Energy Stores and Transfers
Energy Stores
Energy is stored in different ways. The WJEC Eduqas specification uses the "energy store" model:
| Energy Store | Examples |
|---|---|
| Kinetic | Any moving object (car, ball, electron, air molecule) |
| Gravitational potential | An object raised above the ground (apple on a shelf, water in a reservoir) |
| Elastic potential | A stretched spring, compressed gas, bent ruler |
| Chemical | Bonds in food, fuel, batteries |
| Thermal (internal) | Hot objects — increased vibration of particles |
| Electrostatic | Separated charges (a charged Van de Graaff generator) |
| Nuclear | Stored in the nucleus of atoms; released in fission/fusion |
| Magnetic | In magnets and magnetic fields |
Energy Transfers
Energy is transferred between stores by:
- Mechanically (work done by forces — pushing, pulling)
- Electrically (by an electric current)
- By heating (thermal transfer — conduction, convection, radiation)
- By radiation (electromagnetic waves, e.g., light, infrared, microwaves)
Conservation of energy: Energy cannot be created or destroyed — only transferred between stores. The total energy in a closed system remains constant.
Key Calculations
Kinetic Energy (KE)
$$E_k = rac{1}{2}mv^2$$
Where:
- $E_k$ = kinetic energy (J)
- $m$ = mass (kg)
- $v$ = velocity (m/s)
Example: Calculate the KE of a 2 kg ball travelling at 5 m/s. $E_k = rac{1}{2} imes 2 imes 5^2 = rac{1}{2} imes 2 imes 25 = 25$ J
Gravitational Potential Energy (GPE)
$$E_p = mgh$$
Where:
- $E_p$ = GPE (J)
- $m$ = mass (kg)
- $g$ = gravitational field strength (9.8 N/kg on Earth; use 10 N/kg if specified)
- $h$ = height above reference point (m)
Example: Calculate the GPE of a 5 kg object raised 3 m above the ground (g = 10 N/kg). $E_p = 5 imes 10 imes 3 = 150$ J
Elastic Potential Energy (EPE)
$$E_e = rac{1}{2}ke^2$$
Where:
- $E_e$ = elastic PE (J)
- $k$ = spring constant (N/m) — found on the equation sheet
- $e$ = extension (m) — how much the spring is stretched from its natural length
Example: A spring with k = 100 N/m is extended by 0.2 m. $E_e = rac{1}{2} imes 100 imes 0.2^2 = rac{1}{2} imes 100 imes 0.04 = 2$ J
Applying Conservation of Energy
When a ball is dropped:
- Initial state: high GPE, zero KE
- As it falls: GPE decreases, KE increases
- At the bottom (ignoring air resistance): all GPE has converted to KE
Example: A 2 kg ball falls from 5 m height (g = 10 N/kg). What is its speed just before hitting the ground?
GPE at top = $mgh = 2 imes 10 imes 5 = 100$ J KE at bottom = 100 J (all GPE converted to KE) $100 = rac{1}{2} imes 2 imes v^2$ $100 = v^2$ $v = 10$ m/s
Efficiency
$$ ext{Efficiency} = rac{ ext{Useful energy output}}{ ext{Total energy input}} imes 100%$$
Example: A motor transfers 200 J of input energy; 150 J are useful (mechanical) energy. Efficiency = 150/200 × 100 = 75%.
AI-generated · claude-opus-4-7 · v3-wjec-combined-science