Energy and Waves
Energy Stores and Transfers
Energy is stored in different ways and transferred between stores:
| Store | Example |
|---|---|
| Kinetic (KE) | Moving car: KE = ½mv² |
| Gravitational potential (GPE) | Object lifted: GPE = mgh |
| Elastic potential | Stretched spring: E = ½ke² |
| Thermal (internal) | Hot object |
| Chemical | Fuel, food, battery |
| Nuclear | Nucleus of an atom |
| Electrostatic | Charged plates |
| Magnetic | Magnetic field |
Energy is conserved — it cannot be created or destroyed, only transferred between stores (by heating, work done by a force, by waves, by electrical current).
Waves
Transverse and Longitudinal Waves
- Transverse: oscillations perpendicular to direction of travel. Examples: all EM waves, water waves, S-waves.
- Longitudinal: oscillations parallel to direction of travel (compressions and rarefactions). Examples: sound, P-waves.
Key Wave Properties
- Amplitude A: maximum displacement from equilibrium. Related to energy (larger amplitude = more energy).
- Wavelength (λ): distance between two successive identical points (e.g. crest to crest).
- Frequency (f): number of complete waves per second. Unit: hertz (Hz).
- Period (T): time for one complete wave. T = 1/f.
- Wave speed (v): v = fλ (speed = frequency × wavelength).
The Electromagnetic Spectrum
All EM waves travel at 3 × 10⁸ m/s in a vacuum. Listed from longest to shortest wavelength (and lowest to highest frequency):
| Type | Typical wavelength | Uses |
|---|---|---|
| Radio | > 10 cm | Radio/TV broadcasting |
| Microwave | ~cm | Cooking, satellite comms |
| Infrared | ~µm | Thermal imaging, TV remotes |
| Visible | 400–700 nm | Sight, optical fibres |
| Ultraviolet | ~100 nm | Sterilisation, detecting forged notes |
| X-ray | ~0.1 nm | Medical imaging |
| Gamma | < 0.01 nm | Cancer treatment, sterilising equipment |
Higher frequency = higher energy per photon = more potentially harmful radiation.
Sound
Sound is a longitudinal mechanical wave (requires a medium — cannot travel in a vacuum). The compressions and rarefactions travel through air (or other materials).
Speed of sound: ~340 m/s in air, ~1500 m/s in water, ~5000 m/s in steel. Sound travels faster in denser/stiffer materials because the particles interact more strongly.
Hearing range: humans hear approximately 20 Hz to 20 000 Hz (20 kHz). Ultrasound is above 20 kHz (used in medical scanning, sonar, quality control).
Calculating distance using echoes: distance = (speed × time) / 2.
⚠Common mistakes
- Confusing frequency and wavelength relationship: higher frequency → shorter wavelength (they are inversely proportional for a given speed).
- Wave speed depends on medium, not on frequency/amplitude: changing the frequency of a sound does not change its speed in air (at a given temperature).
- Transverse vs longitudinal: all electromagnetic waves are transverse; sound is longitudinal.
- EM spectrum order: students often miss infrared between microwave and visible, or place UV and X-rays in the wrong order.
AI-generated · claude-opus-4-7 · v3-wjec-physics