Energy, Work & Power

Energy Stores & Transfers
  • Energy: A property of an object that can be stored or transferred.
    • Measured in joules (J).
    • Must be transferred to perform work or heat an object.
Systems in Physics
  • System Definition: An object or group of objects defined for observation in physics.
    • Range: From a small object (like an apple) to large systems (like the Universe).
    • In equilibrium, nothing changes and thus nothing happens.
    • Energy transfers occur when changes happen within the system.
Energy Stores
  • Energy is stored in various forms:
    • Kinetic Energy: Energy of moving objects.
    • Gravitational Potential Energy: Gained when an object is lifted through a gravitational field.
    • Elastic Potential Energy: Stored when objects are stretched, squashed, or bent.
    • Magnetic Energy: Energy stored when magnetic materials interact.
    • Electrostatic Energy: Energy stored in charged objects interacting (e.g., electrons and protons).
    • Chemical Energy: Energy transferred during chemical reactions.
    • Nuclear Energy: Released from atomic nuclei during reactions.
    • Thermal Energy: All objects have thermal energy, increasing with temperature.
Energy Transfers
  • Energy Transfer Pathways:
    • Mechanical: Through a force (e.g., pulling, pushing).
    • Electrical: By moving charges (current).
    • Heating: From a hot object to a colder one (e.g., conduction).
    • Radiation: Through electromagnetic waves (e.g., light).
Worked Examples

  • Battery powering a torch:

    • Initial store: Chemical (battery) -> Final store: Thermal (bulb).
    • Transfer pathway: Electrical.

  • Falling object:

    • Initial store: Gravitational Potential -> Final store: Kinetic.
    • Transfer pathway: Mechanical.
Kinetic Energy (KE)
  • Definition: Energy due to mass and speed of an object.
    • Given by equation: Ek=12mv2E_k = \frac{1}{2} mv^2
    • Where:
      • EkE_k = Kinetic energy (J)
      • mm = Mass (kg)
      • vv = Speed (m/s)
    • Doubling mass doubles KE; doubling speed quadruples KE.
Gravitational Potential Energy (GPE)
  • Definition: Energy due to height in a gravitational field.
  • Change calculated using: ΔEp=mgΔh\Delta E_p = mg\Delta h
    • Where:
    • ΔEp\Delta E_p = Change in GPE (J)
    • mm = Mass (kg)
    • gg = Gravitational field strength (N/kg)
    • Δh\Delta h = Change in height (m)
Conservation of Energy
  • Principle: Energy cannot be created or destroyed, just transferred.
    • Total energy in a closed system remains constant.
    • Wasted energy examples include heat loss during processes.
Work Done
  • Work is defined as: A force acting on an object over a distance.
    • Formula: W=FdW = Fd or ΔE\Delta E
    • Where:
      • WW = Work done (N∙m or J)
      • FF = Force (N)
      • dd = Distance (m)
Power
  • Definition: Rate of doing work or transferring energy.
    • Formula: P=WtP = \frac{W}{t} or P=ΔEtP = \frac{\Delta E}{t}
    • Where:
      • PP = Power (W)
      • WW = Work done (J)
      • ΔE\Delta E = Energy transferred (J)
      • tt = Time (s)
Efficiency
  • Definition: Ratio of useful energy output to total energy input, expressed as a percentage:
    • Efficiency = Useful Energy OutputTotal Energy Input×100%\frac{\text{Useful Energy Output}}{\text{Total Energy Input}} \times 100\%
Energy from the Sun
  • Solar Energy: Main source of energy on Earth, impacts wind, wave energy, and biomass.
  • Solar cells: Convert sunlight to electricity, advantages include renewability and reduced greenhouse gas emissions.
Wind and Water Energy
  • Wind turbines convert wind’s kinetic energy into electricity.
  • Wave and tidal energy harness ocean motions for electricity generation.
Fossil Fuels and Biofuels
  • Fossil fuels (coal, oil, gas) are non-renewable, though reliable.
  • Biofuels are renewable but have their own environmental issues.
Nuclear Energy
  • Comes from fission (splitting atoms) and fusion (joining atoms), with potential for large energy outputs but also waste management issues.