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: E_k = \frac{1}{2} mv^2
  • Where:
    • E_k = Kinetic energy (J)
    • m = Mass (kg)
    • v = 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: \Delta E_p = mg\Delta h
  • Where:
  • \Delta E_p = Change in GPE (J)
  • m = Mass (kg)
  • g = Gravitational field strength (N/kg)
  • \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 = Fd or \Delta E
  • Where:
    • W = Work done (N∙m or J)
    • F = Force (N)
    • d = Distance (m)

Power

• Definition: Rate of doing work or transferring energy.
  • Formula: P = \frac{W}{t} or P = \frac{\Delta E}{t}
  • Where:
    • P = Power (W)
    • W = Work done (J)
    • \Delta E = Energy transferred (J)
    • t = Time (s)

Efficiency

• Definition: Ratio of useful energy output to total energy input, expressed as a percentage:
  • Efficiency = \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.