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Chapter 1 - Energy stores and transfers (Form 1-3)
Key points Checklist Energy store Energy transfer – different bodies and body within body Law of conversation of energy Energy calculations Energy can be described as being in different 'stores'. It cannot be created or destroyed but it can be transferred, dissipated 消消 or stored in different ways. 消消 Energy can be stored or transferred but it cannot be used up. For example, energy is stored in the chemical bonds of molecules in diesel oil and oxygen molecules in the air. This energy is transfered in combustion and allows the car to move and accelerate, or reach a certain distance. Where there are energy transfers in a system, the total energy associated with the system stays the same.消消 Energy is measured in joules (J). A store is not a physical place, and energy is not a physical substance. The idea of an energy store is to allow calculations to be made and to state how many joules are available for an action to happen. Each of the stores has an equation associated with
it. Potential energy=mgh, kinetic energy=1/2mv
Part A: Energy stores
There are many energy stores which are used to keep track of energy in a system. Energy store Description Examples Form 1-3 GCSE Food Fuel/ gasoline Chemical The energy store associated with chemical bonds, such as those between molecules. Food, muscles, electrical cells. Potential Gravitatio nal The energy associated with an object at height. Aeroplanes, kites, mugs on a table. Elastic The energy stored when an object is stretched, squashed or twisted. compressed springs, inflated balloons. Nuclear The energy associated with nuclear interactions. Nuclear fusion of hydrogen and helium in the Sun, Uranium- 245 undergoing fission. Kinetic Mechanica l The energy associated with a moving object. Runner, car Heat Thermal In most cases this is the vibrations of the particles in the object. In hotter objects, the particles vibrate faster. The human bodies, hot coffees, stoves or hobs. Also: ice particles vibrate slower, but still have energy. Electricity Electrostat ic The energy stored when like charges are moved closer together Thunderclouds, Robbed ruler
Fuel Chemical Car mechanical Electricity supply Electrical Bulb light Electricity supply electrical Microwave heat Earth Potential Apple Kinetic GCSE System Body 1 Body 2 Body 3 Body within body Iron Heat Electron Kinetic energy Three bodies Bunsen burner Chemical energy Water Heat energy Particle Kinetic energy Energy can transfer from one form to several forms during the process During the process of energy transfer, some energy may be wasted
Energy transfer from one body to another body (From body 1 to body 2 and body
Energy transfer within a body Energy can transfer from one form to several forms during the process During the process of energy transfer, some energy may be wasted Doing 'work'消消 is the scientific way of saying that energy has been transferred. For example, a grazing cow, a firing catapult and a boiling kettle are all doing 'work' because energy is being transferred from one store to another.
Part C: The conservation of energy
Energy cannot be created or destroyed, so it is always conserved. This means that energy is regularly transferred from one store to another, although sometimes it is considered to be 'wasted' when it is transferred to a store that is not useful. All the energy in the Universe was present at the Big Bang and will still be around at the very end of time. Examples of conservation of energy The skydiver When a skydiver jumps out of a plane, the following energy transfers take place:
- the skydiver's gravitational potential energy decreases as his altitude decreases
- his kinetic energy store increases as his speed increases
For a moving object: kinetic energy (J) = 0.5 × mass (kg) × (speed)2 (m/s) Example A racecar has a mass of 500 kg. Calculate its kinetic energy when it travels at 60 m/s. K.E. = 1/2 mv^2 = 0.5 × 500 × 602 = 250 × 3,600= 900,000 J This is 900 kJ or 0.9 MJ. A stretched spring For a stretched spring: energy transferred in stretching (J) = 0.5 × spring constant (N/m) × (extension)2 (m) Example A spring is stretched elastically by 10 cm. Calculate the energy transferred if its spring constant is 200 N/m. 10 cm = 10/100 = 0.10 m energy transferred in stretching = 0.5 × spring constant × (extension) = 0.5 × 200 × 0. = 100 × 0. = 1.0 J
KE =
1/2mv^2 By the law of conversation of energy PE=KE (assume 100% energy transfer) mgh=1/2mv
PES =
1/2kd^2
