1.7.1 Energy
Energy Transfer: The Big Picture ⚡️
Energy is the ability to do work. During any event or process, energy moves from one store (or form) to another. Think of it like a game of passing a ball – the ball (energy) travels from one player (store) to the next until it reaches the goal (useful work).
Mechanical Work (Force × Distance) 🏋️♂️
When a force is applied to an object and the object moves, mechanical work is done. The amount of work is given by:
\$W = F \cdot d \cdot \cos\theta\$
- Example: Lifting a box 2 m high with a 50 N force (θ = 0°) does \$W = 50 \times 2 = 100\$ J.
- Analogy: Pushing a car uphill is like pulling a sled up a slope – the harder you push, the more energy you transfer.
Electrical Work (Current × Voltage) 🔋
Electricity transfers energy when a current flows through a voltage difference. The work done is:
\$W = V \cdot Q = I \cdot V \cdot t\$
- Example: A 12 V battery supplying 2 A for 3 s does \$W = 12 \times 2 \times 3 = 72\$ J.
- Analogy: Think of electrons as tiny runners carrying energy down a track; the voltage is the slope that pushes them.
Heat Transfer (Thermal Energy) 🔥
Heat moves from a hotter body to a cooler one until temperatures equalise. The energy transferred is:
\$Q = m \cdot c \cdot \Delta T\$
- Example: Heating 0.5 kg of water from 20 °C to 80 °C (c = 4.18 kJ kg⁻¹ K⁻¹) transfers \$Q = 0.5 \times 4.18 \times 60 \approx 125\$ kJ.
- Analogy: Heat is like a crowd of energetic dancers; when they bump into each other, they spread their energy around.
Wave Energy (Electromagnetic, Sound, etc.) 🎵🌊
Energy can travel through space as waves. For electromagnetic waves, the power passing through an area is:
\$P = I \cdot A\$
- Electromagnetic: Sunlight reaching Earth delivers energy that powers photosynthesis and keeps us warm.
- Sound: Musical instruments convert mechanical vibrations into sound waves that carry energy to our ears.
- Other waves: Seismic waves transfer energy through the Earth during earthquakes.
Summary Table of Energy Transfer
| Store | Transfer Mechanism | Key Formula | Typical Example |
|---|
| Mechanical | Work by force | \$W = F \cdot d \cdot \cos\theta\$ | Lifting a box |
| Electrical | Electrical work | \$W = I \cdot V \cdot t\$ | Charging a phone |
| Thermal | Heat flow | \$Q = m \cdot c \cdot \Delta T\$ | Boiling water |
| Wave | Wave propagation | \$P = I \cdot A\$ (EM) | Sunlight, radio |