State that energy may be stored as kinetic, gravitational potential, chemical, elastic (strain), nuclear, electrostatic and internal (thermal)

Topic 1.7.1 – Energy (Stored Forms)

Learning Objective

State that energy may be stored as kinetic, gravitational potential, chemical, elastic (strain), nuclear, electrostatic and internal (thermal) energy.

Definition of Energy (IGCSE)

Energy is the capacity to cause a change in a system. In the Cambridge IGCSE syllabus it is considered to be stored in seven distinct forms, each of which can be transformed into another form while the total amount of energy in an isolated system remains constant.

1. Stored Forms of Energy

• Kinetic energy (E_k) – energy of motion.
• Gravitational potential energy (E_g) – energy stored because of an object’s position in a gravitational field.
• Chemical energy – energy stored in the chemical bonds between atoms and molecules.
• Elastic (strain) energy (E_e) – energy stored when a material is stretched or compressed.
• Nuclear energy (E_n) – energy stored in the nucleus of an atom.
• Electrostatic energy (E_es) – energy stored due to the separation of electric charges.
• Internal (thermal) energy (E_th) – energy associated with the random motion of particles within a substance.

2. Key Equations

• Kinetic energy: \(E_k = \dfrac{1}{2}mv^{2}\)
• Gravitational potential energy: \(E_g = mgh\)
• Elastic (spring) energy: \(E_e = \dfrac{1}{2}kx^{2}\)
• Chemical energy: represented by the change in enthalpy, \(\Delta H\) (energy released or absorbed in a reaction).
• Nuclear energy (mass–energy equivalence): \(E_n = \Delta mc^{2}\)
• Electrostatic potential energy (point charges): \(E{es} = \dfrac{k\,q{1}q_{2}}{r}\)
• Internal (thermal) energy of an ideal gas (per mole): \(E_{th} = \dfrac{3}{2}nRT\)

3. Energy‑Transfer Processes (required by the syllabus)

The four basic transfer mechanisms that convert energy from one store to another are:

1. Mechanical work – transfer by a force acting through a distance.

   
   Example: a falling ball loses gravitational potential energy and gains kinetic energy.

2. Electrical work – transfer by the movement of electric charge (current).

   
   Example: a battery (chemical energy) supplies current to a motor, converting chemical energy into kinetic and then into thermal energy.

3. Heating (thermal transfer) – transfer by a temperature difference (conduction, convection, radiation).

   
   Example: a hot kettle transfers internal (thermal) energy to the water, raising its temperature.

4. Wave energy – transfer by a propagating disturbance. The syllabus expects the two main types:

   • Sound waves – e.g. a loudspeaker converts electrical energy into acoustic (wave) energy.
   • Electromagnetic waves (light) – e.g. sunlight delivers electromagnetic energy to a solar cell, which stores it as chemical energy.

4. Conservation of Energy

• In any isolated system the total amount of energy remains constant; energy can only be transferred or transformed between the seven stored forms.
• This principle underpins all the transfer processes listed above and is a core requirement of the IGCSE physics syllabus.

5. Summary Table

6. Conceptual Connections

1. All seven stored‑energy forms can be inter‑converted; the total energy of an isolated system is conserved.
2. Real‑world systems often contain several stores simultaneously – e.g., a roller‑coaster car possesses kinetic and gravitational potential energy, while friction continuously transfers part of the total energy into internal (thermal) energy.
3. Identifying the dominant storage form and the relevant transfer mechanism allows you to predict how energy will be released, stored, or lost in a given situation.
4. Efficiency considerations (energy resources) arise because some transfer processes (e.g., friction, resistance) convert useful energy into internal (thermal) energy that may be difficult to recover.