will only gain marks for 'kinetic energy'.

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Useful Energy

Key Points

  • Useful energy is energy in the place we want it and in the form we need it
  • Wasted energy is energy that is not useful energy
  • Useful energy and wasted energy both end up being transferred to the surroundings, which become warmer
  • As energy spreads out it gets more and more difficult to use for further energy transfers

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Energy and Efficiency

Key Points

  • The efficency of a device = Useful energy / total energy supplied = answer x100
  • No machine can be more than 100% efficient
  • Measures to make machines more efficient include reducing friction, air resistance, electrical resistance and noise due to vibrations

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Weight is measured in Newtons (N). The weight of a 1kg object of earth is about 10N

Energy is measured in Joules (J). The energy need to lift a weight of 1N by 1 metre is 1J

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Exam Tip!

  • The greater the percentage of energy that is usefully transferred in a device, the more efficient the device is
  • Efficiency and percentage efficiency are numbers without units. The maximum efficiency is 1 or 100%, so if the calculation produces a number greater than this is must be wrong.
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Energy and Efficiency (Continued)

Why machines waste energy- How to reduce this problem

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  • Friction between the moving parts causes friction- Lubricate the parts to reduce friction
  • Sound created by machine- Cut out noise (Secure loose parts)

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Exam Tip!

The greater the percentage of energy that is usefully transferred in a device, the more efficient the device is

Efficiency and percentage are numbers without units. The maximum efficiency is 1 or 100%, so if a calculation produces a number greater than this then it must be wrong

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Wasted Energy

We call it 'Wasted heat' because we cant do anything useful with it

  • Useful energy is concentrated energy. The entire energy output of a machine will eventually become heat energy
  • Heat is transferred to cooler areas which will become warmer. As energy is transferred it becomes less concetrated
  • The total amount of energy stays the same . The energy is still there but as it spreads out it cannot be used or collected again.

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Sometimes 'Waste' energy can be useful

  • Heat exchangers reduce the amount of heat energy lost
  • They do this by pumping cool fluid through escaping heat
  • The temprature of the fluid rises and it gains heat energy
  • The heat energy in the fluid can be converted back into a useful form
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Energy Transformation Diagrams

The thickness of the arrow represents the amount of energy

  • Sankey diagrams are made to make it easier to see how much of the total energy is being usefully employed compared to wasted

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The Cost of Electricty

  •  The amount of energy transferred by an appliance depends on its power (how fast it can transfer it) and how long its on.       Energy= Power X Time  
  • Energy is measure in Joules (J)- 1J is the amount of energy transferred by a 1W applicance in 1s
  • Power is usually measured in Watts (W) or Kilowatts (kW)- A 5kW appliance transfers 5000J in 1s

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Two formulas for calculatubg the cost of electricity

  • Units (kWh)= Power (kW) x Time (Hours)
  • Cost= Units x Price
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