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P1: Conservation and dissipation of energy (1.3: Energy and work (To make…
P1: Conservation and dissipation of energy
1.1: Changes in energy stores
Energy can be stored in different ways
Chemical energy stores
Fuels
Foods
Chemicals found in batteries
Kinetic energy stores describe the energy an object has because it is moving
Gravitational potential energy stores describe the energy an object has because of its position relative to the ground
Elastic potential energy stores describe the energy an object has because it is being stretched or squashed
Thermal energy stores describe the energy an object has because of its temperature
Energy can be transferred from one store to another
Energy can be transferred by
Heating
Waves
An electric current
A force moving an object
1.2: Conservation of energy
Energy cannot be created or destroyed - this is called conservation of energy and it applies to all energy changes
Energy can be transferred usefully, stored, or dissipated
A closed system is an object or group of objects in which no energy transfers take place out of or into the energy stores of the system
However, as changes occur in a closed system, energy can be transferred to different stores within the closed system
1.3: Energy and work
To make a stationary object move, you need to apply a force to the object
When a force moves an object, energy is transferred to the object and work is done on it
When work is done to move an object, energy supplied to the object is equal to the work done in moving the object
Both work and energy have the unit Joule (J)
The work done on an object is calculated using the equation: work done (J) = force (N) x distance moved (M)
If an object does not move when a force is applied to it, no work is done on the object
Friction is the force that opposes the motion of 2 surfaces in contact with each other
Work done to overcome friction is mainly transferred to thermal energy stores by heating
1.4: Gravitational potential energy stores
Gravitational potential energy is energy associated with an object because of its position in the earth's gravitational field
Whenever an object is moved upwards, the energy in its GPE store increases -this increase is equal to the work done on the object by the lifting force
Whenever an object is moved downwards, the energy in its GPE store decreases - this decrease is equal to the work done by the gravitational force acting on it
Change in GPE (J) = mass (Kg) x gravitational field strength (N/Kg) x change in height (M)
The gravitational field strength at the surface of the earth is 9.8 N/Kg - the gravitational field strength at the surface of the moon is about one sixth of this
1.5: Kinetic energy and elastic energy stores
Kinetic energy
All moving objects have kinetic energy - the greater the mass and speed of an object, the more kinetic energy it has
Kinetic energy (J) = 0.5 x Mass (Kg) x velocity squared (M/S)
Elastic potential energy
Elastic potential energy is the energy stored in an elastic object that has been stretched or squashed
An object is described as being elastic if it regains its original shape after being stretched or squashed
When an elastic object is stretched or squashed, the work done on it is stored as elastic potential energy
When the object returns to its original shape, this energy becomes available for other transfers
Elastic potential energy (J) = 0.5 x spring constant (N/M) x extension squared (M)
1.6: Energy dissipation
Machines transfer energy for a purpose
Useful energy is energy transferred to where it is wanted in the form that it is wanted - wasted energy is energy that is not usefully transferred
Both the useful and wasted energy will eventually be transferred to the surroundings, which will warm up - the energy is dissipated
As the energy spreads out it becomes more difficult to use for further energy transfers
Energy is often wasted because of friction between the moving parts of a machine - however sometimes friction may be useful
1.7: Energy and efficiency
The energy supplied to a machine is called input energy
The energy usefully transferred by the machine is called the useful output energy
From the principle of conservation of energy
Input energy = useful output energy + energy wasted
The less energy wasted by a machine, the more efficient it is
Efficiency = useful energy transferred/total energy supplied x 100
Efficiency is a ratio, so it doesn't have a unit
No device can have an efficiency that is greater than 1 (100%)
Different machines waste energy in different ways
Including
Friction
Air resistance
Electrical resistance
1.8: Electrical appliances
Energy in the home is mostly supplied by electricity, gas, and oil - electrical appliances are extremely useful because they transfer energy at the flick of a switch
Common electrical appliances include
Appliance
Useful transfer
Light bulb
Light waves emitted from filament
Electric mixer
Work done by the blades of the mixer
Speaker
Sound waves from vibrations of speaker cone
Television
Light waves and sound waves
Many electrical appliances transfer energy by heating - this could be useful (e.g. a kettle) however for many other appliances, energy is wasted by heating
Appliances should be designed to waste as little energy as possible in order to make them as efficient as possible
1.9: Energy and power
The power of an appliance is the rate at which it transfers energy
Unit of power is the watt (W) - 1W = 1 J/S
Often a watt is too small a unit to be useful, so power is given in kilowatts (KW) - 1KW = 1000 W
Power (W) = energy transferred (J) / time (S)
Power wasted = total power supplied to device - useful power output from device
Efficiency = useful power output (W) / total power input (W) x 100