Describe how energy is transferred between stores during events and processes, including examples of transfer by forces (mechanical work done), electrical currents (electrical work done), heating, and by electromagnetic, sound and other waves

1.7 Energy – Transfer Between Stores

In the Cambridge IGCSE (0625) syllabus an energy store is a form in which energy can be held. During any event or process energy is transferred from one store to another by a specific mechanism. The six mechanisms required for the exam are:

• Mechanical work (forces)
• Electrical work (currents)
• Heating (temperature difference)
• Electromagnetic waves
• Sound (acoustic) waves
• Other mechanical waves

Energy Stores (as listed in the syllabus)

Useful Auxiliary Formulas

• Current: \(I=\dfrac{Q}{t}\) (A)
• Ohm’s law: \(I=\dfrac{V}{R}\) (V = voltage, R = resistance)
• Power (general): \(P=\dfrac{E}{t}\) (W)
• Intensity of a wave: \(I=\dfrac{P}{A}\) (W m\(^{-2}\)) (energy per unit time per unit area)

1. Mechanical work (forces)

• Definition: Energy transferred when a force acts through a distance.
• Work: \(W = Fd\cos\theta\) (J)
• Power: \(P = \dfrac{W}{t}=Fv\) (W)
• Typical stores involved: chemical ↔ kinetic ↔ gravitational ↔ elastic
• IGCSE examples

  • Lifting a book: chemical energy in muscles → gravitational potential energy.
  • Compressing a spring: mechanical work → elastic potential energy.
  • Pushing a sled on level ground: chemical energy → kinetic energy of the sled.

2. Electrical work (currents)

• Definition: Energy transferred when a charge moves through a potential difference.
• Work: \(W = VQ\) (J)
• Power: \(P = VI = I^{2}R = \dfrac{V^{2}}{R}\) (W)
• Typical stores involved: chemical ↔ electrical ↔ thermal ↔ radiant
• IGCSE examples

  • Battery powering a torch: chemical → electrical → light (radiant) + heat (thermal).
  • Electric heater: electrical → thermal.
  • Solar panel (photovoltaic): radiant → electrical.

3. Heating (temperature difference)

• Definition: Energy transferred as heat from a hotter body to a cooler one.
• Heat transferred: \(Q = mc\Delta T\) (J)
• Power of heat flow: \(P = \dfrac{Q}{t}\) (W)
• Typical stores involved: thermal ↔ thermal (or thermal ↔ other stores when a phase change occurs)
• IGCSE examples

  • Boiling water on a stove: electrical/chemical → thermal energy of water.
  • Conduction through a metal rod: thermal energy moves from hot end to cold end.
  • Melting ice: thermal energy → latent heat (change of state).

4. Electromagnetic waves

• Definition: Energy carried by oscillating electric and magnetic fields; can travel through vacuum.
• Intensity (energy flux): \(I = \dfrac{P}{A}\) (W m\(^{-2}\))
• Typical stores involved: radiant ↔ thermal ↔ chemical ↔ electrical
• IGCSE examples

  • Sunlight warming the Earth: radiant → thermal.
  • Microwave oven: microwave radiation → thermal energy in food.
  • Solar panel: radiant → electrical.

5. Sound (acoustic) waves

• Definition: Longitudinal pressure waves that propagate by particle vibration in a material medium.
• Intensity: \(I = \dfrac{P}{A}\) (W m\(^{-2}\))
• Typical stores involved: acoustic ↔ thermal (and ↔ electrical when a transducer is involved)
• IGCSE examples

  • Loudspeaker: electrical → acoustic → small amount of thermal loss.
  • Echo in a canyon: sound energy reflected back to the source.
  • Ultrasound in medical imaging: acoustic → minor heating of tissue.

6. Other mechanical waves (e.g., water, seismic)

• Definition: Waves that involve the motion of particles in fluids or solids; energy is transferred by the combined kinetic and potential energy of the particles.
• Typical stores involved: kinetic ↔ potential ↔ electrical (when a generator converts the mechanical energy)
• IGCSE examples

  • Ocean surface wave: wind → kinetic + gravitational potential of water particles → electricity via a wave‑energy converter.
  • Seismic P‑ and S‑waves: elastic energy in Earth’s crust → kinetic energy of particles → heat and damage.

Summary Table of Transfer Mechanisms

Key Points to Remember for the IGCSE Exam

1. Energy is never created or destroyed – it only changes form (Law of Conservation of Energy).
2. Mechanical work transfers energy between mechanical stores; use \(W = Fd\cos\theta\).
3. Electrical work transfers energy via voltage and charge; power is \(P = VI\).
4. Heat always flows from a higher to a lower temperature; the quantity transferred is \(Q = mc\Delta T\).
5. Electromagnetic and sound waves carry energy without a material medium; intensity \(I = P/A\) is useful for exam calculations.
6. When answering a question:

   • Identify the initial and final energy stores.
   • State the transfer mechanism (mechanical work, electrical work, heating, electromagnetic wave, sound wave, other mechanical wave).
   • Apply the appropriate formula to calculate energy, work, power or intensity.