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