Conservation of energy

• Energy is stored in different ways. Energy stored in food, fuel and batteries is often called chemical energy. Energy can also be stored in moving objects (kinetic energy), hot objects (thermal energy), in stretched, squashed or twisted materials (strain energy or elastic potential energy) and in objects in high positions (gravitational potential energy). Energy stored inside atoms is called atomic energy or nuclear energy.
• Energy transfers


  1. Energy can be transferred between different stores

  2. In photo A some of the kinetic energy stored in the moving bullet is transferred to the egg by forces

  3. Some of this energy is stored in the moving fragments of the egg, and some will heat up the egg

  4. When an electrical kettle is used to heat water, energy transferred to the kettle by electricity ends up as a store of thermal energy in the hot water

  5. As the hot water is at a higher temperature than the kettle and the surroundings, some energy is transferred to these things by heating

  6. Energy can also be transferred by light and sound
• Conservation of energy


  Energy cannot be created or destroyed. It can only be transferred from one store to another. This is called the law of conservation of energy. This means that the total energy transferred by a system is the same as the energy put into the system.
• The units for measuring energy are joules (J).
• Although energy is always conserved, it is not always transferred into forms that are useful. Think of the kettle-the energy stored in the hot water is useful but the energy stored in the kettle itself and in the surroundings is not.
• Energy diagrams


  • We represent energy stores and transfers using diagrams, such as diagram B
• Energy diagram


  • Diagram B - flow diagram showing the energy transfers when a car brakes
• The energy transferred by the braking force can increase the temperature of brakes so much that they glow red.
• 1000 J of energy is transferred to a kettle, 850 J ends up stored as thermal energy in the hot water.
• Energy transferred to the kettle

  850 J ends up as thermal energy in the hot water
• Energy transferred to the kettle

  Remaining energy is transferred to the kettle itself and its surroundings
• Sankey diagram


  • Shows the amount of energy transferred. The width of the arrows represents the amount of energy in joules.
• When a light bulb is switched on, most of the energy supplied to it by electricity is transferred to the surroundings by heating. This energy is dissipated (it spreads out) and cannot be used for other useful energy transfers-it is wasted.
• Most machines waste energy when they get hot. Whenever two moving parts touch each other friction causes them to heat up. The thermal energy stored in the hot machine is transferred to the surroundings by heating which dissipates the energy. This energy is wasted energy.
• Efficiency


  A way of describing how good a machine is at transferring energy into useful forms. The efficiency of a machine is given as a number between 0 and 1. The higher the number, the more efficient the machine.
• Efficiency can be calculated using the equation: efficiency = (useful energy transferred by the device) / (total energy supplied to the device)
• Oiling the chain on a bicycle makes pedalling it much easier.
• Pedalling a bicycle without oiling the chain

  Harder
• Pedalling a bicycle


  Wasted energy is transferred to the surroundings by heating
• Friction between moving parts can be reduced by lubrication. Oil or other liquids and sometimes even gases, can be used as lubricants.
• Thermal conductivity


  The rate at which energy is transferred through a material by heating
• Factors affecting rate of energy transfer by heating

  • Thickness of material
  • Thermal conductivity of material
  • Temperature difference across material
• Materials that contain air are good insulators because air has a very low thermal conductivity. When air is trapped it cannot form convection currents and so does not transfer much energy.
• Insulating materials


  • Straw bales
  • Bubble wrap
• Aerogel has an extremely low density and is 99.8% air, so it has an extremely low thermal conductivity and is an excellent insulator.
• Thermal conductivity of aerogel


  Lower than straw bales
• Cavity walls in houses have an air gap between the two layers which helps to insulate the house.
• Cavity wall vs single wall

  Cavity wall allows less energy to escape
• Vacuum flasks use a combination of different materials to reduce energy transfer by heating.
• Gravitational potential energy (GPE)


  Energy that is stored because of an object's position in a gravitational field. Any object that is above the surface of the Earth contains a store of gravitational potential energy.
• Factors affecting GPE


  • Mass of object
  • Strength of gravity
  • Distance object is moved upwards
• Equation for GPE


  GPE = m x g x Δh
• The value for gravitational field strength on Earth is approximately 10 N/kg.
• Gravitational field strength on the Moon

  About 1.6 N/kg, so GPE stored by an object lifted 1 metre above the Moon's surface is less than when lifted 1m on Earth
• One weight in the Big Ben clock tower has a mass of 2.5 tonnes.
• There are three heavy weights on steel cables inside the Big Ben clock tower. They are lifted up three times a week to store the energy needed to drive the clock and the bells
• Equation triangle


  Can help you change the subject of the equation
• Delta (Δ)


  The Greek letter delta and stands for change in
• Change in gravitational potential energy (ΔGPE)


  1. ΔGPE = m × g × Δh

  2. Where m = mass, g = gravitational field strength, Δh = change in vertical height
• The value for gravitational field strength on Earth is approximately 10N/kg