Wave Motion and Progressive Waves: MDCAT Physics notes
Wave Motion and Progressive Waves notes for MDCAT: mechanical vs electromagnetic waves, need for an elastic medium, energy transfer and periodic waves.
What is a wave?
A wave is a disturbance that travels through space or a medium and carries energy from one place to another without carrying matter along with it. The particles of the medium only vibrate (oscillate) about their mean positions; the energy is passed on from particle to particle through this oscillatory motion.
Mechanical and electromagnetic waves
| Feature | Mechanical waves | Electromagnetic waves |
|---|---|---|
| Medium | Need a material (elastic) medium | No medium needed; travel through vacuum |
| Produced by | Vibrating material bodies | Oscillating electric and magnetic fields (accelerated charges) |
| Speed | Depends on medium; e.g. sound in air about $340\ \text{m s}^{-1}$ | $3\times10^8\ \text{m s}^{-1}$ in vacuum |
| Examples | Sound, infrasonic and ultrasonic waves, waves on ropes, springs and water | Radio waves, infrared, light, X-rays, gamma rays |
Mechanical waves cannot travel through outer space because there are no particles to interact and pass on the disturbance. The speed of sound in vacuum is therefore zero, at any temperature. Mechanical waves are not produced by electric and magnetic fields.
Conditions for a mechanical wave
- A source of vibration.
- An elastic medium: particles must have inertia and restoring (elastic) forces, and be linked to one another so that one particle's motion affects its neighbour.
Progressive (travelling) waves
A progressive wave is one that moves forward continuously away from its source, transferring energy through the medium. Waves on water, waves along a rope flicked at one end, and sound from a speaker are progressive. They may be transverse or longitudinal (see the next topic).
By contrast, a stationary (standing) wave is formed by superposition of two identical waves moving in opposite directions; its pattern does not travel. Stationary waves on strings and in air columns are mechanical and need a medium; light can also form stationary waves, so strictly it is mechanical stationary waves that need a medium.
Periodic waves
If the source vibrates continuously, regularly and rhythmically, it produces a periodic wave: a train of identical disturbances repeating at equal intervals. A single disturbance produces only a pulse.
Energy in a wave
The energy carried by a mechanical wave depends on its amplitude (energy $\propto$ amplitude$^2$) and on its frequency. A larger amplitude sound is louder; a larger amplitude water wave can do more damage.
Common MDCAT traps
- Waves carry energy, not matter; particles only oscillate about mean positions.
- Energy moves by oscillatory motion of particles, not their translation or rotation.
- Among gamma rays, X-rays, infrared and infrasonic waves, only infrasonic (sound) needs a medium.
- Speed of sound in vacuum is zero, not 332 m/s.
- The medium must be elastic, with particles linked to each other.
Quick revision
- Mechanical waves: need a medium (sound, water, rope, spring).
- EM waves: travel through vacuum at $3\times10^8\ \text{m s}^{-1}$.
- Progressive waves carry energy through the medium.
- Continuous rhythmic vibration gives periodic waves.
- Water waves are progressive waves.