Chapter P1: Conservation and Dissipation of Energy
- Created by: andre
- Created on: 30-10-17 15:53
1.1 Changes in Energy Stores
Chemical energy stores include:
- Fules
- Foods
- The chemicals found in batteries
kinetic energy stores describe the energy the object an object has because it's moving.
Gravitational Potential Energy (GPE) 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 streched or squashed.
Thermal Energy stores describe the energy an object has because of its tempreture.
energy can be transferred from one store to another.
energy can be transferred by heating, waves, an electric current or when a force moves an object.
1.2 Conservation of Energy
Energy cannot be created or destroyed. this is called the peinciples of conservation of energy and it applies to all energy changes.
Energy can be transferred usefully, stored or dissapated.
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 occurs in a closec system, energy can be transferred to different stores within the closed system. For example:
- when an object falls freely, the GPE store of the object decresases and the kinetic energy store of the object increases.
- when a person streches an elastic band, the chemical energy store of the persons muscles decreases and the elasctic potential energy store of the elastic band increases.
- when a pendulum swings towatds the middle, the GPE store of the pendulum decreases and its kinetic energy store increases. As the pendulum moves away from the middle, its kinetic energy store decreases and its GPE store increases.
*make sure you can state the principles of conservation of energy.
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 in moving the object.
Both work and energy have the unit joule, J.
The work done on the object is calculated using the equation W = F s where:
- W is the work done in joules, J
- F is the force in newtons, N
- s is the distance moved in the direstion of the force in metres 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 two surfaces in contact with each other.
Work to overcome friction is mainly transferred to thermal energy stores by heating.
1.4 Gravitational Potential Energy (GPE) stores
GPE is energy associated with an object because of its position in the Earths gravitational field.
Whenever an object is moved upwards, the energy in its GPE stores 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 stores decreaeses.This decrease is equal to the work done by the gravitational force acting on it.
When an object os lifted or lowered, the change in its GPE is E = m x g x h, where:
- E is the change in the GPE in joules, J
- m is the object's mass in kilograms, kg
- g is the gravitational field strength in newtons per kilogram, N/kg
- h is the change in hight in metes m.
The gravitational field strength ath the surface of the Earth is 9.8N/kg (although you can round this number up to 10 N/kg). The gravirtational field strength at the surface of the moon is about 1/6 (one sixth) of this value.
1.5 Kinetic Energy stores
All moving objects have kinetic energy. The greater the mass and the speed of an object, the more kinetic energy it has.
Kinetic energy can me calculated using the equation Ek = 1/2 x m x V² Where:
- Ek is the kinetic energy in joules, J
- m is the mass of the object in kilograms, kg
- V is the speed of the object in metres per second, m/s
1.5 Elastic Energy Stores
Elastic potential energy is the energy stored in an elastic object that has been streched or squashed.
- An object is decribed as being elastic if it regains its original shape after being streched or squashed.
- The elastic potential energy of a spring can be calculated using the equation Ee= 1/2 x K x e² where:
Ee is the elastic potential energy in joules, J
K is the spring constant of the spring in newtons per metre, N/m
e is the extention of tyhe spring in metres, m.
1.6 Energy Dissipation
Useful energy is the energy transferred to where it is wanted in the form that is wanted. Wasted energy is energy that is not usefully transferred.
both useful energy and the 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 dificult for further energy transferres.
energy is often wasted because of friction between the moving parts of a machine. sometimes friction may be useful - for example, in the brakes of a bicycle or a car. The kinetic energy store of the vehicle decrases \nd the thermal energy stores increase as well as its surroundings.
1.7 Energy and Efficiency
The energy supplied to a device is called the input energy.
The energy that is transferred usefully by tha macheine is called the useful output energy.
from the principle of conservation of energy :
- input energy = useful output energy + energy wasted
The less energy that is wasted by the macheine, the more efficient the macheine gets.
The efficency of a device can be calculated using the equation:
- useful energy transferred by the device
- efficiency = ----------------------------------------------------- x100
- total energy supplied to the device
efficiency is a ratio so it dosen't have a unit.
No device can have an efficiency that is greater than 1 (or %100).
1.8 Electrical appliances
Energy in the home
Energy in the home is mostly supplied by electricity, gas and oil. Electrical appliances are extremerny useful because they transfer energy at the flick of a switch.
Many electrical appliances transfer energy by heating. This may be a useful transfer, for example a kettle, but for many other appliances energy is wasted by heating.
Aooliances 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 transferres energy.
The unit of power is watt, W. 1 watt is equal to the rate of tranferring 1 joule of energy in 1 second. (i.e. 1 W = J/s)
Often a watt is too small to be a unit to be useful, so power is given in kilowatts (kW). 1kW = 1000W.
power can be calculated using the equation P = E/t where:
P is the power in watts, W
E is the energy transferred in joules, J
t is the time taken for the transfer in seconds, s
power wasted = total power supplied to the device -- useful power output from device
the efficiency equation can be written in terms of power
efficiency = useful power output /(divided)/ total power input (x100)
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