Energy-physics

• SI units for elastic potential energy: Energy (J), Spring Constant (N/m), Extension (m)
• SI units for gravity potential energy: Energy (J), Mass (kg), Gravitational Field Strength (N/kg), Height (m)
• SI units for kinetic energy: Energy (J), Mass (kg), Velocity (m/s)
• Units for Specific Heat Capacity: J/°C/kg (Joules/Degree Celsius/Kilogram)
• Unit of power: Watt (W)
• Two equations for power: power = energy transferred/time, power = work done/time
• Equation for elastic potential energy: ½ k x²
• Equation for gravity potential energy: mgh
• Definition of ‘Specific Heat Capacity’ of a substance: The amount of energy needed to raise the temperature of 1 kilogram of a substance by 1 degree Celsius
• Equation for kinetic energy: ½ m v²
• SI units for power: Energy (J), Work Done (J), Time (s)
• Definition of ‘Power’: The rate at which energy is transferred (or rate at which work is done)
• Two motors lift the same mass through the same height. Motor A does this in half the time of Motor B. Which dissipates the most power?

  Motor A dissipates the most power
• GPE


  Gravitational Potential Energy
• A bungee jumper slows down once the cord begins to stretch because

  • Kinetic energy decreases since it is converted to elastic potential energy
  • As KE decreases, velocity decreases since KE is proportional to (velocity)²
• Energy changes for a ball thrown upwards and then returns


  1. Upwards: KE is converted to GPE

  2. Peak: Maximum GPE, zero KE

  3. Downwards: GPE is converted to KE
• 4 different stores of energy

  • Kinetic Energy
  • Gravitational Potential Energy
  • Elastic Potential Energy
  • Chemical Energy
• Energy transferred is the same but the time taken is less (P=E/t)
• Examples of chemical energy stores

  • Food
  • Fuel (e.g. wood, coal, petrol)
  • Batteries
• Energy changes involved when a ball is thrown upwards and then returns to its starting position (Ignore air resistance)
• Energy transfers for a bungee jumper

  1. When falling, the GPE is converted to KE of jumper

  2. As the cord tightens, KE is converted and stored as Elastic Potential Energy (EPE)

  3. At lowest point, the jumper’s initial GPE equals the EPE stored in the cord
• KE


  Kinetic Energy
• Total energy of the system

  Remains constant due to the conservation of energy
• Waste energy


  The energy that is not used by the device for its desired purpose
• Law of energy conservation

  Energy cannot be created or destroyed (it can only be transferred into different forms)
• Energy changes in a filament light-bulb


  1. Electrical energy is transferred into light and heat energy

  2. Light is a useful energy form, heat is waste energy
• Material with high thermal conductivity


  The rate of energy transfer through the material is higher than for a material with a lower thermal conductivity
• Equations to calculate efficiency

  • Useful Output Energy Transfer/Total Input Energy Transfer
  • Useful Power Output/Total Power Output
• Ways to increase system efficiency

  • Reducing waste output (by lubrication, thermal insulation etc.)
  • Recycling waste output (e.g., recycling thermal waste energy as input energy)
• Improving the efficiency of a radiator

  Installing metal foil sheets behind the radiator to reflect the heat back into the room rather than it being absorbed into the walls
• Methods of reducing heat loss in a building

  • Double glazing
  • Loft and wall insulation
  • Thicker walls
• Double-glazed windows have a lower thermal conductivity than single-glazed windows
• Consequence for energy transfer of a material with a high thermal conductivity


  The rate of energy transfer through the material is higher than for a material with a lower thermal conductivity
• Key factors affecting the rate of cooling of a building

  • The thickness of the walls
  • The thermal conductivity of the walls
• Improving the efficiency of a mechanical machine with moving parts

  Lubricate any moving parts to reduce the friction and therefore energy loss due to heating
• Improving the efficiency of boiling water in a pan

  By placing a lid on the pan to reduce the heat loss from the top
• Disadvantages of using renewable energy resources to generate power

  • Output often determined by external factors (like wind speed), so supply is uncertain
  • Generating power through other means is often more efficient and economically beneficial
• Advantages of generating power using gas rather than coal

  • Flexible Generation: Gas power stations have short start-up times so can be switched on/off more readily
  • Lower emissions of carbon dioxide
• Advantages of fossil fuels as an energy resource

  • Reliable: Not dependent on external factors so can generate power anytime
  • Can produce large amounts of energy for a given quantity
  • Still relatively abundant, so cost-effective
• Disadvantages of nuclear power

  • Produces nuclear waste which is harmful to humans & must be safely stored for centuries
  • Non-Renewable energy source
  • Risk of nuclear accidents